[Note that this file is a concatenation of more than one RFC.]
RFC 821
SIMPLE MAIL TRANSFER PROTOCOL
Jonathan B. Postel
August 1982
Information Sciences Institute
University of Southern California
4676 Admiralty Way
Marina del Rey, California 90291
(213) 822-1511
RFC 821 August 1982
Simple Mail Transfer Protocol
TABLE OF CONTENTS
1. INTRODUCTION .................................................. 1
2. THE SMTP MODEL ................................................ 2
3. THE SMTP PROCEDURE ............................................ 4
3.1. Mail ..................................................... 4
3.2. Forwarding ............................................... 7
3.3. Verifying and Expanding .................................. 8
3.4. Sending and Mailing ..................................... 11
3.5. Opening and Closing ..................................... 13
3.6. Relaying ................................................ 14
3.7. Domains ................................................. 17
3.8. Changing Roles .......................................... 18
4. THE SMTP SPECIFICATIONS ...................................... 19
4.1. SMTP Commands ........................................... 19
4.1.1. Command Semantics ..................................... 19
4.1.2. Command Syntax ........................................ 27
4.2. SMTP Replies ............................................ 34
4.2.1. Reply Codes by Function Group ......................... 35
4.2.2. Reply Codes in Numeric Order .......................... 36
4.3. Sequencing of Commands and Replies ...................... 37
4.4. State Diagrams .......................................... 39
4.5. Details ................................................. 41
4.5.1. Minimum Implementation ................................ 41
4.5.2. Transparency .......................................... 41
4.5.3. Sizes ................................................. 42
APPENDIX A: TCP ................................................. 44
APPENDIX B: NCP ................................................. 45
APPENDIX C: NITS ................................................ 46
APPENDIX D: X.25 ................................................ 47
APPENDIX E: Theory of Reply Codes ............................... 48
APPENDIX F: Scenarios ........................................... 51
GLOSSARY ......................................................... 64
REFERENCES ....................................................... 67
Network Working Group J. Postel
Request for Comments: DRAFT ISI
Replaces: RFC 788, 780, 772 August 1982
SIMPLE MAIL TRANSFER PROTOCOL
1. INTRODUCTION
The objective of Simple Mail Transfer Protocol (SMTP) is to transfer
mail reliably and efficiently.
SMTP is independent of the particular transmission subsystem and
requires only a reliable ordered data stream channel. Appendices A,
B, C, and D describe the use of SMTP with various transport services.
A Glossary provides the definitions of terms as used in this
document.
An important feature of SMTP is its capability to relay mail across
transport service environments. A transport service provides an
interprocess communication environment (IPCE). An IPCE may cover one
network, several networks, or a subset of a network. It is important
to realize that transport systems (or IPCEs) are not one-to-one with
networks. A process can communicate directly with another process
through any mutually known IPCE. Mail is an application or use of
interprocess communication. Mail can be communicated between
processes in different IPCEs by relaying through a process connected
to two (or more) IPCEs. More specifically, mail can be relayed
between hosts on different transport systems by a host on both
transport systems.
Postel [Page 1]
August 1982 RFC 821
Simple Mail Transfer Protocol
2. THE SMTP MODEL
The SMTP design is based on the following model of communication: as
the result of a user mail request, the sender-SMTP establishes a
two-way transmission channel to a receiver-SMTP. The receiver-SMTP
may be either the ultimate destination or an intermediate. SMTP
commands are generated by the sender-SMTP and sent to the
receiver-SMTP. SMTP replies are sent from the receiver-SMTP to the
sender-SMTP in response to the commands.
Once the transmission channel is established, the SMTP-sender sends a
MAIL command indicating the sender of the mail. If the SMTP-receiver
can accept mail it responds with an OK reply. The SMTP-sender then
sends a RCPT command identifying a recipient of the mail. If the
SMTP-receiver can accept mail for that recipient it responds with an
OK reply; if not, it responds with a reply rejecting that recipient
(but not the whole mail transaction). The SMTP-sender and
SMTP-receiver may negotiate several recipients. When the recipients
have been negotiated the SMTP-sender sends the mail data, terminating
with a special sequence. If the SMTP-receiver successfully processes
the mail data it responds with an OK reply. The dialog is purposely
lock-step, one-at-a-time.
-------------------------------------------------------------
+----------+ +----------+
+------+ | | | |
| User |<-->| | SMTP | |
+------+ | Sender- |Commands/Replies| Receiver-|
+------+ | SMTP |<-------------->| SMTP | +------+
| File |<-->| | and Mail | |<-->| File |
|System| | | | | |System|
+------+ +----------+ +----------+ +------+
Sender-SMTP Receiver-SMTP
Model for SMTP Use
Figure 1
-------------------------------------------------------------
The SMTP provides mechanisms for the transmission of mail; directly
from the sending user's host to the receiving user's host when the
[Page 2] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
two host are connected to the same transport service, or via one or
more relay SMTP-servers when the source and destination hosts are not
connected to the same transport service.
To be able to provide the relay capability the SMTP-server must be
supplied with the name of the ultimate destination host as well as
the destination mailbox name.
The argument to the MAIL command is a reverse-path, which specifies
who the mail is from. The argument to the RCPT command is a
forward-path, which specifies who the mail is to. The forward-path
is a source route, while the reverse-path is a return route (which
may be used to return a message to the sender when an error occurs
with a relayed message).
When the same message is sent to multiple recipients the SMTP
encourages the transmission of only one copy of the data for all the
recipients at the same destination host.
The mail commands and replies have a rigid syntax. Replies also have
a numeric code. In the following, examples appear which use actual
commands and replies. The complete lists of commands and replies
appears in Section 4 on specifications.
Commands and replies are not case sensitive. That is, a command or
reply word may be upper case, lower case, or any mixture of upper and
lower case. Note that this is not true of mailbox user names. For
some hosts the user name is case sensitive, and SMTP implementations
must take case to preserve the case of user names as they appear in
mailbox arguments. Host names are not case sensitive.
Commands and replies are composed of characters from the ASCII
character set [1]. When the transport service provides an 8-bit byte
(octet) transmission channel, each 7-bit character is transmitted
right justified in an octet with the high order bit cleared to zero.
When specifying the general form of a command or reply, an argument
(or special symbol) will be denoted by a meta-linguistic variable (or
constant), for example, "<string>" or "<reverse-path>". Here the
angle brackets indicate these are meta-linguistic variables.
However, some arguments use the angle brackets literally. For
example, an actual reverse-path is enclosed in angle brackets, i.e.,
"<
[email protected]>" is an instance of <reverse-path> (the
angle brackets are actually transmitted in the command or reply).
Postel [Page 3]
August 1982 RFC 821
Simple Mail Transfer Protocol
3. THE SMTP PROCEDURES
This section presents the procedures used in SMTP in several parts.
First comes the basic mail procedure defined as a mail transaction.
Following this are descriptions of forwarding mail, verifying mailbox
names and expanding mailing lists, sending to terminals instead of or
in combination with mailboxes, and the opening and closing exchanges.
At the end of this section are comments on relaying, a note on mail
domains, and a discussion of changing roles. Throughout this section
are examples of partial command and reply sequences, several complete
scenarios are presented in Appendix F.
3.1. MAIL
There are three steps to SMTP mail transactions. The transaction
is started with a MAIL command which gives the sender
identification. A series of one or more RCPT commands follows
giving the receiver information. Then a DATA command gives the
mail data. And finally, the end of mail data indicator confirms
the transaction.
The first step in the procedure is the MAIL command. The
<reverse-path> contains the source mailbox.
MAIL <SP> FROM:<reverse-path> <CRLF>
This command tells the SMTP-receiver that a new mail
transaction is starting and to reset all its state tables and
buffers, including any recipients or mail data. It gives the
reverse-path which can be used to report errors. If accepted,
the receiver-SMTP returns a 250 OK reply.
The <reverse-path> can contain more than just a mailbox. The
<reverse-path> is a reverse source routing list of hosts and
source mailbox. The first host in the <reverse-path> should be
the host sending this command.
The second step in the procedure is the RCPT command.
RCPT <SP> TO:<forward-path> <CRLF>
This command gives a forward-path identifying one recipient.
If accepted, the receiver-SMTP returns a 250 OK reply, and
stores the forward-path. If the recipient is unknown the
receiver-SMTP returns a 550 Failure reply. This second step of
the procedure can be repeated any number of times.
[Page 4] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
The <forward-path> can contain more than just a mailbox. The
<forward-path> is a source routing list of hosts and the
destination mailbox. The first host in the <forward-path>
should be the host receiving this command.
The third step in the procedure is the DATA command.
DATA <CRLF>
If accepted, the receiver-SMTP returns a 354 Intermediate reply
and considers all succeeding lines to be the message text.
When the end of text is received and stored the SMTP-receiver
sends a 250 OK reply.
Since the mail data is sent on the transmission channel the end
of the mail data must be indicated so that the command and
reply dialog can be resumed. SMTP indicates the end of the
mail data by sending a line containing only a period. A
transparency procedure is used to prevent this from interfering
with the user's text (see Section 4.5.2).
Please note that the mail data includes the memo header
items such as Date, Subject, To, Cc, From [2].
The end of mail data indicator also confirms the mail
transaction and tells the receiver-SMTP to now process the
stored recipients and mail data. If accepted, the
receiver-SMTP returns a 250 OK reply. The DATA command should
fail only if the mail transaction was incomplete (for example,
no recipients), or if resources are not available.
The above procedure is an example of a mail transaction. These
commands must be used only in the order discussed above.
Example 1 (below) illustrates the use of these commands in a mail
transaction.
Postel [Page 5]
August 1982 RFC 821
Simple Mail Transfer Protocol
-------------------------------------------------------------
Example of the SMTP Procedure
This SMTP example shows mail sent by Smith at host Alpha.ARPA,
to Jones, Green, and Brown at host Beta.ARPA. Here we assume
that host Alpha contacts host Beta directly.
S: MAIL FROM:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 550 No such user here
S: RCPT TO:<
[email protected]>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: <CRLF>.<CRLF>
R: 250 OK
The mail has now been accepted for Jones and Brown. Green did
not have a mailbox at host Beta.
Example 1
-------------------------------------------------------------
[Page 6] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
3.2. FORWARDING
There are some cases where the destination information in the
<forward-path> is incorrect, but the receiver-SMTP knows the
correct destination. In such cases, one of the following replies
should be used to allow the sender to contact the correct
destination.
251 User not local; will forward to <forward-path>
This reply indicates that the receiver-SMTP knows the user's
mailbox is on another host and indicates the correct
forward-path to use in the future. Note that either the
host or user or both may be different. The receiver takes
responsibility for delivering the message.
551 User not local; please try <forward-path>
This reply indicates that the receiver-SMTP knows the user's
mailbox is on another host and indicates the correct
forward-path to use. Note that either the host or user or
both may be different. The receiver refuses to accept mail
for this user, and the sender must either redirect the mail
according to the information provided or return an error
response to the originating user.
Example 2 illustrates the use of these responses.
-------------------------------------------------------------
Example of Forwarding
Either
S: RCPT TO:<
[email protected]>
R: 251 User not local; will forward to <
[email protected]>
Or
S: RCPT TO:<
[email protected]>
R: 551 User not local; please try <
[email protected]>
Example 2
-------------------------------------------------------------
Postel [Page 7]
August 1982 RFC 821
Simple Mail Transfer Protocol
3.3. VERIFYING AND EXPANDING
SMTP provides as additional features, commands to verify a user
name or expand a mailing list. This is done with the VRFY and
EXPN commands, which have character string arguments. For the
VRFY command, the string is a user name, and the response may
include the full name of the user and must include the mailbox of
the user. For the EXPN command, the string identifies a mailing
list, and the multiline response may include the full name of the
users and must give the mailboxes on the mailing list.
"User name" is a fuzzy term and used purposely. If a host
implements the VRFY or EXPN commands then at least local mailboxes
must be recognized as "user names". If a host chooses to
recognize other strings as "user names" that is allowed.
In some hosts the distinction between a mailing list and an alias
for a single mailbox is a bit fuzzy, since a common data structure
may hold both types of entries, and it is possible to have mailing
lists of one mailbox. If a request is made to verify a mailing
list a positive response can be given if on receipt of a message
so addressed it will be delivered to everyone on the list,
otherwise an error should be reported (e.g., "550 That is a
mailing list, not a user"). If a request is made to expand a user
name a positive response can be formed by returning a list
containing one name, or an error can be reported (e.g., "550 That
is a user name, not a mailing list").
In the case of a multiline reply (normal for EXPN) exactly one
mailbox is to be specified on each line of the reply. In the case
of an ambiguous request, for example, "VRFY Smith", where there
are two Smith's the response must be "553 User ambiguous".
The case of verifying a user name is straightforward as shown in
example 3.
[Page 8] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
-------------------------------------------------------------
Example of Verifying a User Name
Either
S: VRFY Smith
R: 250 Fred Smith <
[email protected]>
Or
S: VRFY Smith
R: 251 User not local; will forward to <
[email protected]>
Or
S: VRFY Jones
R: 550 String does not match anything.
Or
S: VRFY Jones
R: 551 User not local; please try <
[email protected]>
Or
S: VRFY Gourzenkyinplatz
R: 553 User ambiguous.
Example 3
-------------------------------------------------------------
Postel [Page 9]
August 1982 RFC 821
Simple Mail Transfer Protocol
The case of expanding a mailbox list requires a multiline reply as
shown in example 4.
-------------------------------------------------------------
Example of Expanding a Mailing List
Either
S: EXPN Example-People
R: 250-Jon Postel <
[email protected]>
R: 250-Fred Fonebone <
[email protected]>
R: 250-Sam Q. Smith <
[email protected]>
R: 250-Quincy Smith <@USC-ISIF.ARPA:
[email protected]>
R: 250-<
[email protected]>
R: 250 <
[email protected]>
Or
S: EXPN Executive-Washroom-List
R: 550 Access Denied to You.
Example 4
-------------------------------------------------------------
The character string arguments of the VRFY and EXPN commands
cannot be further restricted due to the variety of implementations
of the user name and mailbox list concepts. On some systems it
may be appropriate for the argument of the EXPN command to be a
file name for a file containing a mailing list, but again there is
a variety of file naming conventions in the Internet.
The VRFY and EXPN commands are not included in the minimum
implementation (Section 4.5.1), and are not required to work
across relays when they are implemented.
[Page 10] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
3.4. SENDING AND MAILING
The main purpose of SMTP is to deliver messages to user's
mailboxes. A very similar service provided by some hosts is to
deliver messages to user's terminals (provided the user is active
on the host). The delivery to the user's mailbox is called
"mailing", the delivery to the user's terminal is called
"sending". Because in many hosts the implementation of sending is
nearly identical to the implementation of mailing these two
functions are combined in SMTP. However the sending commands are
not included in the required minimum implementation
(Section 4.5.1). Users should have the ability to control the
writing of messages on their terminals. Most hosts permit the
users to accept or refuse such messages.
The following three command are defined to support the sending
options. These are used in the mail transaction instead of the
MAIL command and inform the receiver-SMTP of the special semantics
of this transaction:
SEND <SP> FROM:<reverse-path> <CRLF>
The SEND command requires that the mail data be delivered to
the user's terminal. If the user is not active (or not
accepting terminal messages) on the host a 450 reply may
returned to a RCPT command. The mail transaction is
successful if the message is delivered the terminal.
SOML <SP> FROM:<reverse-path> <CRLF>
The Send Or MaiL command requires that the mail data be
delivered to the user's terminal if the user is active (and
accepting terminal messages) on the host. If the user is
not active (or not accepting terminal messages) then the
mail data is entered into the user's mailbox. The mail
transaction is successful if the message is delivered either
to the terminal or the mailbox.
SAML <SP> FROM:<reverse-path> <CRLF>
The Send And MaiL command requires that the mail data be
delivered to the user's terminal if the user is active (and
accepting terminal messages) on the host. In any case the
mail data is entered into the user's mailbox. The mail
transaction is successful if the message is delivered the
mailbox.
Postel [Page 11]
August 1982 RFC 821
Simple Mail Transfer Protocol
The same reply codes that are used for the MAIL commands are used
for these commands.
[Page 12] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
3.5. OPENING AND CLOSING
At the time the transmission channel is opened there is an
exchange to ensure that the hosts are communicating with the hosts
they think they are.
The following two commands are used in transmission channel
opening and closing:
HELO <SP> <domain> <CRLF>
QUIT <CRLF>
In the HELO command the host sending the command identifies
itself; the command may be interpreted as saying "Hello, I am
<domain>".
-------------------------------------------------------------
Example of Connection Opening
R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIF.ARPA
R: 250 BBN-UNIX.ARPA
Example 5
-------------------------------------------------------------
-------------------------------------------------------------
Example of Connection Closing
S: QUIT
R: 221 BBN-UNIX.ARPA Service closing transmission channel
Example 6
-------------------------------------------------------------
Postel [Page 13]
August 1982 RFC 821
Simple Mail Transfer Protocol
3.6. RELAYING
The forward-path may be a source route of the form
"@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE are hosts. This
form is used to emphasize the distinction between an address and a
route. The mailbox is an absolute address, and the route is
information about how to get there. The two concepts should not
be confused.
Conceptually the elements of the forward-path are moved to the
reverse-path as the message is relayed from one server-SMTP to
another. The reverse-path is a reverse source route, (i.e., a
source route from the current location of the message to the
originator of the message). When a server-SMTP deletes its
identifier from the forward-path and inserts it into the
reverse-path, it must use the name it is known by in the
environment it is sending into, not the environment the mail came
from, in case the server-SMTP is known by different names in
different environments.
If when the message arrives at an SMTP the first element of the
forward-path is not the identifier of that SMTP the element is not
deleted from the forward-path and is used to determine the next
SMTP to send the message to. In any case, the SMTP adds its own
identifier to the reverse-path.
Using source routing the receiver-SMTP receives mail to be relayed
to another server-SMTP The receiver-SMTP may accept or reject the
task of relaying the mail in the same way it accepts or rejects
mail for a local user. The receiver-SMTP transforms the command
arguments by moving its own identifier from the forward-path to
the beginning of the reverse-path. The receiver-SMTP then becomes
a sender-SMTP, establishes a transmission channel to the next SMTP
in the forward-path, and sends it the mail.
The first host in the reverse-path should be the host sending the
SMTP commands, and the first host in the forward-path should be
the host receiving the SMTP commands.
Notice that the forward-path and reverse-path appear in the SMTP
commands and replies, but not necessarily in the message. That
is, there is no need for these paths and especially this syntax to
appear in the "To:" , "From:", "CC:", etc. fields of the message
header.
If a server-SMTP has accepted the task of relaying the mail and
[Page 14] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
later finds that the forward-path is incorrect or that the mail
cannot be delivered for whatever reason, then it must construct an
"undeliverable mail" notification message and send it to the
originator of the undeliverable mail (as indicated by the
reverse-path).
This notification message must be from the server-SMTP at this
host. Of course, server-SMTPs should not send notification
messages about problems with notification messages. One way to
prevent loops in error reporting is to specify a null reverse-path
in the MAIL command of a notification message. When such a
message is relayed it is permissible to leave the reverse-path
null. A MAIL command with a null reverse-path appears as follows:
MAIL FROM:<>
An undeliverable mail notification message is shown in example 7.
This notification is in response to a message originated by JOE at
HOSTW and sent via HOSTX to HOSTY with instructions to relay it on
to HOSTZ. What we see in the example is the transaction between
HOSTY and HOSTX, which is the first step in the return of the
notification message.
Postel [Page 15]
August 1982 RFC 821
Simple Mail Transfer Protocol
-------------------------------------------------------------
Example Undeliverable Mail Notification Message
S: MAIL FROM:<>
R: 250 ok
S: RCPT TO:<@HOSTX.ARPA:
[email protected]>
R: 250 ok
S: DATA
R: 354 send the mail data, end with .
S: Date: 23 Oct 81 11:22:33
S: From:
[email protected]
S: To:
[email protected]
S: Subject: Mail System Problem
S:
S: Sorry JOE, your message to
[email protected] lost.
S: HOSTZ.ARPA said this:
S: "550 No Such User"
S: .
R: 250 ok
Example 7
-------------------------------------------------------------
[Page 16] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
3.7. DOMAINS
Domains are a recently introduced concept in the ARPA Internet
mail system. The use of domains changes the address space from a
flat global space of simple character string host names to a
hierarchically structured rooted tree of global addresses. The
host name is replaced by a domain and host designator which is a
sequence of domain element strings separated by periods with the
understanding that the domain elements are ordered from the most
specific to the most general.
For example, "USC-ISIF.ARPA", "Fred.Cambridge.UK", and
"PC7.LCS.MIT.ARPA" might be host-and-domain identifiers.
Whenever domain names are used in SMTP only the official names are
used, the use of nicknames or aliases is not allowed.
Postel [Page 17]
August 1982 RFC 821
Simple Mail Transfer Protocol
3.8. CHANGING ROLES
The TURN command may be used to reverse the roles of the two
programs communicating over the transmission channel.
If program-A is currently the sender-SMTP and it sends the TURN
command and receives an ok reply (250) then program-A becomes the
receiver-SMTP.
If program-B is currently the receiver-SMTP and it receives the
TURN command and sends an ok reply (250) then program-B becomes
the sender-SMTP.
To refuse to change roles the receiver sends the 502 reply.
Please note that this command is optional. It would not normally
be used in situations where the transmission channel is TCP.
However, when the cost of establishing the transmission channel is
high, this command may be quite useful. For example, this command
may be useful in supporting be mail exchange using the public
switched telephone system as a transmission channel, especially if
some hosts poll other hosts for mail exchanges.
[Page 18] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
4. THE SMTP SPECIFICATIONS
4.1. SMTP COMMANDS
4.1.1. COMMAND SEMANTICS
The SMTP commands define the mail transfer or the mail system
function requested by the user. SMTP commands are character
strings terminated by <CRLF>. The command codes themselves are
alphabetic characters terminated by <SP> if parameters follow
and <CRLF> otherwise. The syntax of mailboxes must conform to
receiver site conventions. The SMTP commands are discussed
below. The SMTP replies are discussed in the Section 4.2.
A mail transaction involves several data objects which are
communicated as arguments to different commands. The
reverse-path is the argument of the MAIL command, the
forward-path is the argument of the RCPT command, and the mail
data is the argument of the DATA command. These arguments or
data objects must be transmitted and held pending the
confirmation communicated by the end of mail data indication
which finalizes the transaction. The model for this is that
distinct buffers are provided to hold the types of data
objects, that is, there is a reverse-path buffer, a
forward-path buffer, and a mail data buffer. Specific commands
cause information to be appended to a specific buffer, or cause
one or more buffers to be cleared.
HELLO (HELO)
This command is used to identify the sender-SMTP to the
receiver-SMTP. The argument field contains the host name of
the sender-SMTP.
The receiver-SMTP identifies itself to the sender-SMTP in
the connection greeting reply, and in the response to this
command.
This command and an OK reply to it confirm that both the
sender-SMTP and the receiver-SMTP are in the initial state,
that is, there is no transaction in progress and all state
tables and buffers are cleared.
Postel [Page 19]
August 1982 RFC 821
Simple Mail Transfer Protocol
MAIL (MAIL)
This command is used to initiate a mail transaction in which
the mail data is delivered to one or more mailboxes. The
argument field contains a reverse-path.
The reverse-path consists of an optional list of hosts and
the sender mailbox. When the list of hosts is present, it
is a "reverse" source route and indicates that the mail was
relayed through each host on the list (the first host in the
list was the most recent relay). This list is used as a
source route to return non-delivery notices to the sender.
As each relay host adds itself to the beginning of the list,
it must use its name as known in the IPCE to which it is
relaying the mail rather than the IPCE from which the mail
came (if they are different). In some types of error
reporting messages (for example, undeliverable mail
notifications) the reverse-path may be null (see Example 7).
This command clears the reverse-path buffer, the
forward-path buffer, and the mail data buffer; and inserts
the reverse-path information from this command into the
reverse-path buffer.
RECIPIENT (RCPT)
This command is used to identify an individual recipient of
the mail data; multiple recipients are specified by multiple
use of this command.
The forward-path consists of an optional list of hosts and a
required destination mailbox. When the list of hosts is
present, it is a source route and indicates that the mail
must be relayed to the next host on the list. If the
receiver-SMTP does not implement the relay function it may
user the same reply it would for an unknown local user
(550).
When mail is relayed, the relay host must remove itself from
the beginning forward-path and put itself at the beginning
of the reverse-path. When mail reaches its ultimate
destination (the forward-path contains only a destination
mailbox), the receiver-SMTP inserts it into the destination
mailbox in accordance with its host mail conventions.
[Page 20] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
For example, mail received at relay host A with arguments
FROM:<
[email protected]>
TO:<@HOSTA.ARPA,@HOSTB.ARPA:
[email protected]>
will be relayed on to host B with arguments
FROM:<@HOSTA.ARPA:
[email protected]>
TO:<@HOSTB.ARPA:
[email protected]>.
This command causes its forward-path argument to be appended
to the forward-path buffer.
DATA (DATA)
The receiver treats the lines following the command as mail
data from the sender. This command causes the mail data
from this command to be appended to the mail data buffer.
The mail data may contain any of the 128 ASCII character
codes.
The mail data is terminated by a line containing only a
period, that is the character sequence "<CRLF>.<CRLF>" (see
Section 4.5.2 on Transparency). This is the end of mail
data indication.
The end of mail data indication requires that the receiver
must now process the stored mail transaction information.
This processing consumes the information in the reverse-path
buffer, the forward-path buffer, and the mail data buffer,
and on the completion of this command these buffers are
cleared. If the processing is successful the receiver must
send an OK reply. If the processing fails completely the
receiver must send a failure reply.
When the receiver-SMTP accepts a message either for relaying
or for final delivery it inserts at the beginning of the
mail data a time stamp line. The time stamp line indicates
the identity of the host that sent the message, and the
identity of the host that received the message (and is
inserting this time stamp), and the date and time the
message was received. Relayed messages will have multiple
time stamp lines.
When the receiver-SMTP makes the "final delivery" of a
message it inserts at the beginning of the mail data a
Postel [Page 21]
August 1982 RFC 821
Simple Mail Transfer Protocol
return path line. The return path line preserves the
information in the <reverse-path> from the MAIL command.
Here, final delivery means the message leaves the SMTP
world. Normally, this would mean it has been delivered to
the destination user, but in some cases it may be further
processed and transmitted by another mail system.
It is possible for the mailbox in the return path be
different from the actual sender's mailbox, for example,
if error responses are to be delivered a special error
handling mailbox rather than the message senders.
The preceding two paragraphs imply that the final mail data
will begin with a return path line, followed by one or more
time stamp lines. These lines will be followed by the mail
data header and body [2]. See Example 8.
Special mention is needed of the response and further action
required when the processing following the end of mail data
indication is partially successful. This could arise if
after accepting several recipients and the mail data, the
receiver-SMTP finds that the mail data can be successfully
delivered to some of the recipients, but it cannot be to
others (for example, due to mailbox space allocation
problems). In such a situation, the response to the DATA
command must be an OK reply. But, the receiver-SMTP must
compose and send an "undeliverable mail" notification
message to the originator of the message. Either a single
notification which lists all of the recipients that failed
to get the message, or separate notification messages must
be sent for each failed recipient (see Example 7). All
undeliverable mail notification messages are sent using the
MAIL command (even if they result from processing a SEND,
SOML, or SAML command).
[Page 22] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
-------------------------------------------------------------
Example of Return Path and Received Time Stamps
Return-Path: <@GHI.ARPA,@DEF.ARPA,@ABC.ARPA:
[email protected]>
Received: from GHI.ARPA by JKL.ARPA ; 27 Oct 81 15:27:39 PST
Received: from DEF.ARPA by GHI.ARPA ; 27 Oct 81 15:15:13 PST
Received: from ABC.ARPA by DEF.ARPA ; 27 Oct 81 15:01:59 PST
Date: 27 Oct 81 15:01:01 PST
From:
[email protected]
Subject: Improved Mailing System Installed
To:
[email protected]
This is to inform you that ...
Example 8
-------------------------------------------------------------
SEND (SEND)
This command is used to initiate a mail transaction in which
the mail data is delivered to one or more terminals. The
argument field contains a reverse-path. This command is
successful if the message is delivered to a terminal.
The reverse-path consists of an optional list of hosts and
the sender mailbox. When the list of hosts is present, it
is a "reverse" source route and indicates that the mail was
relayed through each host on the list (the first host in the
list was the most recent relay). This list is used as a
source route to return non-delivery notices to the sender.
As each relay host adds itself to the beginning of the list,
it must use its name as known in the IPCE to which it is
relaying the mail rather than the IPCE from which the mail
came (if they are different).
This command clears the reverse-path buffer, the
forward-path buffer, and the mail data buffer; and inserts
the reverse-path information from this command into the
reverse-path buffer.
SEND OR MAIL (SOML)
This command is used to initiate a mail transaction in which
the mail data is delivered to one or more terminals or
Postel [Page 23]
August 1982 RFC 821
Simple Mail Transfer Protocol
mailboxes. For each recipient the mail data is delivered to
the recipient's terminal if the recipient is active on the
host (and accepting terminal messages), otherwise to the
recipient's mailbox. The argument field contains a
reverse-path. This command is successful if the message is
delivered to a terminal or the mailbox.
The reverse-path consists of an optional list of hosts and
the sender mailbox. When the list of hosts is present, it
is a "reverse" source route and indicates that the mail was
relayed through each host on the list (the first host in the
list was the most recent relay). This list is used as a
source route to return non-delivery notices to the sender.
As each relay host adds itself to the beginning of the list,
it must use its name as known in the IPCE to which it is
relaying the mail rather than the IPCE from which the mail
came (if they are different).
This command clears the reverse-path buffer, the
forward-path buffer, and the mail data buffer; and inserts
the reverse-path information from this command into the
reverse-path buffer.
SEND AND MAIL (SAML)
This command is used to initiate a mail transaction in which
the mail data is delivered to one or more terminals and
mailboxes. For each recipient the mail data is delivered to
the recipient's terminal if the recipient is active on the
host (and accepting terminal messages), and for all
recipients to the recipient's mailbox. The argument field
contains a reverse-path. This command is successful if the
message is delivered to the mailbox.
The reverse-path consists of an optional list of hosts and
the sender mailbox. When the list of hosts is present, it
is a "reverse" source route and indicates that the mail was
relayed through each host on the list (the first host in the
list was the most recent relay). This list is used as a
source route to return non-delivery notices to the sender.
As each relay host adds itself to the beginning of the list,
it must use its name as known in the IPCE to which it is
relaying the mail rather than the IPCE from which the mail
came (if they are different).
This command clears the reverse-path buffer, the
[Page 24] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
forward-path buffer, and the mail data buffer; and inserts
the reverse-path information from this command into the
reverse-path buffer.
RESET (RSET)
This command specifies that the current mail transaction is
to be aborted. Any stored sender, recipients, and mail data
must be discarded, and all buffers and state tables cleared.
The receiver must send an OK reply.
VERIFY (VRFY)
This command asks the receiver to confirm that the argument
identifies a user. If it is a user name, the full name of
the user (if known) and the fully specified mailbox are
returned.
This command has no effect on any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.
EXPAND (EXPN)
This command asks the receiver to confirm that the argument
identifies a mailing list, and if so, to return the
membership of that list. The full name of the users (if
known) and the fully specified mailboxes are returned in a
multiline reply.
This command has no effect on any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.
HELP (HELP)
This command causes the receiver to send helpful information
to the sender of the HELP command. The command may take an
argument (e.g., any command name) and return more specific
information as a response.
This command has no effect on any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.
Postel [Page 25]
August 1982 RFC 821
Simple Mail Transfer Protocol
NOOP (NOOP)
This command does not affect any parameters or previously
entered commands. It specifies no action other than that
the receiver send an OK reply.
This command has no effect on any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.
QUIT (QUIT)
This command specifies that the receiver must send an OK
reply, and then close the transmission channel.
The receiver should not close the transmission channel until
it receives and replies to a QUIT command (even if there was
an error). The sender should not close the transmission
channel until it send a QUIT command and receives the reply
(even if there was an error response to a previous command).
If the connection is closed prematurely the receiver should
act as if a RSET command had been received (canceling any
pending transaction, but not undoing any previously
completed transaction), the sender should act as if the
command or transaction in progress had received a temporary
error (4xx).
TURN (TURN)
This command specifies that the receiver must either (1)
send an OK reply and then take on the role of the
sender-SMTP, or (2) send a refusal reply and retain the role
of the receiver-SMTP.
If program-A is currently the sender-SMTP and it sends the
TURN command and receives an OK reply (250) then program-A
becomes the receiver-SMTP. Program-A is then in the initial
state as if the transmission channel just opened, and it
then sends the 220 service ready greeting.
If program-B is currently the receiver-SMTP and it receives
the TURN command and sends an OK reply (250) then program-B
becomes the sender-SMTP. Program-B is then in the initial
state as if the transmission channel just opened, and it
then expects to receive the 220 service ready greeting.
To refuse to change roles the receiver sends the 502 reply.
[Page 26] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
There are restrictions on the order in which these command may
be used.
The first command in a session must be the HELO command.
The HELO command may be used later in a session as well. If
the HELO command argument is not acceptable a 501 failure
reply must be returned and the receiver-SMTP must stay in
the same state.
The NOOP, HELP, EXPN, and VRFY commands can be used at any
time during a session.
The MAIL, SEND, SOML, or SAML commands begin a mail
transaction. Once started a mail transaction consists of
one of the transaction beginning commands, one or more RCPT
commands, and a DATA command, in that order. A mail
transaction may be aborted by the RSET command. There may
be zero or more transactions in a session.
If the transaction beginning command argument is not
acceptable a 501 failure reply must be returned and the
receiver-SMTP must stay in the same state. If the commands
in a transaction are out of order a 503 failure reply must
be returned and the receiver-SMTP must stay in the same
state.
The last command in a session must be the QUIT command. The
QUIT command can not be used at any other time in a session.
4.1.2. COMMAND SYNTAX
The commands consist of a command code followed by an argument
field. Command codes are four alphabetic characters. Upper
and lower case alphabetic characters are to be treated
identically. Thus, any of the following may represent the mail
command:
MAIL Mail mail MaIl mAIl
This also applies to any symbols representing parameter values,
such as "TO" or "to" for the forward-path. Command codes and
the argument fields are separated by one or more spaces.
However, within the reverse-path and forward-path arguments
case is important. In particular, in some hosts the user
"smith" is different from the user "Smith".
Postel [Page 27]
August 1982 RFC 821
Simple Mail Transfer Protocol
The argument field consists of a variable length character
string ending with the character sequence <CRLF>. The receiver
is to take no action until this sequence is received.
Square brackets denote an optional argument field. If the
option is not taken, the appropriate default is implied.
[Page 28] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
The following are the SMTP commands:
HELO <SP> <domain> <CRLF>
MAIL <SP> FROM:<reverse-path> <CRLF>
RCPT <SP> TO:<forward-path> <CRLF>
DATA <CRLF>
RSET <CRLF>
SEND <SP> FROM:<reverse-path> <CRLF>
SOML <SP> FROM:<reverse-path> <CRLF>
SAML <SP> FROM:<reverse-path> <CRLF>
VRFY <SP> <string> <CRLF>
EXPN <SP> <string> <CRLF>
HELP [<SP> <string>] <CRLF>
NOOP <CRLF>
QUIT <CRLF>
TURN <CRLF>
Postel [Page 29]
August 1982 RFC 821
Simple Mail Transfer Protocol
The syntax of the above argument fields (using BNF notation
where applicable) is given below. The "..." notation indicates
that a field may be repeated one or more times.
<reverse-path> ::= <path>
<forward-path> ::= <path>
<path> ::= "<" [ <a-d-l> ":" ] <mailbox> ">"
<a-d-l> ::= <at-domain> | <at-domain> "," <a-d-l>
<at-domain> ::= "@" <domain>
<domain> ::= <element> | <element> "." <domain>
<element> ::= <name> | "#" <number> | "[" <dotnum> "]"
<mailbox> ::= <local-part> "@" <domain>
<local-part> ::= <dot-string> | <quoted-string>
<name> ::= <a> <ldh-str> <let-dig>
<ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
<let-dig> ::= <a> | <d>
<let-dig-hyp> ::= <a> | <d> | "-"
<dot-string> ::= <string> | <string> "." <dot-string>
<string> ::= <char> | <char> <string>
<quoted-string> ::= """ <qtext> """
<qtext> ::= "\" <x> | "\" <x> <qtext> | <q> | <q> <qtext>
<char> ::= <c> | "\" <x>
<dotnum> ::= <snum> "." <snum> "." <snum> "." <snum>
<number> ::= <d> | <d> <number>
<CRLF> ::= <CR> <LF>
[Page 30] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
<CR> ::= the carriage return character (ASCII code 13)
<LF> ::= the line feed character (ASCII code 10)
<SP> ::= the space character (ASCII code 32)
<snum> ::= one, two, or three digits representing a decimal
integer value in the range 0 through 255
<a> ::= any one of the 52 alphabetic characters A through Z
in upper case and a through z in lower case
<c> ::= any one of the 128 ASCII characters, but not any
<special> or <SP>
<d> ::= any one of the ten digits 0 through 9
<q> ::= any one of the 128 ASCII characters except <CR>,
<LF>, quote ("), or backslash (\)
<x> ::= any one of the 128 ASCII characters (no exceptions)
<special> ::= "<" | ">" | "(" | ")" | "[" | "]" | "\" | "."
| "," | ";" | ":" | "@" """ | the control
characters (ASCII codes 0 through 31 inclusive and
127)
Note that the backslash, "\", is a quote character, which is
used to indicate that the next character is to be used
literally (instead of its normal interpretation). For example,
"Joe\,Smith" could be used to indicate a single nine character
user field with comma being the fourth character of the field.
Hosts are generally known by names which are translated to
addresses in each host. Note that the name elements of domains
are the official names -- no use of nicknames or aliases is
allowed.
Sometimes a host is not known to the translation function and
communication is blocked. To bypass this barrier two numeric
forms are also allowed for host "names". One form is a decimal
integer prefixed by a pound sign, "#", which indicates the
number is the address of the host. Another form is four small
decimal integers separated by dots and enclosed by brackets,
e.g., "[123.255.37.2]", which indicates a 32-bit ARPA Internet
Address in four 8-bit fields.
Postel [Page 31]
August 1982 RFC 821
Simple Mail Transfer Protocol
The time stamp line and the return path line are formally
defined as follows:
<return-path-line> ::= "Return-Path:" <SP><reverse-path><CRLF>
<time-stamp-line> ::= "Received:" <SP> <stamp> <CRLF>
<stamp> ::= <from-domain> <by-domain> <opt-info> ";"
<daytime>
<from-domain> ::= "FROM" <SP> <domain> <SP>
<by-domain> ::= "BY" <SP> <domain> <SP>
<opt-info> ::= [<via>] [<with>] [<id>] [<for>]
<via> ::= "VIA" <SP> <link> <SP>
<with> ::= "WITH" <SP> <protocol> <SP>
<id> ::= "ID" <SP> <string> <SP>
<for> ::= "FOR" <SP> <path> <SP>
<link> ::= The standard names for links are registered with
the Network Information Center.
<protocol> ::= The standard names for protocols are
registered with the Network Information Center.
<daytime> ::= <SP> <date> <SP> <time>
<date> ::= <dd> <SP> <mon> <SP> <yy>
<time> ::= <hh> ":" <mm> ":" <ss> <SP> <zone>
<dd> ::= the one or two decimal integer day of the month in
the range 1 to 31.
<mon> ::= "JAN" | "FEB" | "MAR" | "APR" | "MAY" | "JUN" |
"JUL" | "AUG" | "SEP" | "OCT" | "NOV" | "DEC"
<yy> ::= the two decimal integer year of the century in the
range 00 to 99.
[Page 32] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
<hh> ::= the two decimal integer hour of the day in the
range 00 to 24.
<mm> ::= the two decimal integer minute of the hour in the
range 00 to 59.
<ss> ::= the two decimal integer second of the minute in the
range 00 to 59.
<zone> ::= "UT" for Universal Time (the default) or other
time zone designator (as in [2]).
-------------------------------------------------------------
Return Path Example
Return-Path: <@CHARLIE.ARPA,@BAKER.ARPA:
[email protected]>
Example 9
-------------------------------------------------------------
-------------------------------------------------------------
Time Stamp Line Example
Received: FROM ABC.ARPA BY XYZ.ARPA ; 22 OCT 81 09:23:59 PDT
Received: from ABC.ARPA by XYZ.ARPA via TELENET with X25
id M12345 for
[email protected] ; 22 OCT 81 09:23:59 PDT
Example 10
-------------------------------------------------------------
Postel [Page 33]
August 1982 RFC 821
Simple Mail Transfer Protocol
4.2. SMTP REPLIES
Replies to SMTP commands are devised to ensure the synchronization
of requests and actions in the process of mail transfer, and to
guarantee that the sender-SMTP always knows the state of the
receiver-SMTP. Every command must generate exactly one reply.
The details of the command-reply sequence are made explicit in
Section 5.3 on Sequencing and Section 5.4 State Diagrams.
An SMTP reply consists of a three digit number (transmitted as
three alphanumeric characters) followed by some text. The number
is intended for use by automata to determine what state to enter
next; the text is meant for the human user. It is intended that
the three digits contain enough encoded information that the
sender-SMTP need not examine the text and may either discard it or
pass it on to the user, as appropriate. In particular, the text
may be receiver-dependent and context dependent, so there are
likely to be varying texts for each reply code. A discussion of
the theory of reply codes is given in Appendix E. Formally, a
reply is defined to be the sequence: a three-digit code, <SP>,
one line of text, and <CRLF>, or a multiline reply (as defined in
Appendix E). Only the EXPN and HELP commands are expected to
result in multiline replies in normal circumstances, however
multiline replies are allowed for any command.
[Page 34] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
4.2.1. REPLY CODES BY FUNCTION GROUPS
500 Syntax error, command unrecognized
[This may include errors such as command line too long]
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not implemented
211 System status, or system help reply
214 Help message
[Information on how to use the receiver or the meaning of a
particular non-standard command; this reply is useful only
to the human user]
220 <domain> Service ready
221 <domain> Service closing transmission channel
421 <domain> Service not available,
closing transmission channel
[This may be a reply to any command if the service knows it
must shut down]
250 Requested mail action okay, completed
251 User not local; will forward to <forward-path>
450 Requested mail action not taken: mailbox unavailable
[E.g., mailbox busy]
550 Requested action not taken: mailbox unavailable
[E.g., mailbox not found, no access]
451 Requested action aborted: error in processing
551 User not local; please try <forward-path>
452 Requested action not taken: insufficient system storage
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
[E.g., mailbox syntax incorrect]
354 Start mail input; end with <CRLF>.<CRLF>
554 Transaction failed
Postel [Page 35]
August 1982 RFC 821
Simple Mail Transfer Protocol
4.2.2. NUMERIC ORDER LIST OF REPLY CODES
211 System status, or system help reply
214 Help message
[Information on how to use the receiver or the meaning of a
particular non-standard command; this reply is useful only
to the human user]
220 <domain> Service ready
221 <domain> Service closing transmission channel
250 Requested mail action okay, completed
251 User not local; will forward to <forward-path>
354 Start mail input; end with <CRLF>.<CRLF>
421 <domain> Service not available,
closing transmission channel
[This may be a reply to any command if the service knows it
must shut down]
450 Requested mail action not taken: mailbox unavailable
[E.g., mailbox busy]
451 Requested action aborted: local error in processing
452 Requested action not taken: insufficient system storage
500 Syntax error, command unrecognized
[This may include errors such as command line too long]
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not implemented
550 Requested action not taken: mailbox unavailable
[E.g., mailbox not found, no access]
551 User not local; please try <forward-path>
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
[E.g., mailbox syntax incorrect]
554 Transaction failed
[Page 36] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
4.3. SEQUENCING OF COMMANDS AND REPLIES
The communication between the sender and receiver is intended to
be an alternating dialogue, controlled by the sender. As such,
the sender issues a command and the receiver responds with a
reply. The sender must wait for this response before sending
further commands.
One important reply is the connection greeting. Normally, a
receiver will send a 220 "Service ready" reply when the connection
is completed. The sender should wait for this greeting message
before sending any commands.
Note: all the greeting type replies have the official name of
the server host as the first word following the reply code.
For example,
220 <SP> USC-ISIF.ARPA <SP> Service ready <CRLF>
The table below lists alternative success and failure replies for
each command. These must be strictly adhered to; a receiver may
substitute text in the replies, but the meaning and action implied
by the code numbers and by the specific command reply sequence
cannot be altered.
COMMAND-REPLY SEQUENCES
Each command is listed with its possible replies. The prefixes
used before the possible replies are "P" for preliminary (not
used in SMTP), "I" for intermediate, "S" for success, "F" for
failure, and "E" for error. The 421 reply (service not
available, closing transmission channel) may be given to any
command if the SMTP-receiver knows it must shut down. This
listing forms the basis for the State Diagrams in Section 4.4.
CONNECTION ESTABLISHMENT
S: 220
F: 421
HELO
S: 250
E: 500, 501, 504, 421
MAIL
S: 250
F: 552, 451, 452
E: 500, 501, 421
Postel [Page 37]
August 1982 RFC 821
Simple Mail Transfer Protocol
RCPT
S: 250, 251
F: 550, 551, 552, 553, 450, 451, 452
E: 500, 501, 503, 421
DATA
I: 354 -> data -> S: 250
F: 552, 554, 451, 452
F: 451, 554
E: 500, 501, 503, 421
RSET
S: 250
E: 500, 501, 504, 421
SEND
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
SOML
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
SAML
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
VRFY
S: 250, 251
F: 550, 551, 553
E: 500, 501, 502, 504, 421
EXPN
S: 250
F: 550
E: 500, 501, 502, 504, 421
HELP
S: 211, 214
E: 500, 501, 502, 504, 421
NOOP
S: 250
E: 500, 421
QUIT
S: 221
E: 500
TURN
S: 250
F: 502
E: 500, 503
[Page 38] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
4.4. STATE DIAGRAMS
Following are state diagrams for a simple-minded SMTP
implementation. Only the first digit of the reply codes is used.
There is one state diagram for each group of SMTP commands. The
command groupings were determined by constructing a model for each
command and then collecting together the commands with
structurally identical models.
For each command there are three possible outcomes: "success"
(S), "failure" (F), and "error" (E). In the state diagrams below
we use the symbol B for "begin", and the symbol W for "wait for
reply".
First, the diagram that represents most of the SMTP commands:
1,3 +---+
----------->| E |
| +---+
|
+---+ cmd +---+ 2 +---+
| B |---------->| W |---------->| S |
+---+ +---+ +---+
|
| 4,5 +---+
----------->| F |
+---+
This diagram models the commands:
HELO, MAIL, RCPT, RSET, SEND, SOML, SAML, VRFY, EXPN, HELP,
NOOP, QUIT, TURN.
Postel [Page 39]
August 1982 RFC 821
Simple Mail Transfer Protocol
A more complex diagram models the DATA command:
+---+ DATA +---+ 1,2 +---+
| B |---------->| W |-------------------->| E |
+---+ +---+ ------------>+---+
3| |4,5 |
| | |
-------------- ----- |
| | | +---+
| ---------- -------->| S |
| | | | +---+
| | ------------
| | | |
V 1,3| |2 |
+---+ data +---+ --------------->+---+
| |---------->| W | | F |
+---+ +---+-------------------->+---+
4,5
Note that the "data" here is a series of lines sent from the
sender to the receiver with no response expected until the last
line is sent.
[Page 40] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
4.5. DETAILS
4.5.1. MINIMUM IMPLEMENTATION
In order to make SMTP workable, the following minimum
implementation is required for all receivers:
COMMANDS -- HELO
MAIL
RCPT
DATA
RSET
NOOP
QUIT
4.5.2. TRANSPARENCY
Without some provision for data transparency the character
sequence "<CRLF>.<CRLF>" ends the mail text and cannot be sent
by the user. In general, users are not aware of such
"forbidden" sequences. To allow all user composed text to be
transmitted transparently the following procedures are used.
1. Before sending a line of mail text the sender-SMTP checks
the first character of the line. If it is a period, one
additional period is inserted at the beginning of the line.
2. When a line of mail text is received by the receiver-SMTP
it checks the line. If the line is composed of a single
period it is the end of mail. If the first character is a
period and there are other characters on the line, the first
character is deleted.
The mail data may contain any of the 128 ASCII characters. All
characters are to be delivered to the recipient's mailbox
including format effectors and other control characters. If
the transmission channel provides an 8-bit byte (octets) data
stream, the 7-bit ASCII codes are transmitted right justified
in the octets with the high order bits cleared to zero.
In some systems it may be necessary to transform the data as
it is received and stored. This may be necessary for hosts
that use a different character set than ASCII as their local
character set, or that store data in records rather than
Postel [Page 41]
August 1982 RFC 821
Simple Mail Transfer Protocol
strings. If such transforms are necessary, they must be
reversible -- especially if such transforms are applied to
mail being relayed.
4.5.3. SIZES
There are several objects that have required minimum maximum
sizes. That is, every implementation must be able to receive
objects of at least these sizes, but must not send objects
larger than these sizes.
****************************************************
* *
* TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION *
* TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
* OF THESE OBJECTS SHOULD BE USED. *
* *
****************************************************
user
The maximum total length of a user name is 64 characters.
domain
The maximum total length of a domain name or number is 64
characters.
path
The maximum total length of a reverse-path or
forward-path is 256 characters (including the punctuation
and element separators).
command line
The maximum total length of a command line including the
command word and the <CRLF> is 512 characters.
reply line
The maximum total length of a reply line including the
reply code and the <CRLF> is 512 characters.
[Page 42] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
text line
The maximum total length of a text line including the
<CRLF> is 1000 characters (but not counting the leading
dot duplicated for transparency).
recipients buffer
The maximum total number of recipients that must be
buffered is 100 recipients.
****************************************************
* *
* TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION *
* TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
* OF THESE OBJECTS SHOULD BE USED. *
* *
****************************************************
Errors due to exceeding these limits may be reported by using
the reply codes, for example:
500 Line too long.
501 Path too long
552 Too many recipients.
552 Too much mail data.
Postel [Page 43]
August 1982 RFC 821
Simple Mail Transfer Protocol
APPENDIX A
TCP Transport service
The Transmission Control Protocol [3] is used in the ARPA
Internet, and in any network following the US DoD standards for
internetwork protocols.
Connection Establishment
The SMTP transmission channel is a TCP connection established
between the sender process port U and the receiver process port
L. This single full duplex connection is used as the
transmission channel. This protocol is assigned the service
port 25 (31 octal), that is L=25.
Data Transfer
The TCP connection supports the transmission of 8-bit bytes.
The SMTP data is 7-bit ASCII characters. Each character is
transmitted as an 8-bit byte with the high-order bit cleared to
zero.
[Page 44] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
APPENDIX B
NCP Transport service
The ARPANET Host-to-Host Protocol [4] (implemented by the Network
Control Program) may be used in the ARPANET.
Connection Establishment
The SMTP transmission channel is established via NCP between
the sender process socket U and receiver process socket L. The
Initial Connection Protocol [5] is followed resulting in a pair
of simplex connections. This pair of connections is used as
the transmission channel. This protocol is assigned the
contact socket 25 (31 octal), that is L=25.
Data Transfer
The NCP data connections are established in 8-bit byte mode.
The SMTP data is 7-bit ASCII characters. Each character is
transmitted as an 8-bit byte with the high-order bit cleared to
zero.
Postel [Page 45]
August 1982 RFC 821
Simple Mail Transfer Protocol
APPENDIX C
NITS
The Network Independent Transport Service [6] may be used.
Connection Establishment
The SMTP transmission channel is established via NITS between
the sender process and receiver process. The sender process
executes the CONNECT primitive, and the waiting receiver
process executes the ACCEPT primitive.
Data Transfer
The NITS connection supports the transmission of 8-bit bytes.
The SMTP data is 7-bit ASCII characters. Each character is
transmitted as an 8-bit byte with the high-order bit cleared to
zero.
[Page 46] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
APPENDIX D
X.25 Transport service
It may be possible to use the X.25 service [7] as provided by the
Public Data Networks directly, however, it is suggested that a
reliable end-to-end protocol such as TCP be used on top of X.25
connections.
Postel [Page 47]
August 1982 RFC 821
Simple Mail Transfer Protocol
APPENDIX E
Theory of Reply Codes
The three digits of the reply each have a special significance.
The first digit denotes whether the response is good, bad or
incomplete. An unsophisticated sender-SMTP will be able to
determine its next action (proceed as planned, redo, retrench,
etc.) by simply examining this first digit. A sender-SMTP that
wants to know approximately what kind of error occurred (e.g.,
mail system error, command syntax error) may examine the second
digit, reserving the third digit for the finest gradation of
information.
There are five values for the first digit of the reply code:
1yz Positive Preliminary reply
The command has been accepted, but the requested action
is being held in abeyance, pending confirmation of the
information in this reply. The sender-SMTP should send
another command specifying whether to continue or abort
the action.
[Note: SMTP does not have any commands that allow this
type of reply, and so does not have the continue or
abort commands.]
2yz Positive Completion reply
The requested action has been successfully completed. A
new request may be initiated.
3yz Positive Intermediate reply
The command has been accepted, but the requested action
is being held in abeyance, pending receipt of further
information. The sender-SMTP should send another command
specifying this information. This reply is used in
command sequence groups.
4yz Transient Negative Completion reply
The command was not accepted and the requested action did
not occur. However, the error condition is temporary and
the action may be requested again. The sender should
[Page 48] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
return to the beginning of the command sequence (if any).
It is difficult to assign a meaning to "transient" when
two different sites (receiver- and sender- SMTPs) must
agree on the interpretation. Each reply in this category
might have a different time value, but the sender-SMTP is
encouraged to try again. A rule of thumb to determine if
a reply fits into the 4yz or the 5yz category (see below)
is that replies are 4yz if they can be repeated without
any change in command form or in properties of the sender
or receiver. (E.g., the command is repeated identically
and the receiver does not put up a new implementation.)
5yz Permanent Negative Completion reply
The command was not accepted and the requested action did
not occur. The sender-SMTP is discouraged from repeating
the exact request (in the same sequence). Even some
"permanent" error conditions can be corrected, so the
human user may want to direct the sender-SMTP to
reinitiate the command sequence by direct action at some
point in the future (e.g., after the spelling has been
changed, or the user has altered the account status).
The second digit encodes responses in specific categories:
x0z Syntax -- These replies refer to syntax errors,
syntactically correct commands that don't fit any
functional category, and unimplemented or superfluous
commands.
x1z Information -- These are replies to requests for
information, such as status or help.
x2z Connections -- These are replies referring to the
transmission channel.
x3z Unspecified as yet.
x4z Unspecified as yet.
x5z Mail system -- These replies indicate the status of
the receiver mail system vis-a-vis the requested
transfer or other mail system action.
The third digit gives a finer gradation of meaning in each
category specified by the second digit. The list of replies
Postel [Page 49]
August 1982 RFC 821
Simple Mail Transfer Protocol
illustrates this. Each reply text is recommended rather than
mandatory, and may even change according to the command with
which it is associated. On the other hand, the reply codes
must strictly follow the specifications in this section.
Receiver implementations should not invent new codes for
slightly different situations from the ones described here, but
rather adapt codes already defined.
For example, a command such as NOOP whose successful execution
does not offer the sender-SMTP any new information will return
a 250 reply. The response is 502 when the command requests an
unimplemented non-site-specific action. A refinement of that
is the 504 reply for a command that is implemented, but that
requests an unimplemented parameter.
The reply text may be longer than a single line; in these cases
the complete text must be marked so the sender-SMTP knows when it
can stop reading the reply. This requires a special format to
indicate a multiple line reply.
The format for multiline replies requires that every line,
except the last, begin with the reply code, followed
immediately by a hyphen, "-" (also known as minus), followed by
text. The last line will begin with the reply code, followed
immediately by <SP>, optionally some text, and <CRLF>.
For example:
123-First line
123-Second line
123-234 text beginning with numbers
123 The last line
In many cases the sender-SMTP then simply needs to search for
the reply code followed by <SP> at the beginning of a line, and
ignore all preceding lines. In a few cases, there is important
data for the sender in the reply "text". The sender will know
these cases from the current context.
[Page 50] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
APPENDIX F
Scenarios
This section presents complete scenarios of several types of SMTP
sessions.
A Typical SMTP Transaction Scenario
This SMTP example shows mail sent by Smith at host USC-ISIF, to
Jones, Green, and Brown at host BBN-UNIX. Here we assume that
host USC-ISIF contacts host BBN-UNIX directly. The mail is
accepted for Jones and Brown. Green does not have a mailbox at
host BBN-UNIX.
-------------------------------------------------------------
R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIF.ARPA
R: 250 BBN-UNIX.ARPA
S: MAIL FROM:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 550 No such user here
S: RCPT TO:<
[email protected]>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 BBN-UNIX.ARPA Service closing transmission channel
Scenario 1
-------------------------------------------------------------
Postel [Page 51]
August 1982 RFC 821
Simple Mail Transfer Protocol
Aborted SMTP Transaction Scenario
-------------------------------------------------------------
R: 220 MIT-Multics.ARPA Simple Mail Transfer Service Ready
S: HELO ISI-VAXA.ARPA
R: 250 MIT-Multics.ARPA
S: MAIL FROM:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 550 No such user here
S: RSET
R: 250 OK
S: QUIT
R: 221 MIT-Multics.ARPA Service closing transmission channel
Scenario 2
-------------------------------------------------------------
[Page 52] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
Relayed Mail Scenario
-------------------------------------------------------------
Step 1 -- Source Host to Relay Host
R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
S: HELO MIT-AI.ARPA
R: 250 USC-ISIE.ARPA
S: MAIL FROM:<
[email protected]>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:
[email protected]>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Date: 2 Nov 81 22:33:44
S: From: John Q. Public <
[email protected]>
S: Subject: The Next Meeting of the Board
S: To:
[email protected]
S:
S: Bill:
S: The next meeting of the board of directors will be
S: on Tuesday.
S: John.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE.ARPA Service closing transmission channel
Postel [Page 53]
August 1982 RFC 821
Simple Mail Transfer Protocol
Step 2 -- Relay Host to Destination Host
R: 220 BBN-VAX.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIE.ARPA
R: 250 BBN-VAX.ARPA
S: MAIL FROM:<@USC-ISIE.ARPA:
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Received: from MIT-AI.ARPA by USC-ISIE.ARPA ;
2 Nov 81 22:40:10 UT
S: Date: 2 Nov 81 22:33:44
S: From: John Q. Public <
[email protected]>
S: Subject: The Next Meeting of the Board
S: To:
[email protected]
S:
S: Bill:
S: The next meeting of the board of directors will be
S: on Tuesday.
S: John.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE.ARPA Service closing transmission channel
Scenario 3
-------------------------------------------------------------
[Page 54] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
Verifying and Sending Scenario
-------------------------------------------------------------
R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
S: HELO MIT-MC.ARPA
R: 250 SU-SCORE.ARPA
S: VRFY Crispin
R: 250 Mark Crispin <
[email protected]>
S: SEND FROM:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE.ARPA Service closing transmission channel
Scenario 4
-------------------------------------------------------------
Postel [Page 55]
August 1982 RFC 821
Simple Mail Transfer Protocol
Sending and Mailing Scenarios
First the user's name is verified, then an attempt is made to
send to the user's terminal. When that fails, the messages is
mailed to the user's mailbox.
-------------------------------------------------------------
R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
S: HELO MIT-MC.ARPA
R: 250 SU-SCORE.ARPA
S: VRFY Crispin
R: 250 Mark Crispin <
[email protected]>
S: SEND FROM:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 450 User not active now
S: RSET
R: 250 OK
S: MAIL FROM:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE.ARPA Service closing transmission channel
Scenario 5
-------------------------------------------------------------
[Page 56] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
Doing the preceding scenario more efficiently.
-------------------------------------------------------------
R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
S: HELO MIT-MC.ARPA
R: 250 SU-SCORE.ARPA
S: VRFY Crispin
R: 250 Mark Crispin <
[email protected]>
S: SOML FROM:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 250 User not active now, so will do mail.
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE.ARPA Service closing transmission channel
Scenario 6
-------------------------------------------------------------
Postel [Page 57]
August 1982 RFC 821
Simple Mail Transfer Protocol
Mailing List Scenario
First each of two mailing lists are expanded in separate sessions
with different hosts. Then the message is sent to everyone that
appeared on either list (but no duplicates) via a relay host.
-------------------------------------------------------------
Step 1 -- Expanding the First List
R: 220 MIT-AI.ARPA Simple Mail Transfer Service Ready
S: HELO SU-SCORE.ARPA
R: 250 MIT-AI.ARPA
S: EXPN Example-People
R: 250-<
[email protected]>
R: 250-Fred Fonebone <
[email protected]>
R: 250-Xenon Y. Zither <
[email protected]>
R: 250-Quincy Smith <@USC-ISIF.ARPA:
[email protected]>
R: 250-<
[email protected]>
R: 250 <
[email protected]>
S: QUIT
R: 221 MIT-AI.ARPA Service closing transmission channel
[Page 58] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
Step 2 -- Expanding the Second List
R: 220 MIT-MC.ARPA Simple Mail Transfer Service Ready
S: HELO SU-SCORE.ARPA
R: 250 MIT-MC.ARPA
S: EXPN Interested-Parties
R: 250-Al Calico <
[email protected]>
R: 250-<
[email protected]>
R: 250-Quincy Smith <@USC-ISIF.ARPA:
[email protected]>
R: 250-<
[email protected]>
R: 250 <
[email protected]>
S: QUIT
R: 221 MIT-MC.ARPA Service closing transmission channel
Postel [Page 59]
August 1982 RFC 821
Simple Mail Transfer Protocol
Step 3 -- Mailing to All via a Relay Host
R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
S: HELO SU-SCORE.ARPA
R: 250 USC-ISIE.ARPA
S: MAIL FROM:<
[email protected]>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:
[email protected]>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:
[email protected]>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:
[email protected]>
R: 250 OK
S: RCPT
TO:<@USC-ISIE.ARPA,@USC-ISIF.ARPA:
[email protected]>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:
[email protected]>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:
[email protected]>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:
[email protected]>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE.ARPA Service closing transmission channel
Scenario 7
-------------------------------------------------------------
[Page 60] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
Forwarding Scenarios
-------------------------------------------------------------
R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready
S: HELO LBL-UNIX.ARPA
R: 250 USC-ISIF.ARPA
S: MAIL FROM:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 251 User not local; will forward to <
[email protected]>
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIF.ARPA Service closing transmission channel
Scenario 8
-------------------------------------------------------------
Postel [Page 61]
August 1982 RFC 821
Simple Mail Transfer Protocol
-------------------------------------------------------------
Step 1 -- Trying the Mailbox at the First Host
R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready
S: HELO LBL-UNIX.ARPA
R: 250 USC-ISIF.ARPA
S: MAIL FROM:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 251 User not local; will forward to <
[email protected]>
S: RSET
R: 250 OK
S: QUIT
R: 221 USC-ISIF.ARPA Service closing transmission channel
Step 2 -- Delivering the Mail at the Second Host
R: 220 USC-ISI.ARPA Simple Mail Transfer Service Ready
S: HELO LBL-UNIX.ARPA
R: 250 USC-ISI.ARPA
S: MAIL FROM:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISI.ARPA Service closing transmission channel
Scenario 9
-------------------------------------------------------------
[Page 62] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
Too Many Recipients Scenario
-------------------------------------------------------------
R: 220 BERKELEY.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIF.ARPA
R: 250 BERKELEY.ARPA
S: MAIL FROM:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 552 Recipient storage full, try again in another transaction
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: MAIL FROM:<
[email protected]>
R: 250 OK
S: RCPT TO:<
[email protected]>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 BERKELEY.ARPA Service closing transmission channel
Scenario 10
-------------------------------------------------------------
Note that a real implementation must handle many recipients as
specified in Section 4.5.3.
Postel [Page 63]
August 1982 RFC 821
Simple Mail Transfer Protocol
GLOSSARY
ASCII
American Standard Code for Information Interchange [1].
command
A request for a mail service action sent by the sender-SMTP to the
receiver-SMTP.
domain
The hierarchially structured global character string address of a
host computer in the mail system.
end of mail data indication
A special sequence of characters that indicates the end of the
mail data. In particular, the five characters carriage return,
line feed, period, carriage return, line feed, in that order.
host
A computer in the internetwork environment on which mailboxes or
SMTP processes reside.
line
A a sequence of ASCII characters ending with a <CRLF>.
mail data
A sequence of ASCII characters of arbitrary length, which conforms
to the standard set in the Standard for the Format of ARPA
Internet Text Messages (RFC 822 [2]).
mailbox
A character string (address) which identifies a user to whom mail
is to be sent. Mailbox normally consists of the host and user
specifications. The standard mailbox naming convention is defined
to be "user@domain". Additionally, the "container" in which mail
is stored.
[Page 64] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
receiver-SMTP process
A process which transfers mail in cooperation with a sender-SMTP
process. It waits for a connection to be established via the
transport service. It receives SMTP commands from the
sender-SMTP, sends replies, and performs the specified operations.
reply
A reply is an acknowledgment (positive or negative) sent from
receiver to sender via the transmission channel in response to a
command. The general form of a reply is a completion code
(including error codes) followed by a text string. The codes are
for use by programs and the text is usually intended for human
users.
sender-SMTP process
A process which transfers mail in cooperation with a receiver-SMTP
process. A local language may be used in the user interface
command/reply dialogue. The sender-SMTP initiates the transport
service connection. It initiates SMTP commands, receives replies,
and governs the transfer of mail.
session
The set of exchanges that occur while the transmission channel is
open.
transaction
The set of exchanges required for one message to be transmitted
for one or more recipients.
transmission channel
A full-duplex communication path between a sender-SMTP and a
receiver-SMTP for the exchange of commands, replies, and mail
text.
transport service
Any reliable stream-oriented data communication services. For
example, NCP, TCP, NITS.
Postel [Page 65]
August 1982 RFC 821
Simple Mail Transfer Protocol
user
A human being (or a process on behalf of a human being) wishing to
obtain mail transfer service. In addition, a recipient of
computer mail.
word
A sequence of printing characters.
<CRLF>
The characters carriage return and line feed (in that order).
<SP>
The space character.
[Page 66] Postel
RFC 821 August 1982
Simple Mail Transfer Protocol
REFERENCES
[1] ASCII
ASCII, "USA Code for Information Interchange", United States of
America Standards Institute, X3.4, 1968. Also in: Feinler, E.
and J. Postel, eds., "ARPANET Protocol Handbook", NIC 7104, for
the Defense Communications Agency by SRI International, Menlo
Park, California, Revised January 1978.
[2] RFC 822
Crocker, D., "Standard for the Format of ARPA Internet Text
Messages," RFC 822, Department of Electrical Engineering,
University of Delaware, August 1982.
[3] TCP
Postel, J., ed., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC 793, USC/Information Sciences
Institute, NTIS AD Number A111091, September 1981. Also in:
Feinler, E. and J. Postel, eds., "Internet Protocol Transition
Workbook", SRI International, Menlo Park, California, March 1982.
[4] NCP
McKenzie,A., "Host/Host Protocol for the ARPA Network", NIC 8246,
January 1972. Also in: Feinler, E. and J. Postel, eds., "ARPANET
Protocol Handbook", NIC 7104, for the Defense Communications
Agency by SRI International, Menlo Park, California, Revised
January 1978.
[5] Initial Connection Protocol
Postel, J., "Official Initial Connection Protocol", NIC 7101,
11 June 1971. Also in: Feinler, E. and J. Postel, eds., "ARPANET
Protocol Handbook", NIC 7104, for the Defense Communications
Agency by SRI International, Menlo Park, California, Revised
January 1978.
[6] NITS
PSS/SG3, "A Network Independent Transport Service", Study Group 3,
The Post Office PSS Users Group, February 1980. Available from
the DCPU, National Physical Laboratory, Teddington, UK.
Postel [Page 67]
August 1982 RFC 821
Simple Mail Transfer Protocol
[7] X.25
CCITT, "Recommendation X.25 - Interface Between Data Terminal
Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for
Terminals Operating in the Packet Mode on Public Data Networks,"
CCITT Orange Book, Vol. VIII.2, International Telephone and
Telegraph Consultative Committee, Geneva, 1976.
[Page 68] Postel
=========================================================================
Network Working Group Craig Partridge
Request for Comments: 974 CSNET CIC BBN Laboratories Inc
January 1986
MAIL ROUTING AND THE DOMAIN SYSTEM
Status of this Memo
This RFC presents a description of how mail systems on the Internet
are expected to route messages based on information from the domain
system described in RFCs 882, 883 and 973. Distribution of this memo
is unlimited.
Introduction
The purpose of this memo is to explain how mailers are to decide how
to route a message addressed to a given Internet domain name. This
involves a discussion of how mailers interpret MX RRs, which are used
for message routing. Note that this memo makes no statement about
how mailers are to deal with MB and MG RRs, which are used for
interpreting mailbox names.
Under RFC-882 and RFC-883 certain assumptions about mail addresses
have been changed. Up to now, one could usually assume that if a
message was addressed to a mailbox, for example, at LOKI.BBN.COM,
that one could just open an SMTP connection to LOKI.BBN.COM and pass
the message along. This system broke down in certain situations,
such as for certain UUCP and CSNET hosts which were not directly
attached to the Internet, but these hosts could be handled as special
cases in configuration files (for example, most mailers were set up
to automatically forward mail addressed to a CSNET host to
CSNET-RELAY.ARPA).
Under domains, one cannot simply open a connection to LOKI.BBN.COM,
but must instead ask the domain system where messages to LOKI.BBN.COM
are to be delivered. And the domain system may direct a mailer to
deliver messages to an entirely different host, such as SH.CS.NET.
Or, in a more complicated case, the mailer may learn that it has a
choice of routes to LOKI.BBN.COM. This memo is essentially a set of
guidelines on how mailers should behave in this more complex world.
Readers are expected to be familiar with RFCs 882, 883, and the
updates to them (e.g., RFC-973).
Partridge [Page 1]
RFC 974 January 1986
Mail Routing and the Domain System
What the Domain Servers Know
The domain servers store information as a series of resource records
(RRs), each of which contains a particular piece of information about
a given domain name (which is usually, but not always, a host). The
simplest way to think of a RR is as a typed pair of datum, a domain
name matched with relevant data, and stored with some additional type
information to help systems determine when the RR is relevant. For
the purposes of message routing, the system stores RRs known as MX
RRs. Each MX matches a domain name with two pieces of data, a
preference value (an unsigned 16-bit integer), and the name of a
host. The preference number is used to indicate in what order the
mailer should attempt deliver to the MX hosts, with the lowest
numbered MX being the one to try first. Multiple MXs with the same
preference are permitted and have the same priority.
In addition to mail information, the servers store certain other
types of RR's which mailers may encounter or choose to use. These
are: the canonical name (CNAME) RR, which simply states that the
domain name queried for is actually an alias for another domain name,
which is the proper, or canonical, name; and the Well Known Service
(WKS) RR, which stores information about network services (such as
SMTP) a given domain name supports.
General Routing Guidelines
Before delving into a detailed discussion of how mailers are expected
to do mail routing, it would seem to make sense to give a brief
overview of how this memo is approaching the problems that routing
poses.
The first major principle is derived from the definition of the
preference field in MX records, and is intended to prevent mail
looping. If the mailer is on a host which is listed as an MX for the
destination host, the mailer may only deliver to an MX which has a
lower preference count than its own host.
It is also possible to cause mail looping because routing information
is out of date or incomplete. Out of date information is only a
problem when domain tables are changed. The changes will not be
known to all affected hosts until their resolver caches time out.
There is no way to ensure that this will not happen short of
requiring mailers and their resolvers to always send their queries to
an authoritative server, and never use data stored in a cache. This
is an impractical solution, since eliminating resolver caching would
make mailing inordinately expensive. What is more, the out-of-date
RR problem should not happen if, when a domain table is changed,
Partridge [Page 2]
RFC 974 January 1986
Mail Routing and the Domain System
affected hosts (those in the list of MXs) have their resolver caches
flushed. In other words, given proper precautions, mail looping as a
result of domain information should be avoidable, without requiring
mailers to query authoritative servers. (The appropriate precaution
is to check with a host's administrator before adding that host to a
list of MXs).
The incomplete data problem also requires some care when handling
domain queries. If the answer section of a query is incomplete
critical MX RRs may be left out. This may result in mail looping, or
in a message being mistakenly labelled undeliverable. As a result,
mailers may only accept responses from the domain system which have
complete answer sections. Note that this entire problem can be
avoided by only using virtual circuits for queries, but since this
situation is likely to be very rare and datagrams are the preferred
way to interact with the domain system, implementors should probably
just ensure that their mailer will repeat a query with virtual
circuits should the truncation bit ever be set.
Determining Where to Send a Message
The explanation of how mailers should decide how to route a message
is discussed in terms of the problem of a mailer on a host with
domain name LOCAL trying to deliver a message addressed to the domain
name REMOTE. Both LOCAL and REMOTE are assumed to be syntactically
correct domain names. Furthermore, LOCAL is assumed to be the
official name for the host on which the mailer resides (i.e., it is
not a alias).
Issuing a Query
The first step for the mailer at LOCAL is to issue a query for MX RRs
for REMOTE. It is strongly urged that this step be taken every time
a mailer attempts to send the message. The hope is that changes in
the domain database will rapidly be used by mailers, and thus domain
administrators will be able to re-route in-transit messages for
defective hosts by simply changing their domain databases.
Certain responses to the query are considered errors:
Getting no response to the query. The domain server the mailer
queried never sends anything back. (This is distinct from an
answer which contains no answers to the query, which is not an
error).
Getting a response in which the truncation field of the header is
Partridge [Page 3]
RFC 974 January 1986
Mail Routing and the Domain System
set. (Recall discussion of incomplete queries above). Mailers
may not use responses of this type, and should repeat the query
using virtual circuits instead of datagrams.
Getting a response in which the response code is non-zero.
Mailers are expected to do something reasonable in the face of an
error. The behaviour for each type of error is not specified here,
but implementors should note that different types of errors should
probably be treated differently. For example, a response code of
"non-existent domain" should probably cause the message to be
returned to the sender as invalid, while a response code of "server
failure" should probably cause the message to be retried later.
There is one other special case. If the response contains an answer
which is a CNAME RR, it indicates that REMOTE is actually an alias
for some other domain name. The query should be repeated with the
canonical domain name.
If the response does not contain an error response, and does not
contain aliases, its answer section should be a (possibly zero
length) list of MX RRs for domain name REMOTE (or REMOTE's true
domain name if REMOTE was a alias). The next section describes how
this list is interpreted.
Interpreting the List of MX RRs
NOTE: This section only discusses how mailers choose which names to
try to deliver a message to, working from a list of RR's. It does
not discuss how the mailers actually make delivery. Where ever
delivering a message is mentioned, all that is meant is that the
mailer should do whatever it needs to do to transfer a message to a
remote site, given a domain name for that site. (For example, an
SMTP mailer will try to get an address for the domain name, which
involves another query to the domain system, and then, if it gets an
address, connect to the SMTP TCP port). The mechanics of actually
transferring the message over the network to the address associated
with a given domain name is not within the scope of this memo.
It is possible that the list of MXs in the response to the query will
be empty. This is a special case. If the list is empty, mailers
should treat it as if it contained one RR, an MX RR with a preference
value of 0, and a host name of REMOTE. (I.e., REMOTE is its only
MX). In addition, the mailer should do no further processing on the
list, but should attempt to deliver the message to REMOTE. The idea
Partridge [Page 4]
RFC 974 January 1986
Mail Routing and the Domain System
here is that if a domain fails to advertise any information about a
particular name we will give it the benefit of the doubt and attempt
delivery.
If the list is not empty, the mailer should remove irrelevant RR's
from the list according to the following steps. Note that the order
is significant.
For each MX, a WKS query should be issued to see if the domain
name listed actually supports the mail service desired. MX RRs
which list domain names which do not support the service should be
discarded. This step is optional, but strongly encouraged.
If the domain name LOCAL is listed as an MX RR, all MX RRs with a
preference value greater than or equal to that of LOCAL's must be
discarded.
After removing irrelevant RRs, the list can again be empty. This is
now an error condition and can occur in several ways. The simplest
case is that the WKS queries have discovered that none of the hosts
listed supports the mail service desired. The message is thus deemed
undeliverable, though extremely persistent mail systems might want to
try a delivery to REMOTE's address (if it exists) before returning
the message. Another, more dangerous, possibility is that the domain
system believes that LOCAL is handling message for REMOTE, but the
mailer on LOCAL is not set up to handle mail for REMOTE. For
example, if the domain system lists LOCAL as the only MX for REMOTE,
LOCAL will delete all the entries in the list. But LOCAL is
presumably querying the domain system because it didn't know what to
do with a message addressed to REMOTE. Clearly something is wrong.
How a mailer chooses to handle these situations is to some extent
implementation dependent, and is thus left to the implementor's
discretion.
If the list of MX RRs is not empty, the mailer should try to deliver
the message to the MXs in order (lowest preference value tried
first). The mailer is required to attempt delivery to the lowest
valued MX. Implementors are encouraged to write mailers so that they
try the MXs in order until one of the MXs accepts the message, or all
the MXs have been tried. A somewhat less demanding system, in which
a fixed number of MXs is tried, is also reasonable. Note that
multiple MXs may have the same preference value. In this case, all
MXs at with a given value must be tried before any of a higher value
are tried. In addition, in the special case in which there are
several MXs with the lowest preference value, all of them should be
tried before a message is deemed undeliverable.
Partridge [Page 5]
RFC 974 January 1986
Mail Routing and the Domain System
Minor Special Issues
There are a couple of special issues left out of the preceding
section because they complicated the discussion. They are treated
here in no particular order.
Wildcard names, those containing the character '*' in them, may be
used for mail routing. There are likely to be servers on the network
which simply state that any mail to a domain is to be routed through
a relay. For example, at the time that this RFC is being written, all
mail to hosts in the domain IL is routed through RELAY.CS.NET. This
is done by creating a wildcard RR, which states that *.IL has an MX
of RELAY.CS.NET. This should be transparent to the mailer since the
domain servers will hide this wildcard match. (If it matches *.IL
with HUJI.IL for example, a domain server will return an RR
containing HUJI.IL, not *.IL). If by some accident a mailer receives
an RR with a wildcard domain name in its name or data section it
should discard the RR.
Note that the algorithm to delete irrelevant RRs breaks if LOCAL has
a alias and the alias is listed in the MX records for REMOTE. (E.g.
REMOTE has an MX of ALIAS, where ALIAS has a CNAME of LOCAL). This
can be avoided if aliases are never used in the data section of MX
RRs.
Implementors should understand that the query and interpretation of
the query is only performed for REMOTE. It is not repeated for the
MX RRs listed for REMOTE. You cannot try to support more extravagant
mail routing by building a chain of MXs. (E.g. UNIX.BBN.COM is an MX
for RELAY.CS.NET and RELAY.CS.NET is an MX for all the hosts in .IL,
but this does not mean that UNIX.BBN.COM accepts any responsibility
for mail for .IL).
Finally, it should be noted that this is a standard for routing on
the Internet. Mailers serving hosts which lie on multiple networks
will presumably have to make some decisions about which network to
route through. This decision making is outside the scope of this
memo, although mailers may well use the domain system to help them
decide. However, once a mailer decides to deliver a message via the
Internet it must apply these rules to route the message.
Partridge [Page 6]
RFC 974 January 1986
Mail Routing and the Domain System
Examples
To illustrate the discussion above, here are three examples of how
mailers should route messages. All examples work with the following
database:
A.EXAMPLE.ORG IN MX 10 A.EXAMPLE.ORG
A.EXAMPLE.ORG IN MX 15 B.EXAMPLE.ORG
A.EXAMPLE.ORG IN MX 20 C.EXAMPLE.ORG
A.EXAMPLE.ORG IN WKS 10.0.0.1 TCP SMTP
B.EXAMPLE.ORG IN MX 0 B.EXAMPLE.ORG
B.EXAMPLE.ORG IN MX 10 C.EXAMPLE.ORG
B.EXAMPLE.ORG IN WKS 10.0.0.2 TCP SMTP
C.EXAMPLE.ORG IN MX 0 C.EXAMPLE.ORG
C.EXAMPLE.ORG IN WKS 10.0.0.3 TCP SMTP
D.EXAMPLE.ORG IN MX 0 D.EXAMPLE.ORG
D.EXAMPLE.ORG IN MX 0 C.EXAMPLE.ORG
D.EXAMPLE.ORG IN WKS 10.0.0.4 TCP SMTP
In the first example, an SMTP mailer on D.EXAMPLE.ORG is trying to
deliver a message addressed to A.EXAMPLE.ORG. From the answer to its
query, it learns that A.EXAMPLE.ORG has three MX RRs. D.EXAMPLE.ORG
is not one of the MX RRs and all three MXs support SMTP mail
(determined from the WKS entries), so none of the MXs are eliminated.
The mailer is obliged to try to deliver to A.EXAMPLE.ORG as the
lowest valued MX. If it cannot reach A.EXAMPLE.ORG it can (but is
not required to) try B.EXAMPLE.ORG. and if B.EXAMPLE.ORG is not
responding, it can try C.EXAMPLE.ORG.
In the second example, the mailer is on B.EXAMPLE.ORG, and is again
trying to deliver a message addressed to A.EXAMPLE.ORG. There are
once again three MX RRs for A.EXAMPLE.ORG, but in this case the
mailer must discard the RRs for itself and C.EXAMPLE.ORG (because the
MX RR for C.EXAMPLE.ORG has a higher preference value than the RR for
B.EXAMPLE.ORG). It is left only with the RR for A.EXAMPLE.ORG, and
can only try delivery to A.EXAMPLE.ORG.
In the third example, consider a mailer on A.EXAMPLE.ORG trying to
deliver a message to D.EXAMPLE.ORG. In this case there are only two
MX RRs, both with the same preference value. Either MX will accept
messages for D.EXAMPLE.ORG. The mailer should try one MX first (which
one is up to the mailer, though D.EXAMPLE.ORG seems most reasonable),
and if that delivery fails should try the other MX (e.g.
C.EXAMPLE.ORG).
Partridge [Page 7]
=========================================================================
Network Working Group J. Klensin, WG Chair
Request For Comments: 1869 MCI
STD: 10 N. Freed, Editor
Obsoletes: 1651 Innosoft International, Inc.
Category: Standards Track M. Rose
Dover Beach Consulting, Inc.
E. Stefferud
Network Management Associates, Inc.
D. Crocker
Brandenburg Consulting
November 1995
SMTP Service Extensions
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
1. Abstract
This memo defines a framework for extending the SMTP service by
defining a means whereby a server SMTP can inform a client SMTP as to
the service extensions it supports. Extensions to the SMTP service
are registered with the IANA. This framework does not require
modification of existing SMTP clients or servers unless the features
of the service extensions are to be requested or provided.
2. Introduction
The Simple Mail Transfer Protocol (SMTP) [1] has provided a stable,
effective basis for the relay function of message transfer agents.
Although a decade old, SMTP has proven remarkably resilient.
Nevertheless, the need for a number of protocol extensions has become
evident. Rather than describing these extensions as separate and
haphazard entities, this document enhances SMTP in a straightforward
fashion that provides a framework in which all future extensions can
be built in a single consistent way.
3. Framework for SMTP Extensions
For the purpose of service extensions to SMTP, SMTP relays a mail
object containing an envelope and a content.
Klensin, et al Standards Track [Page 1]
RFC 1869 SMTP Service Extensions November 1995
(1) The SMTP envelope is straightforward, and is sent as a
series of SMTP protocol units: it consists of an
originator address (to which error reports should be
directed); a delivery mode (e.g., deliver to recipient
mailboxes); and, one or more recipient addresses.
(2) The SMTP content is sent in the SMTP DATA protocol unit
and has two parts: the headers and the body. The
headers form a collection of field/value pairs
structured according to RFC 822 [2], whilst the body,
if structured, is defined according to MIME [3]. The
content is textual in nature, expressed using the US
ASCII repertoire (ANSI X3.4-1986). Although extensions
(such as MIME) may relax this restriction for the
content body, the content headers are always encoded
using the US ASCII repertoire. The algorithm defined in
[4] is used to represent header values outside the US
ASCII repertoire, whilst still encoding them using the
US ASCII repertoire.
Although SMTP is widely and robustly deployed, some parts of the
Internet community might wish to extend the SMTP service. This memo
defines a means whereby both an extended SMTP client and server may
recognize each other as such and the server can inform the client as
to the service extensions that it supports.
It must be emphasized that any extension to the SMTP service should
not be considered lightly. SMTP's strength comes primarily from its
simplicity. Experience with many protocols has shown that:
protocols with few options tend towards ubiquity, whilst
protocols with many options tend towards obscurity.
This means that each and every extension, regardless of its benefits,
must be carefully scrutinized with respect to its implementation,
deployment, and interoperability costs. In many cases, the cost of
extending the SMTP service will likely outweigh the benefit.
Given this environment, the framework for the extensions described in
this memo consists of:
(1) a new SMTP command (section 4)
(2) a registry of SMTP service extensions (section 5)
(3) additional parameters to the SMTP MAIL FROM and RCPT TO
commands (section 6).
Klensin, et al Standards Track [Page 2]
RFC 1869 SMTP Service Extensions November 1995
4. The EHLO command
A client SMTP supporting SMTP service extensions should start an SMTP
session by issuing the EHLO command instead of the HELO command. If
the SMTP server supports the SMTP service extensions it will give a
successful response (see section 4.3), a failure response (see 4.4),
or an error response (4.5). If the SMTP server does not support any
SMTP service extensions it will generate an error response (see
section 4.5).
4.1. Changes to STD 10, RFC 821
This specification is intended to extend STD 10, RFC 821 without
impacting existing services in any way. The minor changes needed are
enumerated below.
4.1.1. First command
RFC 821 states that the first command in an SMTP session must be the
HELO command. This requirement is hereby amended to allow a session
to start with either EHLO or HELO.
4.1.2. Maximum command line length
This specification extends the SMTP MAIL FROM and RCPT TO to allow
additional parameters and parameter values. It is possible that the
MAIL FROM and RCPT TO lines that result will exceed the 512 character
limit on command line length imposed by RFC 821. This limit is
hereby amended to only apply to command lines without any parameters.
Each specification that defines new MAIL FROM or RCPT TO parameters
must also specify maximum parameter value lengths for each parameter
so that implementors of some set of extensions know how much buffer
space must be allocated. The maximum command length that must be
supported by an SMTP implementation with extensions is 512 plus the
sum of all the maximum parameter lengths for all the extensions
supported.
4.2. Command syntax
The syntax for this command, using the ABNF notation of [2], is:
ehlo-cmd ::= "EHLO" SP domain CR LF
If successful, the server SMTP responds with code 250. On failure,
the server SMTP responds with code 550. On error, the server SMTP
responds with one of codes 500, 501, 502, 504, or 421.
Klensin, et al Standards Track [Page 3]
RFC 1869 SMTP Service Extensions November 1995
This command is issued instead of the HELO command, and may be issued
at any time that a HELO command would be appropriate. That is, if
the EHLO command is issued, and a successful response is returned,
then a subsequent HELO or EHLO command will result in the server SMTP
replying with code 503. A client SMTP must not cache any information
returned if the EHLO command succeeds. That is, a client SMTP must
issue the EHLO command at the start of each SMTP session if
information about extended facilities is needed.
4.3. Successful response
If the server SMTP implements and is able to perform the EHLO
command, it will return code 250. This indicates that both the
server and client SMTP are in the initial state, that is, there is no
transaction in progress and all state tables and buffers are cleared.
Normally, this response will be a multiline reply. Each line of the
response contains a keyword and, optionally, one or more parameters.
The syntax for a positive response, using the ABNF notation of [2],
is:
ehlo-ok-rsp ::= "250" domain [ SP greeting ] CR LF
/ ( "250-" domain [ SP greeting ] CR LF
*( "250-" ehlo-line CR LF )
"250" SP ehlo-line CR LF )
; the usual HELO chit-chat
greeting ::= 1*<any character other than CR or LF>
ehlo-line ::= ehlo-keyword *( SP ehlo-param )
ehlo-keyword ::= (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
; syntax and values depend on ehlo-keyword
ehlo-param ::= 1*<any CHAR excluding SP and all
control characters (US ASCII 0-31
inclusive)>
ALPHA ::= <any one of the 52 alphabetic characters
(A through Z in upper case, and,
a through z in lower case)>
DIGIT ::= <any one of the 10 numeric characters
(0 through 9)>
CR ::= <the carriage-return character
(ASCII decimal code 13)>
LF ::= <the line-feed character
(ASCII decimal code 10)>
Klensin, et al Standards Track [Page 4]
RFC 1869 SMTP Service Extensions November 1995
SP ::= <the space character
(ASCII decimal code 32)>
Although EHLO keywords may be specified in upper, lower, or mixed
case, they must always be recognized and processed in a case-
insensitive manner. This is simply an extension of practices begun in
RFC 821.
The IANA maintains a registry of SMTP service extensions. Associated
with each such extension is a corresponding EHLO keyword value. Each
service extension registered with the IANA must be defined in an RFC.
Such RFCs must either be on the standards-track or must define an
IESG-approved experimental protocol. The definition must include:
(1) the textual name of the SMTP service extension;
(2) the EHLO keyword value associated with the extension;
(3) the syntax and possible values of parameters associated
with the EHLO keyword value;
(4) any additional SMTP verbs associated with the extension
(additional verbs will usually be, but are not required
to be, the same as the EHLO keyword value);
(5) any new parameters the extension associates with the
MAIL FROM or RCPT TO verbs;
(6) how support for the extension affects the behavior of a
server and client SMTP; and,
(7) the increment by which the extension is increasing the
maximum length of the commands MAIL FROM, RCPT TO, or
both, over that specified in RFC 821.
In addition, any EHLO keyword value that starts with an upper or
lower case "X" refers to a local SMTP service extension, which is
used through bilateral, rather than standardized, agreement. Keywords
beginning with "X" may not be used in a registered service extension.
Any keyword values presented in the EHLO response that do not begin
with "X" must correspond to a standard, standards-track, or IESG-
approved experimental SMTP service extension registered with IANA. A
conforming server must not offer non "X" prefixed keyword values that
are not described in a registered extension.
Klensin, et al Standards Track [Page 5]
RFC 1869 SMTP Service Extensions November 1995
Additional verbs are bound by the same rules as EHLO keywords;
specifically, verbs begining with "X" are local extensions that may
not be registered or standardized and verbs not beginning with "X"
must always be registered.
4.4. Failure response
If for some reason the server SMTP is unable to list the service
extensions it supports, it will return code 554.
In the case of a failure response, the client SMTP should issue
either the HELO or QUIT command.
4.5. Error responses from extended servers
If the server SMTP recognizes the EHLO command, but the command
argument is unacceptable, it will return code 501.
If the server SMTP recognizes, but does not implement, the EHLO
command, it will return code 502.
If the server SMTP determines that the SMTP service is no longer
available (e.g., due to imminent system shutdown), it will return
code 421.
In the case of any error response, the client SMTP should issue
either the HELO or QUIT command.
4.6. Responses from servers without extensions
A server SMTP that conforms to RFC 821 but does not support the
extensions specified here will not recognize the EHLO command and
will consequently return code 500, as specified in RFC 821. The
server SMTP should stay in the same state after returning this code
(see section 4.1.1 of RFC 821). The client SMTP may then issue
either a HELO or a QUIT command.
4.7. Responses from improperly implemented servers
Some SMTP servers are known to disconnect the SMTP transmission
channel upon receipt of the EHLO command. The disconnect can occur
immediately or after sending a response. Such behavior violates
section 4.1.1 of RFC 821, which explicitly states that disconnection
should only occur after a QUIT command is issued.
Nevertheless, in order to achieve maxmimum interoperablity it is
suggested that extended SMTP clients using EHLO be coded to check for
server connection closure after EHLO is sent, either before or after
Klensin, et al Standards Track [Page 6]
RFC 1869 SMTP Service Extensions November 1995
returning a reply. If this happens the client must decide if the
operation can be successfully completed without using any SMTP
extensions. If it can a new connection can be opened and the HELO
command can be used.
Other improperly-implemented servers will not accept a HELO command
after EHLO has been sent and rejected. In some cases, this problem
can be worked around by sending a RSET after the failure response to
EHLO, then sending the HELO. Clients that do this should be aware
that many implementations will return a failure code (e.g., 503 Bad
sequence of commands) in response to the RSET. This code can be
safely ignored.
5. Initial IANA Registry
The IANA's initial registry of SMTP service extensions consists of
these entries:
Service Ext EHLO Keyword Parameters Verb Added Behavior
------------- ------------ ---------- ---------- ------------------
Send SEND none SEND defined in RFC 821
Send or Mail SOML none SOML defined in RFC 821
Send and Mail SAML none SAML defined in RFC 821
Expand EXPN none EXPN defined in RFC 821
Help HELP none HELP defined in RFC 821
Turn TURN none TURN defined in RFC 821
which correspond to those SMTP commands which are defined as optional
in [5]. (The mandatory SMTP commands, according to [5], are HELO,
MAIL, RCPT, DATA, RSET, VRFY, NOOP, and QUIT.)
6. MAIL FROM and RCPT TO Parameters
It is recognized that several of the extensions planned for SMTP will
make use of additional parameters associated with the MAIL FROM and
RCPT TO command. The syntax for these commands, again using the ABNF
notation of [2] as well as underlying definitions from [1], is:
esmtp-cmd ::= inner-esmtp-cmd [SP esmtp-parameters] CR LF
esmtp-parameters ::= esmtp-parameter *(SP esmtp-parameter)
esmtp-parameter ::= esmtp-keyword ["=" esmtp-value]
esmtp-keyword ::= (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
; syntax and values depend on esmtp-keyword
esmtp-value ::= 1*<any CHAR excluding "=", SP, and all
control characters (US ASCII 0-31
inclusive)>
Klensin, et al Standards Track [Page 7]
RFC 1869 SMTP Service Extensions November 1995
; The following commands are extended to
; accept extended parameters.
inner-esmtp-cmd ::= ("MAIL FROM:" reverse-path) /
("RCPT TO:" forward-path)
All esmtp-keyword values must be registered as part of the IANA
registration process described above. This definition only provides
the framework for future extension; no extended MAIL FROM or RCPT TO
parameters are defined by this RFC.
6.1. Error responses
If the server SMTP does not recognize or cannot implement one or more
of the parameters associated with a particular MAIL FROM or RCPT TO
command, it will return code 555.
If for some reason the server is temporarily unable to accomodate one
or more of the parameters associated with a MAIL FROM or RCPT TO
command, and if the definition of the specific parameter does not
mandate the use of another code, it should return code 455.
Errors specific to particular parameters and their values will be
specified in the parameter's defining RFC.
7. Received: Header Field Annotation
SMTP servers are required to add an appropriate Received: field to
the headers of all messages they receive. A "with ESMTP" clause
should be added to this field when any SMTP service extensions are
used. "ESMTP" is hereby added to the list of standard protocol names
registered with IANA.
8. Usage Examples
(1) An interaction of the form:
S: <wait for connection on TCP port 25>
C: <open connection to server>
S: 220 dbc.mtview.ca.us SMTP service ready
C: EHLO ymir.claremont.edu
S: 250 dbc.mtview.ca.us says hello
...
indicates that the server SMTP implements only those
SMTP commands which are defined as mandatory in [5].
Klensin, et al Standards Track [Page 8]
RFC 1869 SMTP Service Extensions November 1995
(2) In contrast, an interaction of the form:
S: <wait for connection on TCP port 25>
C: <open connection to server>
S: 220 dbc.mtview.ca.us SMTP service ready
C: EHLO ymir.claremont.edu
S: 250-dbc.mtview.ca.us says hello
S: 250-EXPN
S: 250-HELP
S: 250-8BITMIME
S: 250-XONE
S: 250 XVRB
...
indicates that the server SMTP also implements the SMTP
EXPN and HELP commands, one standard service extension
(8BITMIME), and two nonstandard and unregistered
service extensions (XONE and XVRB).
(3) Finally, a server that does not support SMTP service
extensions would act as follows:
S: <wait for connection on TCP port 25>
C: <open connection to server>
S: 220 dbc.mtview.ca.us SMTP service ready
C: EHLO ymir.claremont.edu
S: 500 Command not recognized: EHLO
...
The 500 response indicates that the server SMTP does
not implement the extensions specified here. The
client would normally send a HELO command and proceed
as specified in RFC 821. See section 4.7 for
additional discussion.
9. Security Considerations
This RFC does not discuss security issues and is not believed to
raise any security issues not already endemic in electronic mail and
present in fully conforming implementations of RFC-821. It does
provide an announcement of server mail capabilities via the response
to the EHLO verb. However, all information provided by announcement
of any of the initial set of service extensions defined by this RFC
can be readily deduced by selective probing of the verbs required to
transport and deliver mail. The security implications of service
extensions described in other RFCs should be dealt with in those
RFCs.
Klensin, et al Standards Track [Page 9]
RFC 1869 SMTP Service Extensions November 1995
10. Acknowledgements
This document represents a synthesis of the ideas of many people and
reactions to the ideas and proposals of others. Randall Atkinson,
Craig Everhart, Risto Kankkunen, and Greg Vaudreuil contributed ideas
and text sufficient to be considered co-authors. Other important
suggestions, text, or encouragement came from Harald Alvestrand, Jim
Conklin, Mark Crispin, Frank da Cruz, 'Olafur Gudmundsson, Per
Hedeland, Christian Huitma, Neil Katin, Eliot Lear, Harold A.
Miller, Keith Moore, John Myers, Dan Oscarsson, Julian Onions, Rayan
Zachariassen, and the contributions of the entire IETF SMTP Working
Group. Of course, none of the individuals are necessarily responsible
for the combination of ideas represented here. Indeed, in some cases,
the response to a particular criticism was to accept the problem
identification but to include an entirely different solution from the
one originally proposed.
11. References
[1] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
USC/Information Sciences Institute, August 1982.
[2] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, UDEL, August 1982.
[3] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
Extensions", RFC 1521, Bellcore, Innosoft, September 1993.
[4] Moore, K., "Representation of Non-ASCII Text in Internet Message
Headers", RFC 1522, University of Tennessee, September 1993.
[5] Braden, R., "Requirements for Internet Hosts - Application and
Support", STD 3, RFC 1123, USC/Information Sciences Institute,
October 1989.
12. Chair, Editor, and Author Addresses
John Klensin, WG Chair
MCI
2100 Reston Parkway
Reston, VA 22091
Phone: +1 703 715-7361
Fax: +1 703 715-7436
EMail:
[email protected]
Klensin, et al Standards Track [Page 10]
RFC 1869 SMTP Service Extensions November 1995
Ned Freed, Editor
Innosoft International, Inc.
1050 East Garvey Avenue South
West Covina, CA 91790
USA
Phone: +1 818 919 3600
Fax: +1 818 919 3614
EMail:
[email protected]
Marshall T. Rose
Dover Beach Consulting, Inc.
420 Whisman Court
Moutain View, CA 94043-2186
USA
Phone: +1 415 968 1052
Fax: +1 415 968 2510
EMail:
[email protected]
Einar A. Stefferud
Network Management Associates, Inc.
17301 Drey Lane
Huntington Beach, CA, 92647-5615
USA
Phone: +1 714 842 3711
Fax: +1 714 848 2091
EMail:
[email protected]
Dave Crocker
Brandenburg Consulting
675 Spruce Dr.
Sunnyvale, CA 94086 USA
USA
Phone: +1 408 246 8253
Fax: +1 408 249 6205
EMail:
[email protected]
Klensin, et al Standards Track [Page 11]
=========================================================================
Network Working Group J. Klensin, WG Chair
Request For Comments: 1870 MCI
STD: 10 N. Freed, Editor
Obsoletes: 1653 Innosoft International, Inc.
Category: Standards Track K. Moore
University of Tennessee
November 1995
SMTP Service Extension
for Message Size Declaration
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
1. Abstract
This memo defines an extension to the SMTP service whereby an SMTP
client and server may interact to give the server an opportunity to
decline to accept a message (perhaps temporarily) based on the
client's estimate of the message size.
2. Introduction
The MIME extensions to the Internet message protocol provide for the
transmission of many kinds of data which were previously unsupported
in Internet mail. One expected result of the use of MIME is that
SMTP will be expected to carry a much wider range of message sizes
than was previously the case. This has an impact on the amount of
resources (e.g. disk space) required by a system acting as a server.
This memo uses the mechanism defined in [5] to define extensions to
the SMTP service whereby a client ("sender-SMTP") may declare the
size of a particular message to a server ("receiver-SMTP"), after
which the server may indicate to the client that it is or is not
willing to accept the message based on the declared message size and
whereby a server ("receiver-SMTP") may declare the maximum message
size it is willing to accept to a client ("sender-SMTP").
Klensin, et al Standards Track [Page 1]
RFC 1870 SMTP Size Declaration November 1995
3. Framework for the Size Declaration Extension
The following service extension is therefore defined:
(1) the name of the SMTP service extension is "Message Size
Declaration";
(2) the EHLO keyword value associated with this extension is "SIZE";
(3) one optional parameter is allowed with this EHLO keyword value, a
decimal number indicating the fixed maximum message size in bytes
that the server will accept. The syntax of the parameter is as
follows, using the augmented BNF notation of [2]:
size-param ::= [1*DIGIT]
A parameter value of 0 (zero) indicates that no fixed maximum
message size is in force. If the parameter is omitted no
information is conveyed about the server's fixed maximum message
size;
(4) one optional parameter using the keyword "SIZE" is added to the
MAIL FROM command. The value associated with this parameter is a
decimal number indicating the size of the message that is to be
transmitted. The syntax of the value is as follows, using the
augmented BNF notation of [2]:
size-value ::= 1*20DIGIT
(5) the maximum length of a MAIL FROM command line is increased by 26
characters by the possible addition of the SIZE keyword and
value;
(6) no additional SMTP verbs are defined by this extension.
The remainder of this memo specifies how support for the extension
affects the behavior of an SMTP client and server.
4. The Message Size Declaration service extension
An SMTP server may have a fixed upper limit on message size. Any
attempt by a client to transfer a message which is larger than this
fixed upper limit will fail. In addition, a server normally has
limited space with which to store incoming messages. Transfer of a
message may therefore also fail due to a lack of storage space, but
might succeed at a later time.
Klensin, et al Standards Track [Page 2]
RFC 1870 SMTP Size Declaration November 1995
A client using the unextended SMTP protocol defined in [1], can only
be informed of such failures after transmitting the entire message to
the server (which discards the transferred message). If, however,
both client and server support the Message Size Declaration service
extension, such conditions may be detected before any transfer is
attempted.
An SMTP client wishing to relay a large content may issue the EHLO
command to start an SMTP session, to determine if the server supports
any of several service extensions. If the server responds with code
250 to the EHLO command, and the response includes the EHLO keyword
value SIZE, then the Message Size Declaration extension is supported.
If a numeric parameter follows the SIZE keyword value of the EHLO
response, it indicates the size of the largest message that the
server is willing to accept. Any attempt by a client to transfer a
message which is larger than this limit will be rejected with a
permanent failure (552) reply code.
A server that supports the Message Size Declaration extension will
accept the extended version of the MAIL command described below.
When supported by the server, a client may use the extended MAIL
command (instead of the MAIL command as defined in [1]) to declare an
estimate of the size of a message it wishes to transfer. The server
may then return an appropriate error code if it determines that an
attempt to transfer a message of that size would fail.
5. Definitions
The message size is defined as the number of octets, including CR-LF
pairs, but not the SMTP DATA command's terminating dot or doubled
quoting dots, to be transmitted by the SMTP client after receiving
reply code 354 to the DATA command.
The fixed maximum message size is defined as the message size of the
largest message that a server is ever willing to accept. An attempt
to transfer any message larger than the fixed maximum message size
will always fail. The fixed maximum message size may be an
implementation artifact of the SMTP server, or it may be chosen by
the administrator of the server.
The declared message size is defined as a client's estimate of the
message size for a particular message.
Klensin, et al Standards Track [Page 3]
RFC 1870 SMTP Size Declaration November 1995
6. The extended MAIL command
The extended MAIL command is issued by a client when it wishes to
inform a server of the size of the message to be sent. The extended
MAIL command is identical to the MAIL command as defined in [1],
except that a SIZE parameter appears after the address.
The complete syntax of this extended command is defined in [5]. The
esmtp-keyword is "SIZE" and the syntax for esmtp-value is given by
the syntax for size-value shown above.
The value associated with the SIZE parameter is a decimal
representation of the declared message size in octets. This number
should include the message header, body, and the CR-LF sequences
between lines, but not the SMTP DATA command's terminating dot or
doubled quoting dots. Only one SIZE parameter may be specified in a
single MAIL command.
Ideally, the declared message size is equal to the true message size.
However, since exact computation of the message size may be
infeasable, the client may use a heuristically-derived estimate.
Such heuristics should be chosen so that the declared message size is
usually larger than the actual message size. (This has the effect of
making the counting or non-counting of SMTP DATA dots largely an
academic point.)
NOTE: Servers MUST NOT use the SIZE parameter to determine end of
content in the DATA command.
6.1 Server action on receipt of the extended MAIL command
Upon receipt of an extended MAIL command containing a SIZE parameter,
a server should determine whether the declared message size exceeds
its fixed maximum message size. If the declared message size is
smaller than the fixed maximum message size, the server may also wish
to determine whether sufficient resources are available to buffer a
message of the declared message size and to maintain it in stable
storage, until the message can be delivered or relayed to each of its
recipients.
A server may respond to the extended MAIL command with any of the
error codes defined in [1] for the MAIL command. In addition, one of
the following error codes may be returned:
(1) If the server currently lacks sufficient resources to accept a
message of the indicated size, but may be able to accept the
message at a later time, it responds with code "452 insufficient
system storage".
Klensin, et al Standards Track [Page 4]
RFC 1870 SMTP Size Declaration November 1995
(2) If the indicated size is larger than the server's fixed maximum
message size, the server responds with code "552 message size
exceeds fixed maximium message size".
A server is permitted, but not required, to accept a message which
is, in fact, larger than declared in the extended MAIL command, such
as might occur if the client employed a size-estimation heuristic
which was inaccurate.
6.2 Client action on receiving response to extended MAIL command
The client, upon receiving the server's response to the extended MAIL
command, acts as follows:
(1) If the code "452 insufficient system storage" is returned, the
client should next send either a RSET command (if it wishes to
attempt to send other messages) or a QUIT command. The client
should then repeat the attempt to send the message to the server
at a later time.
(2) If the code "552 message exceeds fixed maximum message size" is
received, the client should immediately send either a RSET command
(if it wishes to attempt to send additional messages), or a QUIT
command. The client should then declare the message undeliverable
and return appropriate notification to the sender (if a sender
address was present in the MAIL command).
A successful (250) reply code in response to the extended MAIL
command does not constitute an absolute guarantee that the message
transfer will succeed. SMTP clients using the extended MAIL command
must still be prepared to handle both temporary and permanent error
reply codes (including codes 452 and 552), either immediately after
issuing the DATA command, or after transfer of the message.
6.3 Messages larger than the declared size.
Once a server has agreed (via the extended MAIL command) to accept a
message of a particular size, it should not return a 552 reply code
after the transfer phase of the DATA command, unless the actual size
of the message transferred is greater than the declared message size.
A server may also choose to accept a message which is somewhat larger
than the declared message size.
A client is permitted to declare a message to be smaller than its
actual size. However, in this case, a successful (250) reply code is
no assurance that the server will accept the message or has
sufficient resources to do so. The server may reject such a message
after its DATA transfer.
Klensin, et al Standards Track [Page 5]
RFC 1870 SMTP Size Declaration November 1995
6.4 Per-recipient rejection based on message size.
A server that implements this extension may return a 452 or 552 reply
code in response to a RCPT command, based on its unwillingness to
accept a message of the declared size for a particular recipient.
(1) If a 452 code is returned, the client may requeue the message for
later delivery to the same recipient.
(2) If a 552 code is returned, the client may not requeue the message
for later delivery to the same recipient.
7. Minimal usage
A "minimal" client may use this extension to simply compare its
(perhaps estimated) size of the message that it wishes to relay, with
the server's fixed maximum message size (from the parameter to the
SIZE keyword in the EHLO response), to determine whether the server
will ever accept the message. Such an implementation need not
declare message sizes via the extended MAIL command. However,
neither will it be able to discover temporary limits on message size
due to server resource limitations, nor per-recipient limitations on
message size.
A minimal server that employs this service extension may simply use
the SIZE keyword value to inform the client of the size of the
largest message it will accept, or to inform the client that there is
no fixed limit on message size. Such a server must accept the
extended MAIL command and return a 552 reply code if the client's
declared size exceeds its fixed size limit (if any), but it need not
detect "temporary" limitations on message size.
The numeric parameter to the EHLO SIZE keyword is optional. If the
parameter is omitted entirely it indicates that the server does not
advertise a fixed maximum message size. A server that returns the
SIZE keyword with no parameter in response to the EHLO command may
not issue a positive (250) response to an extended MAIL command
containing a SIZE specification without first checking to see if
sufficient resources are available to transfer a message of the
declared size, and to retain it in stable storage until it can be
relayed or delivered to its recipients. If possible, the server
should actually reserve sufficient storage space to transfer the
message.
Klensin, et al Standards Track [Page 6]
RFC 1870 SMTP Size Declaration November 1995
8. Example
The following example illustrates the use of size declaration with
some permanent and temporary failures.
S: <wait for connection on TCP port 25>
C: <open connection to server>
S: 220 sigurd.innosoft.com -- Server SMTP (PMDF V4.2-6 #1992)
C: EHLO ymir.claremont.edu
S: 250-sigurd.innosoft.com
S: 250-EXPN
S: 250-HELP
S: 250 SIZE 1000000
C: MAIL FROM:<
[email protected]> SIZE=500000
S: 250 Address Ok.
C: RCPT TO:<
[email protected]>
S: 250
[email protected] OK; can accomodate 500000 byte message
C: RCPT TO:<
[email protected]>
S: 552 Channel size limit exceeded:
[email protected]
C: RCPT TO:<
[email protected]>
S: 452 Insufficient channel storage:
[email protected]
C: DATA
S: 354 Send message, ending in CRLF.CRLF.
...
C: .
S: 250 Some recipients OK
C: QUIT
S: 221 Goodbye
9. Security Considerations
The size declaration extensions described in this memo can
conceivably be used to facilitate crude service denial attacks.
Specifically, both the information contained in the SIZE parameter
and use of the extended MAIL command make it somewhat quicker and
easier to devise an efficacious service denial attack. However,
unless implementations are very weak, these extensions do not create
any vulnerability that has not always existed with SMTP. In addition,
no issues are addressed involving trusted systems and possible
release of information via the mechanisms described in this RFC.
10. Acknowledgements
This document was derived from an earlier Working Group work in
progess contribution. Jim Conklin, Dave Crocker, Neil Katin, Eliot
Lear, Marshall T. Rose, and Einar Stefferud provided extensive
comments in response to earlier works in progress of both this and
the previous memo.
Klensin, et al Standards Track [Page 7]
RFC 1870 SMTP Size Declaration November 1995
11. References
[1] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
USC/Information Sciences Institute, August 1982.
[2] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, UDEL, August 1982.
[3] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
Extensions", RFC 1521, Bellcore, Innosoft, September 1993.
[4] Moore, K., "Representation of Non-ASCII Text in Internet Message
Headers", RFC 1522, University of Tennessee, September 1993.
[5] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker,
"SMTP Service Extensions", STD 11, RFC 1869, MCI, Innosoft
International, Inc., Dover Beach Consulting, Inc., Network
Management Associates, Inc., Brandenburg Consulting, November
1995.
[6] Partridge, C., "Mail Routing and the Domain System", STD 14, RFC
974, BBN, January 1986.
Klensin, et al Standards Track [Page 8]
RFC 1870 SMTP Size Declaration November 1995
12. Chair, Editor, and Author Addresses
John Klensin, WG Chair
MCI
2100 Reston Parkway
Reston, VA 22091
Phone: +1 703 715-7361
Fax: +1 703 715-7436
EMail:
[email protected]
Ned Freed, Editor
Innosoft International, Inc.
1050 East Garvey Avenue South
West Covina, CA 91790
USA
Phone: +1 818 919 3600
Fax: +1 818 919 3614
EMail:
[email protected]
Keith Moore
Computer Science Dept.
University of Tennessee
107 Ayres Hall
Knoxville, TN 37996-1301
USA
EMail:
[email protected]
Klensin, et al Standards Track [Page 9]
