Network Working Group                                           J. Myers
Request for Comments: 1939                               Carnegie Mellon
STD: 53                                                          M. Rose
Obsoletes: 1725                             Dover Beach Consulting, Inc.
Category: Standards Track                                       May 1996


                   Post Office Protocol - Version 3

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.

Table of Contents

  1. Introduction ................................................    2
  2. A Short Digression ..........................................    2
  3. Basic Operation .............................................    3
  4. The AUTHORIZATION State .....................................    4
     QUIT Command ................................................    5
  5. The TRANSACTION State .......................................    5
     STAT Command ................................................    6
     LIST Command ................................................    6
     RETR Command ................................................    8
     DELE Command ................................................    8
     NOOP Command ................................................    9
     RSET Command ................................................    9
  6. The UPDATE State ............................................   10
     QUIT Command ................................................   10
  7. Optional POP3 Commands ......................................   11
     TOP Command .................................................   11
     UIDL Command ................................................   12
     USER Command ................................................   13
     PASS Command ................................................   14
     APOP Command ................................................   15
  8. Scaling and Operational Considerations ......................   16
  9. POP3 Command Summary ........................................   18
  10. Example POP3 Session .......................................   19
  11. Message Format .............................................   19
  12. References .................................................   20
  13. Security Considerations ....................................   20
  14. Acknowledgements ...........................................   20
  15. Authors' Addresses .........................................   21
  Appendix A. Differences from RFC 1725 ..........................   22



Myers & Rose                Standards Track                     [Page 1]

RFC 1939                          POP3                          May 1996


  Appendix B. Command Index ......................................   23

1. Introduction

  On certain types of smaller nodes in the Internet it is often
  impractical to maintain a message transport system (MTS).  For
  example, a workstation may not have sufficient resources (cycles,
  disk space) in order to permit a SMTP server [RFC821] and associated
  local mail delivery system to be kept resident and continuously
  running.  Similarly, it may be expensive (or impossible) to keep a
  personal computer interconnected to an IP-style network for long
  amounts of time (the node is lacking the resource known as
  "connectivity").

  Despite this, it is often very useful to be able to manage mail on
  these smaller nodes, and they often support a user agent (UA) to aid
  the tasks of mail handling.  To solve this problem, a node which can
  support an MTS entity offers a maildrop service to these less endowed
  nodes.  The Post Office Protocol - Version 3 (POP3) is intended to
  permit a workstation to dynamically access a maildrop on a server
  host in a useful fashion.  Usually, this means that the POP3 protocol
  is used to allow a workstation to retrieve mail that the server is
  holding for it.

  POP3 is not intended to provide extensive manipulation operations of
  mail on the server; normally, mail is downloaded and then deleted.  A
  more advanced (and complex) protocol, IMAP4, is discussed in
  [RFC1730].

  For the remainder of this memo, the term "client host" refers to a
  host making use of the POP3 service, while the term "server host"
  refers to a host which offers the POP3 service.

2. A Short Digression

  This memo does not specify how a client host enters mail into the
  transport system, although a method consistent with the philosophy of
  this memo is presented here:

     When the user agent on a client host wishes to enter a message
     into the transport system, it establishes an SMTP connection to
     its relay host and sends all mail to it.  This relay host could
     be, but need not be, the POP3 server host for the client host.  Of
     course, the relay host must accept mail for delivery to arbitrary
     recipient addresses, that functionality is not required of all
     SMTP servers.





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RFC 1939                          POP3                          May 1996


3. Basic Operation

  Initially, the server host starts the POP3 service by listening on
  TCP port 110.  When a client host wishes to make use of the service,
  it establishes a TCP connection with the server host.  When the
  connection is established, the POP3 server sends a greeting.  The
  client and POP3 server then exchange commands and responses
  (respectively) until the connection is closed or aborted.

  Commands in the POP3 consist of a case-insensitive keyword, possibly
  followed by one or more arguments.  All commands are terminated by a
  CRLF pair.  Keywords and arguments consist of printable ASCII
  characters.  Keywords and arguments are each separated by a single
  SPACE character.  Keywords are three or four characters long. Each
  argument may be up to 40 characters long.

  Responses in the POP3 consist of a status indicator and a keyword
  possibly followed by additional information.  All responses are
  terminated by a CRLF pair.  Responses may be up to 512 characters
  long, including the terminating CRLF.  There are currently two status
  indicators: positive ("+OK") and negative ("-ERR").  Servers MUST
  send the "+OK" and "-ERR" in upper case.

  Responses to certain commands are multi-line.  In these cases, which
  are clearly indicated below, after sending the first line of the
  response and a CRLF, any additional lines are sent, each terminated
  by a CRLF pair.  When all lines of the response have been sent, a
  final line is sent, consisting of a termination octet (decimal code
  046, ".") and a CRLF pair.  If any line of the multi-line response
  begins with the termination octet, the line is "byte-stuffed" by
  pre-pending the termination octet to that line of the response.
  Hence a multi-line response is terminated with the five octets
  "CRLF.CRLF".  When examining a multi-line response, the client checks
  to see if the line begins with the termination octet.  If so and if
  octets other than CRLF follow, the first octet of the line (the
  termination octet) is stripped away.  If so and if CRLF immediately
  follows the termination character, then the response from the POP
  server is ended and the line containing ".CRLF" is not considered
  part of the multi-line response.

  A POP3 session progresses through a number of states during its
  lifetime.  Once the TCP connection has been opened and the POP3
  server has sent the greeting, the session enters the AUTHORIZATION
  state.  In this state, the client must identify itself to the POP3
  server.  Once the client has successfully done this, the server
  acquires resources associated with the client's maildrop, and the
  session enters the TRANSACTION state.  In this state, the client
  requests actions on the part of the POP3 server.  When the client has



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  issued the QUIT command, the session enters the UPDATE state.  In
  this state, the POP3 server releases any resources acquired during
  the TRANSACTION state and says goodbye.  The TCP connection is then
  closed.

  A server MUST respond to an unrecognized, unimplemented, or
  syntactically invalid command by responding with a negative status
  indicator.  A server MUST respond to a command issued when the
  session is in an incorrect state by responding with a negative status
  indicator.  There is no general method for a client to distinguish
  between a server which does not implement an optional command and a
  server which is unwilling or unable to process the command.

  A POP3 server MAY have an inactivity autologout timer.  Such a timer
  MUST be of at least 10 minutes' duration.  The receipt of any command
  from the client during that interval should suffice to reset the
  autologout timer.  When the timer expires, the session does NOT enter
  the UPDATE state--the server should close the TCP connection without
  removing any messages or sending any response to the client.

4. The AUTHORIZATION State

  Once the TCP connection has been opened by a POP3 client, the POP3
  server issues a one line greeting.  This can be any positive
  response.  An example might be:

     S:  +OK POP3 server ready

  The POP3 session is now in the AUTHORIZATION state.  The client must
  now identify and authenticate itself to the POP3 server.  Two
  possible mechanisms for doing this are described in this document,
  the USER and PASS command combination and the APOP command.  Both
  mechanisms are described later in this document.  Additional
  authentication mechanisms are described in [RFC1734].  While there is
  no single authentication mechanism that is required of all POP3
  servers, a POP3 server must of course support at least one
  authentication mechanism.

  Once the POP3 server has determined through the use of any
  authentication command that the client should be given access to the
  appropriate maildrop, the POP3 server then acquires an exclusive-
  access lock on the maildrop, as necessary to prevent messages from
  being modified or removed before the session enters the UPDATE state.
  If the lock is successfully acquired, the POP3 server responds with a
  positive status indicator.  The POP3 session now enters the
  TRANSACTION state, with no messages marked as deleted.  If the
  maildrop cannot be opened for some reason (for example, a lock can
  not be acquired, the client is denied access to the appropriate



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  maildrop, or the maildrop cannot be parsed), the POP3 server responds
  with a negative status indicator.  (If a lock was acquired but the
  POP3 server intends to respond with a negative status indicator, the
  POP3 server must release the lock prior to rejecting the command.)
  After returning a negative status indicator, the server may close the
  connection.  If the server does not close the connection, the client
  may either issue a new authentication command and start again, or the
  client may issue the QUIT command.

  After the POP3 server has opened the maildrop, it assigns a message-
  number to each message, and notes the size of each message in octets.
  The first message in the maildrop is assigned a message-number of
  "1", the second is assigned "2", and so on, so that the nth message
  in a maildrop is assigned a message-number of "n".  In POP3 commands
  and responses, all message-numbers and message sizes are expressed in
  base-10 (i.e., decimal).

  Here is the summary for the QUIT command when used in the
  AUTHORIZATION state:

     QUIT

        Arguments: none

        Restrictions: none

        Possible Responses:
            +OK

        Examples:
            C: QUIT
            S: +OK dewey POP3 server signing off

5. The TRANSACTION State

  Once the client has successfully identified itself to the POP3 server
  and the POP3 server has locked and opened the appropriate maildrop,
  the POP3 session is now in the TRANSACTION state.  The client may now
  issue any of the following POP3 commands repeatedly.  After each
  command, the POP3 server issues a response.  Eventually, the client
  issues the QUIT command and the POP3 session enters the UPDATE state.










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RFC 1939                          POP3                          May 1996


  Here are the POP3 commands valid in the TRANSACTION state:

     STAT

        Arguments: none

        Restrictions:
            may only be given in the TRANSACTION state

        Discussion:
            The POP3 server issues a positive response with a line
            containing information for the maildrop.  This line is
            called a "drop listing" for that maildrop.

            In order to simplify parsing, all POP3 servers are
            required to use a certain format for drop listings.  The
            positive response consists of "+OK" followed by a single
            space, the number of messages in the maildrop, a single
            space, and the size of the maildrop in octets.  This memo
            makes no requirement on what follows the maildrop size.
            Minimal implementations should just end that line of the
            response with a CRLF pair.  More advanced implementations
            may include other information.

               NOTE: This memo STRONGLY discourages implementations
               from supplying additional information in the drop
               listing.  Other, optional, facilities are discussed
               later on which permit the client to parse the messages
               in the maildrop.

            Note that messages marked as deleted are not counted in
            either total.

        Possible Responses:
            +OK nn mm

        Examples:
            C: STAT
            S: +OK 2 320


     LIST [msg]

        Arguments:
            a message-number (optional), which, if present, may NOT
            refer to a message marked as deleted





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        Restrictions:
            may only be given in the TRANSACTION state

        Discussion:
            If an argument was given and the POP3 server issues a
            positive response with a line containing information for
            that message.  This line is called a "scan listing" for
            that message.

            If no argument was given and the POP3 server issues a
            positive response, then the response given is multi-line.
            After the initial +OK, for each message in the maildrop,
            the POP3 server responds with a line containing
            information for that message.  This line is also called a
            "scan listing" for that message.  If there are no
            messages in the maildrop, then the POP3 server responds
            with no scan listings--it issues a positive response
            followed by a line containing a termination octet and a
            CRLF pair.

            In order to simplify parsing, all POP3 servers are
            required to use a certain format for scan listings.  A
            scan listing consists of the message-number of the
            message, followed by a single space and the exact size of
            the message in octets.  Methods for calculating the exact
            size of the message are described in the "Message Format"
            section below.  This memo makes no requirement on what
            follows the message size in the scan listing.  Minimal
            implementations should just end that line of the response
            with a CRLF pair.  More advanced implementations may
            include other information, as parsed from the message.

               NOTE: This memo STRONGLY discourages implementations
               from supplying additional information in the scan
               listing.  Other, optional, facilities are discussed
               later on which permit the client to parse the messages
               in the maildrop.

            Note that messages marked as deleted are not listed.

        Possible Responses:
            +OK scan listing follows
            -ERR no such message

        Examples:
            C: LIST
            S: +OK 2 messages (320 octets)
            S: 1 120



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RFC 1939                          POP3                          May 1996


            S: 2 200
            S: .
              ...
            C: LIST 2
            S: +OK 2 200
              ...
            C: LIST 3
            S: -ERR no such message, only 2 messages in maildrop


     RETR msg

        Arguments:
            a message-number (required) which may NOT refer to a
            message marked as deleted

        Restrictions:
            may only be given in the TRANSACTION state

        Discussion:
            If the POP3 server issues a positive response, then the
            response given is multi-line.  After the initial +OK, the
            POP3 server sends the message corresponding to the given
            message-number, being careful to byte-stuff the termination
            character (as with all multi-line responses).

        Possible Responses:
            +OK message follows
            -ERR no such message

        Examples:
            C: RETR 1
            S: +OK 120 octets
            S: <the POP3 server sends the entire message here>
            S: .


     DELE msg

        Arguments:
            a message-number (required) which may NOT refer to a
            message marked as deleted

        Restrictions:
            may only be given in the TRANSACTION state






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RFC 1939                          POP3                          May 1996


        Discussion:
            The POP3 server marks the message as deleted.  Any future
            reference to the message-number associated with the message
            in a POP3 command generates an error.  The POP3 server does
            not actually delete the message until the POP3 session
            enters the UPDATE state.

        Possible Responses:
            +OK message deleted
            -ERR no such message

        Examples:
            C: DELE 1
            S: +OK message 1 deleted
               ...
            C: DELE 2
            S: -ERR message 2 already deleted


     NOOP

        Arguments: none

        Restrictions:
            may only be given in the TRANSACTION state

        Discussion:
            The POP3 server does nothing, it merely replies with a
            positive response.

        Possible Responses:
            +OK

        Examples:
            C: NOOP
            S: +OK


     RSET

        Arguments: none

        Restrictions:
            may only be given in the TRANSACTION state

        Discussion:
            If any messages have been marked as deleted by the POP3
            server, they are unmarked.  The POP3 server then replies



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RFC 1939                          POP3                          May 1996


            with a positive response.

        Possible Responses:
            +OK

        Examples:
            C: RSET
            S: +OK maildrop has 2 messages (320 octets)

6. The UPDATE State

  When the client issues the QUIT command from the TRANSACTION state,
  the POP3 session enters the UPDATE state.  (Note that if the client
  issues the QUIT command from the AUTHORIZATION state, the POP3
  session terminates but does NOT enter the UPDATE state.)

  If a session terminates for some reason other than a client-issued
  QUIT command, the POP3 session does NOT enter the UPDATE state and
  MUST not remove any messages from the maildrop.

     QUIT

        Arguments: none

        Restrictions: none

        Discussion:
            The POP3 server removes all messages marked as deleted
            from the maildrop and replies as to the status of this
            operation.  If there is an error, such as a resource
            shortage, encountered while removing messages, the
            maildrop may result in having some or none of the messages
            marked as deleted be removed.  In no case may the server
            remove any messages not marked as deleted.

            Whether the removal was successful or not, the server
            then releases any exclusive-access lock on the maildrop
            and closes the TCP connection.

        Possible Responses:
            +OK
            -ERR some deleted messages not removed

        Examples:
            C: QUIT
            S: +OK dewey POP3 server signing off (maildrop empty)
               ...
            C: QUIT



Myers & Rose                Standards Track                    [Page 10]

RFC 1939                          POP3                          May 1996


            S: +OK dewey POP3 server signing off (2 messages left)
               ...

7. Optional POP3 Commands

  The POP3 commands discussed above must be supported by all minimal
  implementations of POP3 servers.

  The optional POP3 commands described below permit a POP3 client
  greater freedom in message handling, while preserving a simple POP3
  server implementation.

     NOTE: This memo STRONGLY encourages implementations to support
     these commands in lieu of developing augmented drop and scan
     listings.  In short, the philosophy of this memo is to put
     intelligence in the part of the POP3 client and not the POP3
     server.

     TOP msg n

        Arguments:
            a message-number (required) which may NOT refer to to a
            message marked as deleted, and a non-negative number
            of lines (required)

        Restrictions:
            may only be given in the TRANSACTION state

        Discussion:
            If the POP3 server issues a positive response, then the
            response given is multi-line.  After the initial +OK, the
            POP3 server sends the headers of the message, the blank
            line separating the headers from the body, and then the
            number of lines of the indicated message's body, being
            careful to byte-stuff the termination character (as with
            all multi-line responses).

            Note that if the number of lines requested by the POP3
            client is greater than than the number of lines in the
            body, then the POP3 server sends the entire message.

        Possible Responses:
            +OK top of message follows
            -ERR no such message

        Examples:
            C: TOP 1 10
            S: +OK



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RFC 1939                          POP3                          May 1996


            S: <the POP3 server sends the headers of the
               message, a blank line, and the first 10 lines
               of the body of the message>
            S: .
               ...
            C: TOP 100 3
            S: -ERR no such message


     UIDL [msg]

     Arguments:
         a message-number (optional), which, if present, may NOT
         refer to a message marked as deleted

     Restrictions:
         may only be given in the TRANSACTION state.

     Discussion:
         If an argument was given and the POP3 server issues a positive
         response with a line containing information for that message.
         This line is called a "unique-id listing" for that message.

         If no argument was given and the POP3 server issues a positive
         response, then the response given is multi-line.  After the
         initial +OK, for each message in the maildrop, the POP3 server
         responds with a line containing information for that message.
         This line is called a "unique-id listing" for that message.

         In order to simplify parsing, all POP3 servers are required to
         use a certain format for unique-id listings.  A unique-id
         listing consists of the message-number of the message,
         followed by a single space and the unique-id of the message.
         No information follows the unique-id in the unique-id listing.

         The unique-id of a message is an arbitrary server-determined
         string, consisting of one to 70 characters in the range 0x21
         to 0x7E, which uniquely identifies a message within a
         maildrop and which persists across sessions.  This
         persistence is required even if a session ends without
         entering the UPDATE state.  The server should never reuse an
         unique-id in a given maildrop, for as long as the entity
         using the unique-id exists.

         Note that messages marked as deleted are not listed.

         While it is generally preferable for server implementations
         to store arbitrarily assigned unique-ids in the maildrop,



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         this specification is intended to permit unique-ids to be
         calculated as a hash of the message.  Clients should be able
         to handle a situation where two identical copies of a
         message in a maildrop have the same unique-id.

     Possible Responses:
         +OK unique-id listing follows
         -ERR no such message

     Examples:
         C: UIDL
         S: +OK
         S: 1 whqtswO00WBw418f9t5JxYwZ
         S: 2 QhdPYR:00WBw1Ph7x7
         S: .
            ...
         C: UIDL 2
         S: +OK 2 QhdPYR:00WBw1Ph7x7
            ...
         C: UIDL 3
         S: -ERR no such message, only 2 messages in maildrop


     USER name

        Arguments:
            a string identifying a mailbox (required), which is of
            significance ONLY to the server

        Restrictions:
            may only be given in the AUTHORIZATION state after the POP3
            greeting or after an unsuccessful USER or PASS command

        Discussion:
            To authenticate using the USER and PASS command
            combination, the client must first issue the USER
            command.  If the POP3 server responds with a positive
            status indicator ("+OK"), then the client may issue
            either the PASS command to complete the authentication,
            or the QUIT command to terminate the POP3 session.  If
            the POP3 server responds with a negative status indicator
            ("-ERR") to the USER command, then the client may either
            issue a new authentication command or may issue the QUIT
            command.

            The server may return a positive response even though no
            such mailbox exists.  The server may return a negative
            response if mailbox exists, but does not permit plaintext



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RFC 1939                          POP3                          May 1996


            password authentication.

        Possible Responses:
            +OK name is a valid mailbox
            -ERR never heard of mailbox name

        Examples:
            C: USER frated
            S: -ERR sorry, no mailbox for frated here
               ...
            C: USER mrose
            S: +OK mrose is a real hoopy frood


     PASS string

        Arguments:
            a server/mailbox-specific password (required)

        Restrictions:
            may only be given in the AUTHORIZATION state immediately
            after a successful USER command

        Discussion:
            When the client issues the PASS command, the POP3 server
            uses the argument pair from the USER and PASS commands to
            determine if the client should be given access to the
            appropriate maildrop.

            Since the PASS command has exactly one argument, a POP3
            server may treat spaces in the argument as part of the
            password, instead of as argument separators.

        Possible Responses:
            +OK maildrop locked and ready
            -ERR invalid password
            -ERR unable to lock maildrop

        Examples:
            C: USER mrose
            S: +OK mrose is a real hoopy frood
            C: PASS secret
            S: -ERR maildrop already locked
              ...
            C: USER mrose
            S: +OK mrose is a real hoopy frood
            C: PASS secret
            S: +OK mrose's maildrop has 2 messages (320 octets)



Myers & Rose                Standards Track                    [Page 14]

RFC 1939                          POP3                          May 1996


     APOP name digest

        Arguments:
            a string identifying a mailbox and a MD5 digest string
            (both required)

        Restrictions:
            may only be given in the AUTHORIZATION state after the POP3
            greeting or after an unsuccessful USER or PASS command

        Discussion:
            Normally, each POP3 session starts with a USER/PASS
            exchange.  This results in a server/user-id specific
            password being sent in the clear on the network.  For
            intermittent use of POP3, this may not introduce a sizable
            risk.  However, many POP3 client implementations connect to
            the POP3 server on a regular basis -- to check for new
            mail.  Further the interval of session initiation may be on
            the order of five minutes.  Hence, the risk of password
            capture is greatly enhanced.

            An alternate method of authentication is required which
            provides for both origin authentication and replay
            protection, but which does not involve sending a password
            in the clear over the network.  The APOP command provides
            this functionality.

            A POP3 server which implements the APOP command will
            include a timestamp in its banner greeting.  The syntax of
            the timestamp corresponds to the `msg-id' in [RFC822], and
            MUST be different each time the POP3 server issues a banner
            greeting.  For example, on a UNIX implementation in which a
            separate UNIX process is used for each instance of a POP3
            server, the syntax of the timestamp might be:

               <process-ID.clock@hostname>

            where `process-ID' is the decimal value of the process's
            PID, clock is the decimal value of the system clock, and
            hostname is the fully-qualified domain-name corresponding
            to the host where the POP3 server is running.

            The POP3 client makes note of this timestamp, and then
            issues the APOP command.  The `name' parameter has
            identical semantics to the `name' parameter of the USER
            command. The `digest' parameter is calculated by applying
            the MD5 algorithm [RFC1321] to a string consisting of the
            timestamp (including angle-brackets) followed by a shared



Myers & Rose                Standards Track                    [Page 15]

RFC 1939                          POP3                          May 1996


            secret.  This shared secret is a string known only to the
            POP3 client and server.  Great care should be taken to
            prevent unauthorized disclosure of the secret, as knowledge
            of the secret will allow any entity to successfully
            masquerade as the named user.  The `digest' parameter
            itself is a 16-octet value which is sent in hexadecimal
            format, using lower-case ASCII characters.

            When the POP3 server receives the APOP command, it verifies
            the digest provided.  If the digest is correct, the POP3
            server issues a positive response, and the POP3 session
            enters the TRANSACTION state.  Otherwise, a negative
            response is issued and the POP3 session remains in the
            AUTHORIZATION state.

            Note that as the length of the shared secret increases, so
            does the difficulty of deriving it.  As such, shared
            secrets should be long strings (considerably longer than
            the 8-character example shown below).

        Possible Responses:
            +OK maildrop locked and ready
            -ERR permission denied

        Examples:
            S: +OK POP3 server ready <[email protected]>
            C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
            S: +OK maildrop has 1 message (369 octets)

            In this example, the shared  secret  is  the  string  `tan-
            staaf'.  Hence, the MD5 algorithm is applied to the string

               <[email protected]>tanstaaf

            which produces a digest value of

               c4c9334bac560ecc979e58001b3e22fb

8. Scaling and Operational Considerations

  Since some of the optional features described above were added to the
  POP3 protocol, experience has accumulated in using them in large-
  scale commercial post office operations where most of the users are
  unrelated to each other.  In these situations and others, users and
  vendors of POP3 clients have discovered that the combination of using
  the UIDL command and not issuing the DELE command can provide a weak
  version of the "maildrop as semi-permanent repository" functionality
  normally associated with IMAP.  Of course the other capabilities of



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RFC 1939                          POP3                          May 1996


  IMAP, such as polling an existing connection for newly arrived
  messages and supporting multiple folders on the server, are not
  present in POP3.

  When these facilities are used in this way by casual users, there has
  been a tendency for already-read messages to accumulate on the server
  without bound.  This is clearly an undesirable behavior pattern from
  the standpoint of the server operator.  This situation is aggravated
  by the fact that the limited capabilities of the POP3 do not permit
  efficient handling of maildrops which have hundreds or thousands of
  messages.

  Consequently, it is recommended that operators of large-scale multi-
  user servers, especially ones in which the user's only access to the
  maildrop is via POP3, consider such options as:

  *  Imposing a per-user maildrop storage quota or the like.

     A disadvantage to this option is that accumulation of messages may
     result in the user's inability to receive new ones into the
     maildrop.  Sites which choose this option should be sure to inform
     users of impending or current exhaustion of quota, perhaps by
     inserting an appropriate message into the user's maildrop.

  *  Enforce a site policy regarding mail retention on the server.

     Sites are free to establish local policy regarding the storage and
     retention of messages on the server, both read and unread.  For
     example, a site might delete unread messages from the server after
     60 days and delete read messages after 7 days.  Such message
     deletions are outside the scope of the POP3 protocol and are not
     considered a protocol violation.

     Server operators enforcing message deletion policies should take
     care to make all users aware of the policies in force.

     Clients must not assume that a site policy will automate message
     deletions, and should continue to explicitly delete messages using
     the DELE command when appropriate.

     It should be noted that enforcing site message deletion policies
     may be confusing to the user community, since their POP3 client
     may contain configuration options to leave mail on the server
     which will not in fact be supported by the server.

     One special case of a site policy is that messages may only be
     downloaded once from the server, and are deleted after this has
     been accomplished.  This could be implemented in POP3 server



Myers & Rose                Standards Track                    [Page 17]

RFC 1939                          POP3                          May 1996


     software by the following mechanism: "following a POP3 login by a
     client which was ended by a QUIT, delete all messages downloaded
     during the session with the RETR command".  It is important not to
     delete messages in the event of abnormal connection termination
     (ie, if no QUIT was received from the client) because the client
     may not have successfully received or stored the messages.
     Servers implementing a download-and-delete policy may also wish to
     disable or limit the optional TOP command, since it could be used
     as an alternate mechanism to download entire messages.

9. POP3 Command Summary

     Minimal POP3 Commands:

        USER name               valid in the AUTHORIZATION state
        PASS string
        QUIT

        STAT                    valid in the TRANSACTION state
        LIST [msg]
        RETR msg
        DELE msg
        NOOP
        RSET
        QUIT

     Optional POP3 Commands:

        APOP name digest        valid in the AUTHORIZATION state

        TOP msg n               valid in the TRANSACTION state
        UIDL [msg]

     POP3 Replies:

        +OK
        -ERR

     Note that with the exception of the STAT, LIST, and UIDL commands,
     the reply given by the POP3 server to any command is significant
     only to "+OK" and "-ERR".  Any text occurring after this reply
     may be ignored by the client.









Myers & Rose                Standards Track                    [Page 18]

RFC 1939                          POP3                          May 1996


10. Example POP3 Session

     S: <wait for connection on TCP port 110>
     C: <open connection>
     S:    +OK POP3 server ready <[email protected]>
     C:    APOP mrose c4c9334bac560ecc979e58001b3e22fb
     S:    +OK mrose's maildrop has 2 messages (320 octets)
     C:    STAT
     S:    +OK 2 320
     C:    LIST
     S:    +OK 2 messages (320 octets)
     S:    1 120
     S:    2 200
     S:    .
     C:    RETR 1
     S:    +OK 120 octets
     S:    <the POP3 server sends message 1>
     S:    .
     C:    DELE 1
     S:    +OK message 1 deleted
     C:    RETR 2
     S:    +OK 200 octets
     S:    <the POP3 server sends message 2>
     S:    .
     C:    DELE 2
     S:    +OK message 2 deleted
     C:    QUIT
     S:    +OK dewey POP3 server signing off (maildrop empty)
     C:  <close connection>
     S:  <wait for next connection>

11. Message Format

  All messages transmitted during a POP3 session are assumed to conform
  to the standard for the format of Internet text messages [RFC822].

  It is important to note that the octet count for a message on the
  server host may differ from the octet count assigned to that message
  due to local conventions for designating end-of-line.  Usually,
  during the AUTHORIZATION state of the POP3 session, the POP3 server
  can calculate the size of each message in octets when it opens the
  maildrop.  For example, if the POP3 server host internally represents
  end-of-line as a single character, then the POP3 server simply counts
  each occurrence of this character in a message as two octets.  Note
  that lines in the message which start with the termination octet need
  not (and must not) be counted twice, since the POP3 client will
  remove all byte-stuffed termination characters when it receives a
  multi-line response.



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RFC 1939                          POP3                          May 1996


12. References

  [RFC821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
      821, USC/Information Sciences Institute, August 1982.

  [RFC822] Crocker, D., "Standard for the Format of ARPA-Internet Text
      Messages", STD 11, RFC 822, University of Delaware, August 1982.

  [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
      MIT Laboratory for Computer Science, April 1992.

  [RFC1730] Crispin, M., "Internet Message Access Protocol - Version
      4", RFC 1730, University of Washington, December 1994.

  [RFC1734] Myers, J., "POP3 AUTHentication command", RFC 1734,
      Carnegie Mellon, December 1994.

13. Security Considerations

  It is conjectured that use of the APOP command provides origin
  identification and replay protection for a POP3 session.
  Accordingly, a POP3 server which implements both the PASS and APOP
  commands should not allow both methods of access for a given user;
  that is, for a given mailbox name, either the USER/PASS command
  sequence or the APOP command is allowed, but not both.

  Further, note that as the length of the shared secret increases, so
  does the difficulty of deriving it.

  Servers that answer -ERR to the USER command are giving potential
  attackers clues about which names are valid.

  Use of the PASS command sends passwords in the clear over the
  network.

  Use of the RETR and TOP commands sends mail in the clear over the
  network.

  Otherwise, security issues are not discussed in this memo.

14. Acknowledgements

  The POP family has a long and checkered history.  Although primarily
  a minor revision to RFC 1460, POP3 is based on the ideas presented in
  RFCs 918, 937, and 1081.

  In addition, Alfred Grimstad, Keith McCloghrie, and Neil Ostroff
  provided significant comments on the APOP command.



Myers & Rose                Standards Track                    [Page 20]

RFC 1939                          POP3                          May 1996


15. Authors' Addresses

  John G. Myers
  Carnegie-Mellon University
  5000 Forbes Ave
  Pittsburgh, PA 15213

  EMail: [email protected]


  Marshall T. Rose
  Dover Beach Consulting, Inc.
  420 Whisman Court
  Mountain View, CA  94043-2186

  EMail: [email protected]



































Myers & Rose                Standards Track                    [Page 21]

RFC 1939                          POP3                          May 1996


Appendix A. Differences from RFC 1725

  This memo is a revision to RFC 1725, a Draft Standard.  It makes the
  following changes from that document:

     - clarifies that command keywords are case insensitive.

     - specifies that servers must send "+OK" and "-ERR" in
       upper case.

     - specifies that the initial greeting is a positive response,
       instead of any string which should be a positive response.

     - clarifies behavior for unimplemented commands.

     - makes the USER and PASS commands optional.

     - clarified the set of possible responses to the USER command.

     - reverses the order of the examples in the USER and PASS
       commands, to reduce confusion.

     - clarifies that the PASS command may only be given immediately
       after a successful USER command.

     - clarified the persistence requirements of UIDs and added some
       implementation notes.

     - specifies a UID length limitation of one to 70 octets.

     - specifies a status indicator length limitation
       of 512 octets, including the CRLF.

     - clarifies that LIST with no arguments on an empty mailbox
       returns success.

     - adds a reference from the LIST command to the Message Format
       section

     - clarifies the behavior of QUIT upon failure

     - clarifies the security section to not imply the use of the
       USER command with the APOP command.

     - adds references to RFCs 1730 and 1734

     - clarifies the method by which a UA may enter mail into the
       transport system.



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RFC 1939                          POP3                          May 1996


     - clarifies that the second argument to the TOP command is a
       number of lines.

     - changes the suggestion in the Security Considerations section
       for a server to not accept both PASS and APOP for a given user
       from a "must" to a "should".

     - adds a section on scaling and operational considerations

Appendix B. Command Index

      APOP .......................................................   15
      DELE .......................................................    8
      LIST .......................................................    6
      NOOP .......................................................    9
      PASS .......................................................   14
      QUIT .......................................................    5
      QUIT .......................................................   10
      RETR .......................................................    8
      RSET .......................................................    9
      STAT .......................................................    6
      TOP ........................................................   11
      UIDL .......................................................   12
      USER .......................................................   13



























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