Network Working Group                                       N. Freed
Request for Comments: RFC 2034                              Innosoft
Category: Standards Track                               October 1996


                      SMTP Service Extension for
                    Returning Enhanced Error Codes

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 [RFC-821, RFC-
  1869] whereby an SMTP server augments its responses with the enhanced
  mail system status codes defined in RFC 1893.  These codes can then
  be used to provide more informative explanations of error conditions,
  especially in the context of the delivery status notifications format
  defined in RFC 1894.

2.  Introduction

  Although SMTP is widely and robustly deployed, various extensions
  have been requested by parts of the Internet community. In
  particular, in the modern, international, and multilingual Internet a
  need exists to assign codes to specific error conditions that can be
  translated into different languages. RFC 1893 defines such a set of
  status codes and RFC 1894 defines a mechanism to send such coded
  material to users. However, in many cases the agent creating the RFC
  1894 delivery status notification is doing so in response to errors
  it received from a remote SMTP server.

  As such, remote servers need a mechanism for embedding enhanced
  status codes in their responses as well as a way to indicate to a
  client when they are in fact doing this. This memo uses the SMTP
  extension mechanism described in RFC 1869 to define such a mechanism.










Freed                       Standards Track                     [Page 1]

RFC 2034               SMTP Enhanced Error Codes            October 1996


3.  Framework for the Enhanced Error Statuses Extension

  The enhanced error statuses transport extension is laid out as
  follows:

  (1)   the name of the SMTP service extension defined here is
        Enhanced-Status-Codes;

  (2)   the EHLO keyword value associated with the extension is
        ENHANCEDSTATUSCODES;

  (3)   no parameter is used with the ENHANCEDSTATUSCODES EHLO
        keyword;

  (4)   the text part of all 2xx, 4xx, and 5xx SMTP responses
        other than the initial greeting and any response to
        HELO or EHLO are prefaced with a status code as defined
        in RFC 1893. This status code is always followed by one
        or more spaces.

  (5)   no additional SMTP verbs are defined by this extension;
        and,

  (6)   the next section specifies how support for the
        extension affects the behavior of a server and client
        SMTP.

4.  The Enhanced-Status-Codes service extension

  Servers supporting the Enhanced-Status-Codes extension must preface
  the text part of almost all response lines with a status code. As in
  RFC 1893, the syntax of these status codes is given by the ABNF:

       status-code ::= class "." subject "." detail
       class       ::= "2" / "4" / "5"
       subject     ::= 1*3digit
       detail      ::= 1*3digit

  These codes must appear in all 2xx, 4xx, and 5xx response lines other
  than initial greeting and any response to HELO or EHLO. Note that 3xx
  responses are NOT included in this list.

  All status codes returned by the server must agree with the primary
  response code, that is, a 2xx response must incorporate a 2.X.X code,
  a 4xx response must incorporate a 4.X.X code, and a 5xx response must
  incorporate a 5.X.X code.





Freed                       Standards Track                     [Page 2]

RFC 2034               SMTP Enhanced Error Codes            October 1996


  When responses are continued across multiple lines the same status
  code must appear at the beginning of the text in each line of the
  response.

  Servers supporting this extension must attach enhanced status codes
  to their responses regardless of whether or not EHLO is employed by
  the client.

5.  Status Codes and Negotiation

  This specification does not provide a means for clients to request
  that status codes be returned or that they not be returned; a
  compliant server includes these codes in the responses it sends
  regardless of whether or not the client expects them.  This is
  somewhat different from most other SMTP extensions, where generally
  speaking a client must specifically make a request before the
  extended server behaves any differently than an unextended server.
  The omission of client negotiation in this case is entirely
  intentional: Given the generally poor state of SMTP server error code
  implementation it is felt that any step taken towards more
  comprehensible error codes is something that all clients, extended or
  not, should benefit from.

  IMPORTANT NOTE:  The use of this approach in this extension should be
  seen as a very special case.  It MUST NOT be taken as a license for
  future SMTP extensions to dramatically change the nature of SMTP
  client-server interaction without proper announcement from the server
  and a corresponding enabling command from the client.

6.  Usage Example

  The following dialogue illustrates the use of enhanced status codes
  by a server:

  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 ENHANCEDSTATUSCODES
  C: MAIL FROM:<[email protected]>
  S: 250 2.1.0 Originator <[email protected]> ok
  C: RCPT TO:<[email protected]>
  S: 250 2.1.5 Recipient <[email protected]> ok
  C: RCPT TO:<[email protected]>
  S: 550 5.1.1 Mailbox "nosuchuser" does not exist
  C: RCPT TO:<[email protected]>
  S: 551-5.7.1 Forwarding to remote hosts disabled



Freed                       Standards Track                     [Page 3]

RFC 2034               SMTP Enhanced Error Codes            October 1996


  S: 551 5.7.1 Select another host to act as your forwarder
  C: DATA
  S: 354 Send message, ending in CRLF.CRLF.
   ...
  C: .
  S: 250 2.6.0 Message accepted
  C: QUIT
  S: 221 2.0.0 Goodbye

  The client that receives these responses might then send a
  nondelivery notification of the general form:

     Date: Mon, 11 Mar 1996 09:21:47 -0400
     From: Mail Delivery Subsystem <[email protected]>
     Subject: Returned mail
     To: <[email protected]>
     MIME-Version: 1.0
     Content-Type: multipart/report; report-type=delivery-status;
           boundary="JAA13167.773673707/YMIR.CLAREMONT.EDU"

     --JAA13167.773673707/YMIR.CLAREMONT.EDU
     content-type: text/plain; charset=us-ascii

        ----- Mail was successfully relayed to
              the following addresses -----

     <[email protected]>

        ----- The following addresses had delivery problems -----
     <[email protected]>
       (Mailbox "nosuchuser" does not exist)
     <[email protected]>
       (Forwarding to remote hosts disabled)

     --JAA13167.773673707/YMIR.CLAREMONT.EDU
     content-type: message/delivery-status

     Reporting-MTA: dns; ymir.claremont.edu

     Original-Recipient: rfc822;[email protected]
     Final-Recipient: rfc822;[email protected]
     Action: relayed
     Status: 2.1.5 (Destination address valid)
     Diagnostic-Code: smtp;
      250 Recipient <[email protected]> ok
     Remote-MTA: dns; dbc.mtview.ca.us





Freed                       Standards Track                     [Page 4]

RFC 2034               SMTP Enhanced Error Codes            October 1996


     Original-Recipient: rfc822;[email protected]
     Final-Recipient: rfc822;[email protected]
     Action: failed
     Status: 5.1.1 (Bad destination mailbox address)
     Diagnostic-Code: smtp;
      550 Mailbox "nosuchuser" does not exist
     Remote-MTA: dns; dbc.mtview.ca.us

     Original-Recipient: rfc822;[email protected]
     Final-Recipient: rfc822;[email protected]
     Action: failed
     Status: 5.7.1 (Delivery not authorized, message refused)
     Diagnostic-Code: smtp;
       551 Forwarding to remote hosts disabled
       Select another host to act as your forwarder
     Remote-MTA: dns; dbc.mtview.ca.us

     --JAA13167.773673707/YMIR.CLAREMONT.EDU
     content-type: message/rfc822

     [original message goes here]
     --JAA13167.773673707/YMIR.CLAREMONT.EDU--

  Note that in order to reduce clutter the reporting MTA has omitted
  enhanced status code information from the diagnostic-code fields it
  has generated.

7.  Security Considerations

  Additional detail in server responses axiomatically provides
  additional information about the server.  It is conceivable that
  additional information of this sort may be of assistance in
  circumventing server security.  The advantages of provides additional
  information must always be weighed against the security implications
  of doing so.
















Freed                       Standards Track                     [Page 5]

RFC 2034               SMTP Enhanced Error Codes            October 1996


8.  References

  [RFC-821]
       Postel, J., "Simple Mail Transfer Protocol", RFC 821,
       August, 1982.  (August, 1982).

  [RFC-1869]
       Rose, M., Stefferud, E., Crocker, C., Klensin, J., Freed,
       N., "SMTP Service Extensions", RFC 1869, November, 1995.

  [RFC-1893]
       Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
       1893, January, 1996.

  [RFC-1894]
       Moore, K., Vaudreuil, G., "An Extensible Message Format
       for Delivery Status Notifications", RFC 1894, January,
       1996.

9.  Author Address

  Ned Freed
  Innosoft International, Inc.
  1050 East Garvey Avenue South
  West Covina, CA 91790
  USA
   tel: +1 818 919 3600           fax: +1 818 919 3614
   email: [email protected]























Freed                       Standards Track                     [Page 6]


You are viewing proxied material from gopher.meulie.net. The copyright of proxied material belongs to its original authors. Any comments or complaints in relation to proxied material should be directed to the original authors of the content concerned. Please see the disclaimer for more details.