SMTP 521 and 556 Reply Codes
draft-klensin-smtp-521code-02

Network Working Group                                         J. Klensin
Internet-Draft
Updates: 1846, 5321 (if approved)                      September 7, 2014
Intended status: Standards Track
Expires: March 11, 2015

                      SMTP 521 and 556 Reply Codes
                   draft-klensin-smtp-521code-02.txt

Abstract

   This memo defines two Simple Mail Transfer Protocol (SMTP) reply
   code, 521 and 556.  The 521 code was originally described in an
   Experimental RFC in 1995 and is in wide use, but has not previously
   been formally incorporated into SMTP.  The 556 code was created for
   [[RFC nullMX]].  These codes are used to indicate that an Internet
   host does not accept incoming mail at all (not just under particular
   circumstances).

Status of This Memo

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   This Internet-Draft will expire on March 11, 2015.

Copyright Notice

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   include Simplified BSD License text as described in Section 4.e of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Discussion List . . . . . . . . . . . . . . . . . . . . .   3
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  The 521 Reply Code  . . . . . . . . . . . . . . . . . . . . .   3
   4.  The 556 Reply Code  . . . . . . . . . . . . . . . . . . . . .   4
   5.  Small details to avoid loose ends . . . . . . . . . . . . . .   4
     5.1.  Specific changes to RFC 5321  . . . . . . . . . . . . . .   4
     5.2.  The RFC 1846 Experiment . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .   6
     A.1.  Changss from -00 to -01 . . . . . . . . . . . . . . . . .   6
     A.2.  Changes from -01 to -02 . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   The SMTP specification [2] (referred to, along with its various
   updates, as "SMTP" below) contains a list and discussion of reply
   codes.  This document updates that list with a new code, 521, for use
   in response to an initial connection.  In that context, it
   specifically denotes a system that does not receive email or
   otherwise handle SMTP mail or inquiry transactions.  That code
   differs from the use of reply code 554, recommended by RFC 5321,
   because the latter code can be used in a larger variety of
   situations, including mail that is not accepted for, or from,
   particular sources, destinations, or addresses.  It also introduces a
   second reply code, 556, for use when an SMTP client encounters a
   domain in a forward-pointing address that it can determine (e.g.,
   from the DNS "null MX" convention [4]) does not support receipt of
   email and has to report that condition to a host that delivered the
   message to it for further processing.

   This specification updates RFC 5321 to add the new codes.  The 521
   code was first formally proposed in the Experimental RFC 1846 [3];
   this document updates that specification to standardize the code and
   provide more specific treatment of it.

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1.1.  Terminology

   The reader of this document is expected to have reasonable
   familiarity with the SMTP specification in RFC 5321, particularly its
   discussion of reply codes and their use and theory.  [[CREF1: For
   those who do not, it is safe to assume that it adds the two codes and
   that everything else is just details.]]

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

1.2.  Discussion List

   [[CREF2: RFC Editor: please remove this subsection.]]

   Discussion of the SMTP aspects and relationships of this
   specification should occur on the ietf-smtp list,
   https://www.ietf.org/mailman/listinfo/ietf-smtp.  Discussions of
   "null MX" and the relationship of this specification to it occur on
   the apps-discuss list, https://www.ietf.org/mailman/listinfo/apps-
   discuss.

2.  Background

   Many Internet hosts are not in a position -- whether technically,
   operationally, or administratively-- to offer email service.  If an
   SMTP client (sender) attempts to open a mail connection to a system
   that does not have an SMTP server, the connection attempt will time
   out.  SMTP requires that timeouts result in the client queuing the
   message and retrying it for an extended period.  That behavior will
   result in wasted resources and long delays in getting an error
   message back to its originator.

   One alternative is to run a dummy SMTP server on hosts that do not
   receive mail under any circumstances, having that dummy server return
   a fatal error reply code in response to any connection-opening
   attempt.  Another is to determine, from a separate source such as a
   DNS record, that the host does not receive mail.  This document
   specifies reply codes to be used for those purposes.

3.  The 521 Reply Code

   This specification adds the 521 reply code to the repertoire
   specified in SMTP, reserving it for use at connection-opening time to
   indicate that the host does not accept email under any circumstances.
   It SHOULD be used for dummy SMTP servers whose sole purpose is to
   notify systems that attempt to open mail connections that the host

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   never accepts mail.  It MAY be used in other situations where the
   intent is for the host to indicate that it never accepts email.

   The 521 reply code SHOULD NOT be used for situations in which the
   server rejects mail from particular hosts or addresses or in which
   mail intended to be transferred to a particular destination host is
   not accepted.  As discussed in SMTP, reply code 554 is more
   appropriate for most of those conditions.  One additional case is
   covered in the next section.

   The preferred message to accompany a 521 code is "Host does not
   accept mail".

   Once the 521 reply code is returned instead of the usual 220, the
   SMTP session proceeds normally (i.e., the client will send QUIT and
   close the connection).  If the SMTP client attempts to send
   additional commands other than QUIT, the Server MAY either continue
   sending 521 reply codes or simply close the connection.  If the
   purpose of running a dummy SMTP server that returns a 521 code is to
   conserve resources, the latter will usually be preferable.

4.  The 556 Reply Code

   This specification adds the 556 reply code to the repertoire
   specified in SMTP.  The code is intended for situations in which an
   SMTP client can determine that a particular server or domain that is
   referred to in a forward-pointing address does not accept mail
   connections without attempting to open a connection to a mail server
   associated with that domain.  Interpretation of a special DNS record,
   found when a lookup is performed in conjunction with a RCPT command
   [4], is one such method (and the only standardized one at the time
   this specification was written).

   When an SMTP server returns a 556 response code after receiving a
   command, such as RCPT, containing a forward-pointing address because
   it has information, as discussed above, that mail is not accepted,
   the SMTP client is, consistent with the SMTP specification, expected
   to handle the response like any other permanent negative completion
   reply to the command.

5.  Small details to avoid loose ends

5.1.  Specific changes to RFC 5321

   This document adds the 521 code, with message "Host does not accept
   mail", and the 556 code, with message "Domain does not accept mail",
   to the function group and numerical lists (Sections 4.2.2 and 4.2.3
   respectively) of RFC 5321.  It also adds the 521 code to the

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   "CONNECTION ESTABLISHMENT" portion of Section 4.3.2 ("Command-Reply
   Sequences"), preceding the 554 code, and the 556 code to the "RCPT"
   portion of that same section.

5.2.  The RFC 1846 Experiment

   By formalizing Response Code 521, this specification ends the
   experiment proposed in RFC 1846.  That earlier document also
   discusses general strategies for hosts that do not accept mail
   directly.  The latter discussion is out of scope for the present
   document.

6.  Security Considerations

   Not running any SMTP server, or running an SMTP server which simply
   emits fixed strings in response to incoming connections, should
   provide significantly fewer opportunities for security problems than
   running a complete SMTP implementation.  See the Security
   Considerations section of [[RFC nullMX]] [4] for a discussion of
   security issues with that approach.  Use of the specific code
   provided here provides more information to the client than a generic
   or arbitrarily-chosen 5yz code but should have no other effect on
   security.

7.  Acknowledgments

   Alain Durand and Francis Dupont proposed the 521 code in RFC 1846
   [3].  They also participated in an extended conversation and provided
   many useful comments that led to this document.  The document also
   contains, with their permission, some text copied from that early
   specification.

   Discussion of the "null MX" approach and proposal [4] inspired the
   creation of this specification and the inclusion of the 556 code in
   it.  Specific comments and suggestions from John Levine (co-author of
   that document) and Ned Freed were also helpful.

   Martin Duerst and Tom Petch identified significant issues in the
   initial draft of the current form of the document.

8.  References

8.1.  Normative References

   [1]        Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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   [2]        Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

8.2.  Informative References

   [3]        Durand, A. and F. Dupont, "SMTP 521 Reply Code", RFC 1846,
              September 1995.

   [4]        Levine, J. and M. Delany, "A "Null MX" No Service Resource
              Record for Domains that Accept No Mail", August 2014,
              <https://datatracker.ietf.org/doc/draft-ietf-appsawg-
              nullmx/>.

Appendix A.  Change Log

   RFC Editor: Please remove this appendix before publication..

   This Internet Draft is the successor to draft-klensin-rfc1846bis-00.
   That document was an attempt to completely update and replace RFC
   1846.  That effort led to the conclusion that it would be better to
   focus narrowly on the 521 code, leaving a more general treatment of
   hosts that do not receive mail to a separate replacement for RFC 1846
   and/or an update to RFC 5321.

A.1.  Changss from -00 to -01

   Revised abstract and cleaned up "error code" terminology.

A.2.  Changes from -01 to -02

   Added provision for code 556 and associated discussion.

   Several editorial corrections and cleanups.

Author's Address

   John C Klensin
   1770 Massachusetts Ave, Ste 322
   Cambridge, MA  02140
   USA

   Phone: +1 617 245 1457
   Email: john-ietf@jck.com

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