SMTP 521 and 556 Reply Codes
RFC 7504

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Internet Engineering Task Force (IETF)                        J. Klensin
Request for Comments: 7504                                     June 2015
Updates: 1846, 5321
Category: Standards Track
ISSN: 2070-1721

                      SMTP 521 and 556 Reply Codes

Abstract

   This memo defines two Simple Mail Transfer Protocol (SMTP) reply
   codes, 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 to
   support the new tests and actions specified in RFC 7505.  These codes
   are used to indicate that an Internet host does not accept incoming
   mail at all.  This specification is not applicable when the host
   sometimes accepts mail but may reject particular messages, or even
   all messages, under specific circumstances.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7504.

Klensin                      Standards Track                    [Page 1]
RFC 7504              SMTP 521 and 556 Reply Codes             June 2015

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  The 521 Reply Code  . . . . . . . . . . . . . . . . . . . . .   4
   4.  The 556 Reply Code  . . . . . . . . . . . . . . . . . . . . .   4
   5.  Small Details to Avoid Loose Ends . . . . . . . . . . . . . .   5
     5.1.  Specific Changes to RFC 5321  . . . . . . . . . . . . . .   5
     5.2.  The RFC 1846 Experiment . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

Klensin                      Standards Track                    [Page 2]
RFC 7504              SMTP 521 and 556 Reply Codes             June 2015

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 mail 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 [5])
   does not support receipt of mail 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 [4];
   this document updates that specification to standardize the code and
   provide more specific treatment of it.

1.1.  Terminology

   The reader of this document is expected to have reasonable
   familiarity with the SMTP specification in RFC 5321, particularly its