Email Authentication Status Codes
RFC 7372

Internet Engineering Task Force (IETF)                      M. Kucherawy
Request for Comments: 7372                                September 2014
Updates: 7208
Category: Standards Track
ISSN: 2070-1721

                   Email Authentication Status Codes

Abstract

   This document registers code points to allow status codes to be
   returned to an email client to indicate that a message is being
   rejected or deferred specifically because of email authentication
   failures.

   This document updates RFC 7208, since some of the code points
   registered replace the ones recommended for use in that document.

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/rfc7372.

Copyright Notice

   Copyright (c) 2014 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Key Words . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  New Enhanced Status Codes . . . . . . . . . . . . . . . . . .   3
     3.1.  DKIM Failure Codes  . . . . . . . . . . . . . . . . . . .   3
     3.2.  SPF Failure Codes . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Reverse DNS Failure Code  . . . . . . . . . . . . . . . .   5
     3.4.  Multiple Authentication Failures Code . . . . . . . . . .   5
   4.  General Considerations  . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   [RFC3463] introduced Enhanced Mail System Status Codes, and [RFC5248]
   created an IANA registry for these.

   [RFC6376] and [RFC7208] introduced, respectively, DomainKeys
   Identified Mail (DKIM) and Sender Policy Framework (SPF), two
   protocols for conducting message authentication.  Another common
   email acceptance test is the reverse Domain Name System (DNS) check
   on an email client's IP address, as described in Section 3 of
   [RFC7001].

   The current set of enhanced status codes does not include any code
   for indicating that a message is being rejected or deferred due to
   local policy reasons related to any of these mechanisms.  This is
   potentially useful information to agents that need more than
   rudimentary handling information about the reason a message was
   rejected on receipt.  This document introduces enhanced status codes
   for reporting those cases to clients.

   Section 3.2 updates [RFC7208], as new enhanced status codes relevant
   to that specification are being registered and recommended for use.

2.  Key Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

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3.  New Enhanced Status Codes

   The new enhanced status codes are defined in the following
   subsections.

3.1.  DKIM Failure Codes

   In the code point definitions below, the following definitions are
   used:

   passing:  A signature is "passing" if the basic DKIM verification
      algorithm, as defined in [RFC6376], succeeds.

   acceptable:  A signature is "acceptable" if it satisfies all locally
      defined requirements (if any) in addition to passing the basic
      DKIM verification algorithm (e.g., certain header fields are
      included in the signed content, no partial signatures, etc.).

      Code:               X.7.20
      Sample Text:        No passing DKIM signature found
      Associated basic status code:  550
      Description:        This status code is returned when a message
                          did not contain any passing DKIM
                          signatures.  (This violates the
                          advice of Section 6.1 of RFC 6376.)
      Reference:          [RFC7372]; [RFC6376]
      Submitter:          M. Kucherawy
      Change controller:  IESG

      Code:               X.7.21
      Sample Text:        No acceptable DKIM signature found
      Associated basic status code:  550
      Description:        This status code is returned when a message
                          contains one or more passing DKIM signatures,
                          but none are acceptable.  (This violates the
                          advice of Section 6.1 of RFC 6376.)
      Reference:          [RFC7372]; [RFC6376]
      Submitter:          M. Kucherawy
      Change controller:  IESG

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      Code:               X.7.22
      Sample Text:        No valid author-matched DKIM signature found
      Associated basic status code:  550
      Description:        This status code is returned when a message
                          contains one or more passing DKIM
                          signatures, but none are acceptable because
                          none have an identifier(s)
                          that matches the author address(es) found in
                          the From header field.  This is a special
                          case of X.7.21. (This violates the advice
                          of Section 6.1 of RFC 6376.)
      Reference:          [RFC7372]; [RFC6376]
      Submitter:          M. Kucherawy
      Change controller:  IESG

3.2.  SPF Failure Codes

      Code:               X.7.23
      Sample Text:        SPF validation failed
      Associated basic status code:  550
      Description:        This status code is returned when a message
                          completed an SPF check that produced a
                          "fail" result, contrary to local policy
                          requirements.  Used in place of 5.7.1, as
                          described in Section 8.4 of RFC 7208.
      Reference:          [RFC7372]; [RFC7208]
      Submitter:          M. Kucherawy
      Change controller:  IESG

      Code:               X.7.24
      Sample Text:        SPF validation error
      Associated basic status code:  451/550
      Description:        This status code is returned when evaluation
                          of SPF relative to an arriving message
                          resulted in an error.  Used in place of
                          4.4.3 or 5.5.2, as described in Sections
                          8.6 and 8.7 of RFC 7208.
      Reference:          [RFC7372]; [RFC7208]
      Submitter:          M. Kucherawy
      Change controller:  IESG

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3.3.  Reverse DNS Failure Code

      Code:               X.7.25
      Sample Text:        Reverse DNS validation failed
      Associated basic status code:  550
      Description:        This status code is returned when an SMTP
                          client's IP address failed a reverse DNS
                          validation check, contrary to local policy
                          requirements.
      Reference:          [RFC7372]; Section 3 of [RFC7001]
      Submitter:          M. Kucherawy
      Change controller:  IESG

3.4.  Multiple Authentication Failures Code

      Code:               X.7.26
      Sample Text:        Multiple authentication checks failed
      Associated basic status code:  550
      Description:        This status code is returned when a message
                          failed more than one message authentication
                          check, contrary to local policy requirements.
                          The particular mechanisms that failed are not
                          specified.
      Reference:          [RFC7372]
      Submitter:          M. Kucherawy
      Change controller:  IESG

4.  General Considerations

   By the nature of the Simple Mail Transfer Protocol (SMTP), only one
   enhanced status code can be returned for a given exchange between
   client and server.  However, an operator might decide to defer or
   reject a message for a plurality of reasons.  Clients receiving these
   codes need to consider that the failure reflected by one of these
   status codes might not reflect the only reason, or the most important
   reason, for non-acceptance of the message or command.

   It is important to note that Section 6.1 of [RFC6376] discourages
   special treatment of messages bearing no valid DKIM signature.  There
   are some operators that disregard this advice, a few of which go so
   far as to require a valid Author Domain Signature (that is, one
   matching the domain(s) in the From header field) in order to accept
   the message.  Moreover, some nascent technologies built atop SPF and
   DKIM depend on such authentications.  This work does not endorse
   configurations that violate DKIM's recommendations but rather
   acknowledges that they do exist and merely seeks to provide for
   improved interoperability with such operators.

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   A specific use case for these codes is mailing list software, which
   processes rejections in order to remove from the subscriber set those
   addresses that are no longer valid.  There is a need in that case to
   distinguish authentication failures from indications that the
   recipient address is no longer valid.

   If a receiving server performs multiple authentication checks and
   more than one of them fails, thus warranting rejection of the
   message, the SMTP server SHOULD use the code that indicates multiple
   methods failed rather than only reporting the first one that failed.
   It may be the case that one method is always expected to fail; thus,
   returning that method's specific code is not information useful to
   the sending agent.

   The reverse IP DNS check is defined in Section 3 of [RFC7001].

   Any message authentication or policy enforcement technologies
   developed in the future should also include registration of their own
   enhanced status codes so that this kind of specific reporting is
   available to operators that wish to use them.

5.  Security Considerations

   Use of these codes reveals local policy with respect to email
   authentication, which can be useful information to actors attempting
   to deliver undesired mail.  It should be noted that there is no
   specific obligation to use these codes; if an operator wishes not to
   reveal this aspect of local policy, it can continue using a generic
   result code such as 5.7.7, 5.7.1, or even 5.7.0.

6.  IANA Considerations

   Registration of new enhanced status codes, for addition to the
   Enumerated Status Codes sub-registry of the SMTP Enhanced Status
   Codes Registry, can be found in Section 3.

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7.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
              3463, January 2003.

   [RFC5248]  Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
              Mail System Status Codes", BCP 138, RFC 5248, June 2008.

   [RFC6376]  Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
              Identified Mail (DKIM) Signatures", STD 76, RFC 6376,
              September 2011.

   [RFC7001]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 7001, September 2013.

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              April 2014.

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Appendix A.  Acknowledgments

   Claudio Allocchio, Dave Crocker, Ned Freed, Arnt Gulbrandsen, Scott
   Kitterman, Barry Leiba, Alexey Melnikov, S. Moonesamy, Hector Santos,
   and Stephen Turnbull contributed to this work.

Author's Address

   Murray S. Kucherawy
   270 Upland Drive
   San Francisco, CA  94127
   USA

   EMail: superuser@gmail.com

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