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rfc:bcp:bcp138

Network Working Group T. Hansen Request for Comments: 5248 AT&T Laboratories BCP: 138 J. Klensin Updates: 3463, 4468, 4954 June 2008 Category: Best Current Practice

       A Registry for SMTP Enhanced Mail System Status Codes

Status of This Memo

 This document specifies an Internet Best Current Practices for the
 Internet Community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.

Abstract

 The specification for enhanced mail system status codes, RFC 3463,
 establishes a new code model and lists a collection of status codes.
 While it anticipated that more codes would be added over time, it did
 not provide an explicit mechanism for registering and tracking those
 codes.  This document specifies an IANA registry for mail system
 enhanced status codes, and initializes that registry with the codes
 so far established in published standards-track documents, as well as
 other codes that have become established in the industry.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
 2.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 2
   2.1.  SMTP Enhanced Status Codes Registry . . . . . . . . . . . . 2
   2.2.  Review Process for New Values . . . . . . . . . . . . . . . 4
   2.3.  Registration Updates  . . . . . . . . . . . . . . . . . . . 4
   2.4.  Initial Values  . . . . . . . . . . . . . . . . . . . . . . 5
 3.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
 4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 9
 5.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 9
   5.1.  Normative References  . . . . . . . . . . . . . . . . . . . 9
   5.2.  Informative References  . . . . . . . . . . . . . . . . . . 9

Hansen & Klensin Best Current Practice [Page 1] RFC 5248 SMTP Enhanced Status Code Registry June 2008

1. Introduction

 Enhanced Status Codes for SMTP were first defined in [RFC1893], which
 was subsequently replaced by [RFC3463].  While it anticipated that
 more codes would be added over time (see section 2 of [RFC3463]), it
 did not provide an explicit mechanism for registering and tracking
 those codes.  Since then, various RFCs have been published and
 internet drafts proposed that define additional status codes.
 However, without an IANA registry, conflicts in definitions have
 begun to appear.
 This RFC defines such an IANA registry and was written to help
 prevent further conflicts from appearing in the future.  It
 initializes the registry with the established standards-track
 enhanced status codes from [RFC3463], [RFC3886], [RFC4468], and
 [RFC4954].  In addition, this document adds several codes to the
 registry that were established by various internet drafts and have
 come into common use, despite the expiration of the documents
 themselves.
 As specified in [RFC3463], an enhanced status code consists of a
 three-part code, with each part being numeric and separated by a
 period character.  The three portions are known as the class sub-
 code, the subject sub-code, and the detail sub-code.  In the tables,
 a wildcard for the class sub-code is represented by an X, a wildcard
 for a subject sub-code is represented by an XXX, and a wildcard for a
 detail sub-code is represented by a YYY.  For example, 3.XXX.YYY has
 an unspecified subject sub-code and an unspecified status code, and
 X.5.0 is has an unspecified class sub-code.  (This is a change from
 [RFC3463], which uses XXX for both the subject sub-code and detail
 sub-code wildcards.)

2. IANA Considerations

2.1. SMTP Enhanced Status Codes Registry

 IANA has created the registry "SMTP Enhanced Status Codes".  The SMTP
 Enhanced Status Codes registry will have three tables:
 o  Class Sub-Codes
    Each of the entries in this table represent class sub-codes and
    all have an unspecified subject sub-code and an unspecified detail
    sub-code.
 o  Subject Sub-Codes
    Each of the entries in this table represent subject sub-codes and
    all have an unspecified class sub-code and an unspecified detail
    sub-code.

Hansen & Klensin Best Current Practice [Page 2] RFC 5248 SMTP Enhanced Status Code Registry June 2008

 o  Enumerated Status Codes
    Each of the entries in this table represent the combination of a
    subject sub-code and a detail sub-code.  All entries will have an
    unspecified class sub-code, a specified subject sub-code, and a
    specified detail sub-code.
 Each entry in the tables will include the following.  (The sub-code
 tables will not have the Associated Basic Status Code entries.)
 Code:                         The status code.  For example,
                               3.XXX.YYY is a class sub-code with an
                               unspecified subject sub-code and an
                               unspecified detail sub-code, and X.5.0
                               is an enumerated status code with an
                               unspecified class sub-code.
 Summary: or Sample Text:      For class and subject sub-codes, this
                               is the summary of the use for the sub-
                               code shown in section 2 of [RFC3463].
                               For enumerated status codes, this is an
                               example of a message that might be sent
                               along with the code.
 Associated Basic Status Code: For enumerated status codes, the basic
                               status code(s) of [RFC2821] with which
                               it is usually associated.  This may
                               also have a value such as "Any" or "Not
                               given".  NOTE: This is a non-exclusive
                               list.  In particular, the entries that
                               list some basic status codes for an
                               Enhanced Status Code might allow for
                               other basic status codes, while the
                               entries denoted "Not given" can be
                               filled in by updating the IANA registry
                               through updates to this document or at
                               the direction of the IESG.
 Description:                  A short description of the code.
 Reference:                    A reference to the document in which
                               the code is defined.  This reference
                               should note whether the relevant
                               specification is standards-track, best
                               current practice, or neither, using one
                               of "(Standards track)", "(Best current
                               practice)" or "(Not standards track)".

Hansen & Klensin Best Current Practice [Page 3] RFC 5248 SMTP Enhanced Status Code Registry June 2008

 Submitter:                    The identity of the submitter, usually
                               the document author.
 Change Controller:            The identity of the change controller
                               for the specification.  This will be
                               "IESG" in the case of IETF-produced
                               documents.
 An example of an entry in the enumerated status code table would be:
 Code:               X.0.0
 Sample Text:        Other undefined Status
 Associated basic status code:  Any
 Description:        Other undefined status is the only undefined
                     error code.  It should be used for all errors for
                     which only the class of the error is known.
 Reference:          RFC 3463 (Standards track)
 Submitter:          G. Vaudreuil
 Change controller:  IESG.

2.2. Review Process for New Values

 Entries in this registry are expected to follow the "Specification
 Required" model ([RFC5226]) although, in practice, most entries are
 expected to derive from standards-track documents.  Non-standards-
 track documents that specify codes to be registered should be readily
 available.  The principal purpose of this registry is to avoid
 confusion and conflicts among different definitions or uses for the
 same code.

2.3. Registration Updates

 Standards-track registrations may be updated if the relevant
 standards are updated as a consequence of that action.  Non-
 standards-track entries may be updated by the listed change
 controller.  Only the entry's short description or references may be
 modified in this way, not the code or associated text.  In
 exceptional cases, any aspect of any registered entity may be updated
 at the direction of the IESG (for example, to correct a conflict).

Hansen & Klensin Best Current Practice [Page 4] RFC 5248 SMTP Enhanced Status Code Registry June 2008

2.4. Initial Values

 The initial values for the class and subject sub-code tables are to
 be populated from section 2 of [RFC3463].  Specifically, these are
 the values for 2.XXX.YYY, 4.XXX.YYY, and 5.XXX.YYY for the Class Sub-
 Code table, and the values X.0.YYY, X.1.YYY, X.2.YYY, X.3.YYY,
 X.4.YYY, X.5.YYY, X.6.YYY, and X.7.YYY for the Subject Sub-Code
 table.  The code, sample text, and description for each entry are to
 be taken from [RFC3463].  Each entry is to use [RFC3463] as the
 reference, submitted by G. Vaudreuil, and change controlled by the
 IESG.  There are no associated detail sub-code values for the class
 and subject sub-code tables.
 The initial values for the Enumerated Status Code table is to be
 populated from:
 1.  sections 3.1 through 3.8 of [RFC3463], (X.0.0, X.1.0 through
     X.1.8, X.2.0 through X.2.4, X.3.0 through X.3.5, X.4.0 through
     X.4.7, X.5.0 through X.5.5, X.6.0 through X.6.5, and X.7.0
     through X.7.7),
 2.  section 3.3.4 of [RFC3886] (X.1.9),
 3.  X.6.6 found in section 5 of [RFC4468], (but not X.7.8 found in
     the same section),
 4.  and X.5.6, X.7.8, X.7.9, X.7.11, and X.7.12, found in section 6
     of [RFC4954] (using the text from X.5.6, 5.7.8, 5.7.9, 5.7.11,
     and 4.7.12).
 Each entry is to be designated as defined in the corresponding RFC,
 submitted by the corresponding RFC author, and change controlled by
 the IESG.  Each of the above RFCs is a standards-track document.
 The initial values for the Associated Basic Status Code for each of
 the above initial enhanced status codes is given in the following
 table.
 As noted above, this table is incomplete.  In particular, the entries
 that have some basic status codes might allow for other detail sub-
 status codes, while the entries denoted "Not given" can be filled in
 by updating the IANA registry through updates to this document or at
 the direction of the IESG.

Hansen & Klensin Best Current Practice [Page 5] RFC 5248 SMTP Enhanced Status Code Registry June 2008

 +--------+---------------+--------+----------+--------+-------------+
 | Enh.   | Assoc.  Basic | Enh.   | Assoc.   | Enh.   | Assoc.      |
 | Status | Status Code   | Status | Basic    | Status | Basic       |
 | Code   |               | Code   | Status   | Code   | Status Code |
 |        |               |        | Code     |        |             |
 +--------+---------------+--------+----------+--------+-------------+
 | X.0.0  | Any           | X.1.0  | Not      | X.1.1  | 451, 550    |
 |        |               |        | given    |        |             |
 | X.1.2  | Not given     | X.1.3  | 501      | X.1.4  | Not given   |
 | X.1.5  | 250           | X.1.6  | Not      | X.1.7  | Not given   |
 |        |               |        | given    |        |             |
 | X.1.8  | 451, 501      | X.1.9  | Not      | X.2.0  | Not given   |
 |        |               |        | given    |        |             |
 | X.2.1  | Not given     | X.2.2  | 552      | X.2.3  | 552         |
 | X.2.4  | 450, 452      | X.3.0  | 221,     | X.3.1  | 452         |
 |        |               |        | 250,     |        |             |
 |        |               |        | 421,     |        |             |
 |        |               |        | 451,     |        |             |
 |        |               |        | 550, 554 |        |             |
 | X.3.2  | 453           | X.3.3  | Not      | X.3.4  | 552, 554    |
 |        |               |        | given    |        |             |
 | X.3.5  | Not given     | X.4.0  | Not      | X.4.1  | 451         |
 |        |               |        | given    |        |             |
 | X.4.2  | 421           | X.4.3  | 451, 550 | X.4.4  | Not given   |
 | X.4.5  | 451           | X.4.6  | Not      | X.4.7  | Not given   |
 |        |               |        | given    |        |             |
 | X.5.0  | 220, 250,     | X.5.1  | 430,     | X.5.2  | 500, 501,   |
 |        | 251, 252,     |        | 500,     |        | 502, 550,   |
 |        | 253, 451,     |        | 501,     |        | 555         |
 |        | 452, 454,     |        | 503,     |        |             |
 |        | 458, 459,     |        | 530,     |        |             |
 |        | 501, 502,     |        | 550,     |        |             |
 |        | 503, 554      |        | 554, 555 |        |             |
 | X.5.3  | 451           | X.5.4  | 451,     | X.5.5  | Not given   |
 |        |               |        | 501,     |        |             |
 |        |               |        | 502,     |        |             |
 |        |               |        | 503,     |        |             |
 |        |               |        | 504,     |        |             |
 |        |               |        | 550, 555 |        |             |
 | X.5.6  | 500           | X.6.0  | Not      | X.6.1  | Not given   |
 |        |               |        | given    |        |             |
 | X.6.2  | Not given     | X.6.3  | 554      | X.6.4  | 250         |
 | X.6.5  | Not given     | X.6.6  | 554      | X.7.0  | 220, 235,   |
 |        |               |        |          |        | 450, 454,   |
 |        |               |        |          |        | 500, 501,   |
 |        |               |        |          |        | 503, 504,   |
 |        |               |        |          |        | 530, 535,   |
 |        |               |        |          |        | 550         |

Hansen & Klensin Best Current Practice [Page 6] RFC 5248 SMTP Enhanced Status Code Registry June 2008

 | X.7.1  | 451, 454,     | X.7.2  | 550      | X.7.3  | Not given   |
 |        | 502, 503,     |        |          |        |             |
 |        | 533, 550, 551 |        |          |        |             |
 | X.7.4  | 504           | X.7.5  | Not      | X.7.6  | Not given   |
 |        |               |        | given    |        |             |
 | X.7.7  | Not given     | X.7.8  | 535, 554 | X.7.9  | 534         |
 | X.7.10 | 523           | X.7.11 | 524, 538 | X.7.12 | 422, 432    |
 | X.7.13 | 525           | X.7.14 | 535, 554 |        |             |
 +--------+---------------+--------+----------+--------+-------------+
                                Table 1
 The following additional definitions have been registered in the
 enumerated status code table.  These entries have been used in the
 industry without any published specification.
 Code:               X.7.10
 Sample Text:        Encryption Needed
 Associated basic status code:  523
 Description:        This indicates that an external strong privacy
                     layer is needed in order to use the requested
                     authentication mechanism.  This is primarily
                     intended for use with clear text authentication
                     mechanisms.  A client that receives this may
                     activate a security layer such as TLS prior to
                     authenticating, or attempt to use a stronger
                     mechanism.
 Reference:          RFC 5248 (Best current practice)
 Submitter:          T. Hansen, J. Klensin
 Change controller:  IESG

Hansen & Klensin Best Current Practice [Page 7] RFC 5248 SMTP Enhanced Status Code Registry June 2008

 Code:               X.7.13
 Sample Text:        User Account Disabled
 Associated basic status code:  525
 Description:        Sometimes a system administrator will have to
                     disable a user's account (e.g., due to lack of
                     payment, abuse, evidence of a break-in attempt,
                     etc.).  This error code occurs after a successful
                     authentication to a disabled account.  This
                     informs the client that the failure is permanent
                     until the user contacts their system
                     administrator to get the account re-enabled.  It
                     differs from a generic authentication failure
                     where the client's best option is to present the
                     passphrase entry dialog in case the user simply
                     mistyped their passphrase.
 Reference:          RFC 5248 (Best current practice)
 Submitter:          T. Hansen, J. Klensin
 Change controller:  IESG
 Code:               X.7.14
 Sample Text:        Trust relationship required
 Associated basic status code:  535, 554
 Description:        The submission server requires a configured trust
                     relationship with a third-party server in order
                     to access the message content.  This value
                     replaces the prior use of X.7.8 for this error
                     condition, thereby updating [RFC4468].
 Reference:          RFC 5248 (Best current practice)
 Submitter:          T. Hansen, J. Klensin
 Change controller:  IESG

3. Security Considerations

 As stated in [RFC1893], use of enhanced status codes may disclose
 additional information about how an internal mail system is
 implemented beyond that available through the SMTP status codes.
 Many proposed additions to the response code list are security
 related.  Having these registered in one place to prevent collisions
 will improve their value.  Security error responses can leak
 information to active attackers (e.g., the distinction between "user
 not found" and "bad password" during authentication).  Documents
 defining security error codes should make it clear when this is the
 case so SMTP server software subject to such threats can provide
 appropriate controls to restrict exposure.

Hansen & Klensin Best Current Practice [Page 8] RFC 5248 SMTP Enhanced Status Code Registry June 2008

4. Acknowledgements

 While the need for this registry should have become clear shortly
 after [RFC3463] was approved, the growth of the code table through
 additional documents and work done as part of email
 internationalization and [RFC2821] updating efforts made the
 requirement much more clear.  The comments of the participants in
 those efforts are gratefully acknowledged, particularly the members
 of the ietf-smtp@imc.org mailing list.  Chris Newman and Randy
 Gellens provided useful comments and some text for early versions of
 the document.

5. References

5.1. Normative References

 [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
            April 2001.
 [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes",
            RFC 3463, January 2003.
 [RFC3886]  Allman, E., "An Extensible Message Format for Message
            Tracking Responses", RFC 3886, September 2004.
 [RFC4468]  Newman, C., "Message Submission BURL Extension", RFC 4468,
            May 2006.
 [RFC4954]  Siemborski, R. and A. Melnikov, "SMTP Service Extension
            for Authentication", RFC 4954, July 2007.

5.2. Informative References

 [RFC1893]  Vaudreuil, G., "Enhanced Mail System Status Codes",
            RFC 1893, January 1996.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.

Hansen & Klensin Best Current Practice [Page 9] RFC 5248 SMTP Enhanced Status Code Registry June 2008

Authors' Addresses

 Tony Hansen
 AT&T Laboratories
 200 Laurel Ave.
 Middletown, NJ  07748
 USA
 EMail: tony+mailesc@maillennium.att.com
 John C Klensin
 1770 Massachusetts Ave, Ste 322
 Cambridge, MA  02140
 USA
 Phone: +1 617 245 1457
 EMail: john+ietf@jck.com

Hansen & Klensin Best Current Practice [Page 10] RFC 5248 SMTP Enhanced Status Code Registry June 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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Hansen & Klensin Best Current Practice [Page 11]

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