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

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:

• 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.

• 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.

• 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)".
   
   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).

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.

   +--------+---------------+--------+----------+--------+-------------+
   | 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         |
   
   | 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
   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.

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 [email protected] 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.

Authors' Addresses

   Tony Hansen
   AT&T Laboratories
   200 Laurel Ave.
   Middletown, NJ  07748
   USA
   
   EMail: [email protected]
   
   John C Klensin
   1770 Massachusetts Ave, Ste 322
   Cambridge, MA  02140
   USA
   
   Phone: +1 617 245 1457
   EMail: [email protected]

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