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IETF RFC 5248
A Registry for SMTP Enhanced Mail System Status Codes
Last modified on Saturday, June 28th, 2008
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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 [RFC 1893], which
was subsequently replaced by [RFC 3463]. While it anticipated that
more codes would be added over time (see section 2 of [RFC 3463]), 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 [RFC 3463], [RFC 3886], [RFC 4468], and
[RFC 4954]. 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 [RFC 3463], 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
[RFC 3463], 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 [RFC 3463].
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 [RFC 2821] 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 ([RFC 5226]) 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 [RFC 3463]. 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 [RFC 3463]. Each entry is to use [RFC 3463] 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 [RFC 3463], (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 [RFC 3886] (X.1.9),
3. X.6.6 found in section 5 of [RFC 4468], (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 [RFC 4954] (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 [RFC 4468].
Reference: RFC 5248 (Best current practice)
Submitter: T. Hansen, J. Klensin
Change controller: IESG
3. Security Considerations
As stated in [RFC 1893], 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 [RFC 3463] was approved, the growth of the code table through
additional documents and work done as part of email
internationalization and [RFC 2821] 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
[RFC 2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[RFC 3463] Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC 3463, January 2003.
[RFC 3886] Allman, E., "An Extensible Message Format for Message
Tracking Responses", RFC 3886, September 2004.
[RFC 4468] Newman, C., "Message Submission BURL Extension", RFC 4468,
May 2006.
[RFC 4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.
5.2. Informative References
[RFC 1893] Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC 1893, January 1996.
[RFC 5226] 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 © 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.
This document and the information contained herein are provided on an
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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
A Registry for SMTP Enhanced Mail System Status Codes
RFC TOTAL SIZE: 23845 bytes
PUBLICATION DATE: Saturday, June 28th, 2008
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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