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IETF RFC 3463
Enhanced Mail System Status Codes
Last modified on Friday, January 17th, 2003
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Network Working Group G. Vaudreuil
Request for Comments: 3463 Lucent Technologies
Obsoletes: 1893 January 2003
Category: Standards Track
Enhanced Mail System Status Codes
 Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright © The Internet Society (2003). All Rights Reserved.
Abstract
This document defines a set of extended status codes for use within
the mail system for delivery status reports, tracking, and improved
diagnostics. In combination with other information provided in the
Delivery Status Notification (DSN) delivery report, these codes
facilitate media and language independent rendering of message
delivery status.
Table of Contents
1. Overview ......................................................2
2. Status Code Structure .........................................3
3. Enumerated Status Codes .......................................5
3.1 Other or Undefined Status ...................................6
3.2 Address Status ..............................................6
3.3 Mailbox Status ..............................................7
3.4 Mail system status ..........................................8
3.5 Network and Routing Status ..................................9
3.6 Mail Delivery Protocol Status ..............................10
3.7 Message Content or Message Media Status ....................11
3.8 Security or Policy Status ..................................12
4. References ...................................................13
5. Security Considerations ......................................13
Appendix A - Collected Status Codes ..........................14
Appendix B - Changes from RFC 1893 ............................15
Author's Address .............................................15
Full Copyright Statement .....................................16
Vaudreuil Standards Track PAGE 1 
RFC 3463 Enhanced Mail System Status Codes January 2003
1. Overview
There is a need for a standard mechanism for the reporting of mail
system errors richer than the limited set offered by SMTP and the
system specific text descriptions sent in mail messages. There is a
pressing need for a rich machine-readable, human language independent
status code for use in delivery status notifications [DSN]. This
document proposes a new set of status codes for this purpose.
SMTP [SMTP] error codes have historically been used for reporting
mail system errors. Because of limitations in the SMTP code design,
these are not suitable for use in delivery status notifications.
SMTP provides about 12 useful codes for delivery reports. The
majority of the codes are protocol specific response codes such as
the 354 response to the SMTP data command. Each of the 12 useful
codes are overloaded to indicate several error conditions. SMTP
suffers some scars from history, most notably the unfortunate damage
to the reply code extension mechanism by uncontrolled use. This
proposal facilitates future extensibility by requiring the client to
interpret unknown error codes according to the theory of codes while
requiring servers to register new response codes.
The SMTP theory of reply codes are partitioned in the number space in
such a manner that the remaining available codes will not provide the
space needed. The most critical example is the existence of only 5
remaining codes for mail system errors. The mail system
classification includes both host and mailbox error conditions. The
remaining third digit space would be completely consumed as needed to
indicate MIME and media conversion errors and security system errors.
A revision to the SMTP theory of reply codes to better distribute the
error conditions in the number space will necessarily be incompatible
with SMTP. Further, consumption of the remaining reply-code number
space for delivery notification reporting will reduce the available
codes for new ESMTP extensions.
The following status code set is based on the SMTP theory of reply
codes. It adopts the success, permanent error, and transient error
semantics of the first value, with a further description and
classification in the second. This proposal re-distributes the
classifications to better distribute the error conditions, such as
separating mailbox from host errors.
Document Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14 [RFC 2119].
Vaudreuil Standards Track PAGE 2
RFC 3463 Enhanced Mail System Status Codes January 2003
2. Status Code Structure
This document defines a new set of status codes to report mail system
conditions. These status codes are used for media and language
independent status reporting. They are not intended for system
specific diagnostics.
The syntax of the new status codes is defined as:
status-code = class "." subject "." detail
class = "2"/"4"/"5"
subject = 1*3digit
detail = 1*3digit
White-space characters and comments are NOT allowed within a status-
code. Each numeric sub-code within the status-code MUST be expressed
without leading zero digits.
Status codes consist of three numerical fields separated by ".". The
first sub-code indicates whether the delivery attempt was successful.
The second sub-code indicates the probable source of any delivery
anomalies, and the third sub-code indicates a precise error
condition.
Example: 2.1.23
The code space defined is intended to be extensible only by standards
track documents. Mail system specific status codes should be mapped
as close as possible to the standard status codes. Servers should
send only defined, registered status codes. System specific errors
and diagnostics should be carried by means other than status codes.
New subject and detail codes will be added over time. Because the
number space is large, it is not intended that published status codes
will ever be redefined or eliminated. Clients should preserve the
extensibility of the code space by reporting the general error
described in the subject sub-code when the specific detail is
unrecognized.
Vaudreuil Standards Track PAGE 3
RFC 3463 Enhanced Mail System Status Codes January 2003
The class sub-code provides a broad classification of the status.
The enumerated values for each class are defined as:
2.XXX.XXX Success
Success specifies that the DSN is reporting a positive delivery
action. Detail sub-codes may provide notification of
transformations required for delivery.
4.XXX.XXX Persistent Transient Failure
A persistent transient failure is one in which the message as
sent is valid, but persistence of some temporary condition has
caused abandonment or delay of attempts to send the message.
If this code accompanies a delivery failure report, sending in
the future may be successful.
5.XXX.XXX Permanent Failure
A permanent failure is one which is not likely to be resolved
by resending the message in the current form. Some change to
the message or the destination must be made for successful
delivery.
A client must recognize and report class sub-code even where
subsequent subject sub-codes are unrecognized.
The subject sub-code classifies the status. This value applies to
each of the three classifications. The subject sub-code, if
recognized, must be reported even if the additional detail provided
by the detail sub-code is not recognized. The enumerated values for
the subject sub-code are:
X.0.XXX Other or Undefined Status
There is no additional subject information available.
X.1.XXX Addressing Status
The address status reports on the originator or destination
address. It may include address syntax or validity. These
errors can generally be corrected by the sender and retried.
X.2.XXX Mailbox Status
Mailbox status indicates that something having to do with the
mailbox has caused this DSN. Mailbox issues are assumed to be
under the general control of the recipient.
Vaudreuil Standards Track PAGE 4
RFC 3463 Enhanced Mail System Status Codes January 2003
X.3.XXX Mail System Status
Mail system status indicates that something having to do with
the destination system has caused this DSN. System issues are
assumed to be under the general control of the destination
system administrator.
X.4.XXX Network and Routing Status
The networking or routing codes report status about the
delivery system itself. These system components include any
necessary infrastructure such as directory and routing
services. Network issues are assumed to be under the control
of the destination or intermediate system administrator.
X.5.XXX Mail Delivery Protocol Status
The mail delivery protocol status codes report failures
involving the message delivery protocol. These failures
include the full range of problems resulting from
implementation errors or an unreliable connection.
X.6.XXX Message Content or Media Status
The message content or media status codes report failures
involving the content of the message. These codes report
failures due to translation, transcoding, or otherwise
unsupported message media. Message content or media issues are
under the control of both the sender and the receiver, both of
which must support a common set of supported content-types.
X.7.XXX Security or Policy Status
The security or policy status codes report failures involving
policies such as per-recipient or per-host filtering and
cryptographic operations. Security and policy status issues
are assumed to be under the control of either or both the
sender and recipient. Both the sender and recipient must
permit the exchange of messages and arrange the exchange of
necessary keys and certificates for cryptographic operations.
3. Enumerated Status Codes
The following section defines and describes the detail sub-code. The
detail value provides more information about the status and is
defined relative to the subject of the status.
Vaudreuil Standards Track PAGE 5
RFC 3463 Enhanced Mail System Status Codes January 2003
3.1 Other or Undefined Status
X.0.0 Other undefined Status
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.
3.2 Address Status
X.1.0 Other address status
Something about the address specified in the message caused
this DSN.
X.1.1 Bad destination mailbox address
The mailbox specified in the address does not exist. For
Internet mail names, this means the address portion to the left
of the "@" sign is invalid. This code is only useful for
permanent failures.
X.1.2 Bad destination system address
The destination system specified in the address does not exist
or is incapable of accepting mail. For Internet mail names,
this means the address portion to the right of the "@" is
invalid for mail. This code is only useful for permanent
failures.
X.1.3 Bad destination mailbox address syntax
The destination address was syntactically invalid. This can
apply to any field in the address. This code is only useful
for permanent failures.
X.1.4 Destination mailbox address ambiguous
The mailbox address as specified matches one or more recipients
on the destination system. This may result if a heuristic
address mapping algorithm is used to map the specified address
to a local mailbox name.
X.1.5 Destination address valid
This mailbox address as specified was valid. This status code
should be used for positive delivery reports.
Vaudreuil Standards Track PAGE 6
RFC 3463 Enhanced Mail System Status Codes January 2003
X.1.6 Destination mailbox has moved, No forwarding address
The mailbox address provided was at one time valid, but mail is
no longer being accepted for that address. This code is only
useful for permanent failures.
X.1.7 Bad sender's mailbox address syntax
The sender's address was syntactically invalid. This can apply
to any field in the address.
X.1.8 Bad sender's system address
The sender's system specified in the address does not exist or
is incapable of accepting return mail. For domain names, this
means the address portion to the right of the "@" is invalid
for mail.
3.3 Mailbox Status
X.2.0 Other or undefined mailbox status
The mailbox exists, but something about the destination mailbox
has caused the sending of this DSN.
X.2.1 Mailbox disabled, not accepting messages
The mailbox exists, but is not accepting messages. This may be
a permanent error if the mailbox will never be re-enabled or a
transient error if the mailbox is only temporarily disabled.
X.2.2 Mailbox full
The mailbox is full because the user has exceeded a per-mailbox
administrative quota or physical capacity. The general
semantics implies that the recipient can delete messages to
make more space available. This code should be used as a
persistent transient failure.
X.2.3 Message length exceeds administrative limit
A per-mailbox administrative message length limit has been
exceeded. This status code should be used when the per-mailbox
message length limit is less than the general system limit.
This code should be used as a permanent failure.
Vaudreuil Standards Track PAGE 7
RFC 3463 Enhanced Mail System Status Codes January 2003
X.2.4 Mailing list expansion problem
The mailbox is a mailing list address and the mailing list was
unable to be expanded. This code may represent a permanent
failure or a persistent transient failure.
3.4 Mail system status
X.3.0 Other or undefined mail system status
The destination system exists and normally accepts mail, but
something about the system has caused the generation of this
DSN.
X.3.1 Mail system full
Mail system storage has been exceeded. The general semantics
imply that the individual recipient may not be able to delete
material to make room for additional messages. This is useful
only as a persistent transient error.
X.3.2 System not accepting network messages
The host on which the mailbox is resident is not accepting
messages. Examples of such conditions include an immanent
shutdown, excessive load, or system maintenance. This is
useful for both permanent and persistent transient errors.
X.3.3 System not capable of selected features
Selected features specified for the message are not supported
by the destination system. This can occur in gateways when
features from one domain cannot be mapped onto the supported
feature in another.
X.3.4 Message too big for system
The message is larger than per-message size limit. This limit
may either be for physical or administrative reasons. This is
useful only as a permanent error.
X.3.5 System incorrectly configured
The system is not configured in a manner that will permit it to
accept this message.
Vaudreuil Standards Track PAGE 8
RFC 3463 Enhanced Mail System Status Codes January 2003
3.5 Network and Routing Status
X.4.0 Other or undefined network or routing status
Something went wrong with the networking, but it is not clear
what the problem is, or the problem cannot be well expressed
with any of the other provided detail codes.
X.4.1 No answer from host
The outbound connection attempt was not answered, because
either the remote system was busy, or was unable to take a
call. This is useful only as a persistent transient error.
X.4.2 Bad connection
The outbound connection was established, but was unable to
complete the message transaction, either because of time-out,
or inadequate connection quality. This is useful only as a
persistent transient error.
X.4.3 Directory server failure
The network system was unable to forward the message, because a
directory server was unavailable. This is useful only as a
persistent transient error.
The inability to connect to an Internet DNS server is one
example of the directory server failure error.
X.4.4 Unable to route
The mail system was unable to determine the next hop for the
message because the necessary routing information was
unavailable from the directory server. This is useful for both
permanent and persistent transient errors.
A DNS lookup returning only an SOA (Start of Administration)
record for a domain name is one example of the unable to route
error.
X.4.5 Mail system congestion
The mail system was unable to deliver the message because the
mail system was congested. This is useful only as a persistent
transient error.
Vaudreuil Standards Track PAGE 9
RFC 3463 Enhanced Mail System Status Codes January 2003
X.4.6 Routing loop detected
A routing loop caused the message to be forwarded too many
times, either because of incorrect routing tables or a user-
forwarding loop. This is useful only as a persistent transient
error.
X.4.7 Delivery time expired
The message was considered too old by the rejecting system,
either because it remained on that host too long or because the
time-to-live value specified by the sender of the message was
exceeded. If possible, the code for the actual problem found
when delivery was attempted should be returned rather than this
code.
3.6 Mail Delivery Protocol Status
X.5.0 Other or undefined protocol status
Something was wrong with the protocol necessary to deliver the
message to the next hop and the problem cannot be well
expressed with any of the other provided detail codes.
X.5.1 Invalid command
A mail transaction protocol command was issued which was either
out of sequence or unsupported. This is useful only as a
permanent error.
X.5.2 Syntax error
A mail transaction protocol command was issued which could not
be interpreted, either because the syntax was wrong or the
command is unrecognized. This is useful only as a permanent
error.
X.5.3 Too many recipients
More recipients were specified for the message than could have
been delivered by the protocol. This error should normally
result in the segmentation of the message into two, the
remainder of the recipients to be delivered on a subsequent
delivery attempt. It is included in this list in the event
that such segmentation is not possible.
Vaudreuil Standards Track PAGE 10
RFC 3463 Enhanced Mail System Status Codes January 2003
X.5.4 Invalid command arguments
A valid mail transaction protocol command was issued with
invalid arguments, either because the arguments were out of
range or represented unrecognized features. This is useful
only as a permanent error.
X.5.5 Wrong protocol version
A protocol version mis-match existed which could not be
automatically resolved by the communicating parties.
3.7 Message Content or Message Media Status
X.6.0 Other or undefined media error
Something about the content of a message caused it to be
considered undeliverable and the problem cannot be well
expressed with any of the other provided detail codes.
X.6.1 Media not supported
The media of the message is not supported by either the
delivery protocol or the next system in the forwarding path.
This is useful only as a permanent error.
X.6.2 Conversion required and prohibited
The content of the message must be converted before it can be
delivered and such conversion is not permitted. Such
prohibitions may be the expression of the sender in the message
itself or the policy of the sending host.
X.6.3 Conversion required but not supported
The message content must be converted in order to be forwarded
but such conversion is not possible or is not practical by a
host in the forwarding path. This condition may result when an
ESMTP gateway supports 8bit transport but is not able to
downgrade the message to 7 bit as required for the next hop.
X.6.4 Conversion with loss performed
This is a warning sent to the sender when message delivery was
successfully but when the delivery required a conversion in
which some data was lost. This may also be a permanent error
if the sender has indicated that conversion with loss is
prohibited for the message.
Vaudreuil Standards Track PAGE 11
RFC 3463 Enhanced Mail System Status Codes January 2003
X.6.5 Conversion Failed
A conversion was required but was unsuccessful. This may be
useful as a permanent or persistent temporary notification.
3.8 Security or Policy Status
X.7.0 Other or undefined security status
Something related to security caused the message to be
returned, and the problem cannot be well expressed with any of
the other provided detail codes. This status code may also be
used when the condition cannot be further described because of
security policies in force.
X.7.1 Delivery not authorized, message refused
The sender is not authorized to send to the destination. This
can be the result of per-host or per-recipient filtering. This
memo does not discuss the merits of any such filtering, but
provides a mechanism to report such. This is useful only as a
permanent error.
X.7.2 Mailing list expansion prohibited
The sender is not authorized to send a message to the intended
mailing list. This is useful only as a permanent error.
X.7.3 Security conversion required but not possible
A conversion from one secure messaging protocol to another was
required for delivery and such conversion was not possible.
This is useful only as a permanent error.
X.7.4 Security features not supported
A message contained security features such as secure
authentication that could not be supported on the delivery
protocol. This is useful only as a permanent error.
X.7.5 Cryptographic failure
A transport system otherwise authorized to validate or decrypt
a message in transport was unable to do so because necessary
information such as key was not available or such information
was invalid.
Vaudreuil Standards Track PAGE 12
RFC 3463 Enhanced Mail System Status Codes January 2003
X.7.6 Cryptographic algorithm not supported
A transport system otherwise authorized to validate or decrypt
a message was unable to do so because the necessary algorithm
was not supported.
X.7.7 Message integrity failure
A transport system otherwise authorized to validate a message
was unable to do so because the message was corrupted or
altered. This may be useful as a permanent, transient
persistent, or successful delivery code.
4. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SMTP] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
821, August 1982.
[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464, January 2003.
5. Security Considerations
This document describes a status code system with increased
precision. Use of these status codes may disclose additional
information about how an internal mail system is implemented beyond
that currently available.
Vaudreuil Standards Track PAGE 13
RFC 3463 Enhanced Mail System Status Codes January 2003
Appendix A - Collected Status Codes
X.1.0 Other address status
X.1.1 Bad destination mailbox address
X.1.2 Bad destination system address
X.1.3 Bad destination mailbox address syntax
X.1.4 Destination mailbox address ambiguous
X.1.5 Destination mailbox address valid
X.1.6 Mailbox has moved
X.1.7 Bad sender's mailbox address syntax
X.1.8 Bad sender's system address
X.2.0 Other or undefined mailbox status
X.2.1 Mailbox disabled, not accepting messages
X.2.2 Mailbox full
X.2.3 Message length exceeds administrative limit.
X.2.4 Mailing list expansion problem
X.3.0 Other or undefined mail system status
X.3.1 Mail system full
X.3.2 System not accepting network messages
X.3.3 System not capable of selected features
X.3.4 Message too big for system
X.4.0 Other or undefined network or routing status
X.4.1 No answer from host
X.4.2 Bad connection
X.4.3 Routing server failure
X.4.4 Unable to route
X.4.5 Network congestion
X.4.6 Routing loop detected
X.4.7 Delivery time expired
X.5.0 Other or undefined protocol status
X.5.1 Invalid command
X.5.2 Syntax error
X.5.3 Too many recipients
X.5.4 Invalid command arguments
X.5.5 Wrong protocol version
X.6.0 Other or undefined media error
X.6.1 Media not supported
X.6.2 Conversion required and prohibited
X.6.3 Conversion required but not supported
X.6.4 Conversion with loss performed
X.6.5 Conversion failed
Vaudreuil Standards Track PAGE 14
RFC 3463 Enhanced Mail System Status Codes January 2003
X.7.0 Other or undefined security status
X.7.1 Delivery not authorized, message refused
X.7.2 Mailing list expansion prohibited
X.7.3 Security conversion required but not possible
X.7.4 Security features not supported
X.7.5 Cryptographic failure
X.7.6 Cryptographic algorithm not supported
X.7.7 Message integrity failure
Appendix B - Changes from RFC 1893
Changed Authors contact information.
Updated required standards boilerplate.
Edited the text to make it spell-checker and grammar checker
compliant.
Modified the text describing the persistent transient failure to more
closely reflect current practice and understanding.
Eliminated the restriction on the X.4.7 codes limiting them to
persistent transient errors.
Author's Address
Gregory M. Vaudreuil
Lucent Technologies
7291 Williamson Rd
Dallas, Tx. 75214
Phone: +1 214 823 9325
EMail: GregV@ieee.org
Vaudreuil Standards Track PAGE 15
RFC 3463 Enhanced Mail System Status Codes January 2003
Full Copyright Statement
Copyright © The Internet Society (2003). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Vaudreuil Standards Track PAGE 16
Enhanced Mail System Status Codes
RFC TOTAL SIZE: 31832 bytes
PUBLICATION DATE: Friday, January 17th, 2003
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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