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IETF RFC 1893
Enhanced Mail System Status Codes
Last modified on Tuesday, January 9th, 1996
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Network Working Group G. Vaudreuil
Request for Comments: 1893 Octel Network Services
Category: Standards Track January 1996
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.
1. Overview
There currently is not a standard mechanism for the reporting of mail
system errors except for 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 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 each overloaded to indicate several error conditions each.
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 partitioned in the number space 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
Vaudreuil Standards Track PAGE 1 
RFC 1893 Mail System Status Codes January 1996
space for delivery notification reporting will reduce the available
codes for new ESMTP extensions.
The following proposal 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.
2. Status Codes
This document defines a new set of status codes to report mail system
conditions. These status codes are intended to be 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.
The codes 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 2
RFC 1893 Mail System Status Codes January 1996
The class sub-code provides a broad classification of the status.
The enumerated values the class are defined as:
2.X.X Success
Success specifies that the DSN is reporting a positive delivery
action. Detail sub-codes may provide notification of
transformations required for delivery.
4.X.X Persistent Transient Failure
A persistent transient failure is one in which the message as
sent is valid, but some temporary event prevents the successful
sending of the message. Sending in the future may be successful.
5.X.X 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.X Other or Undefined Status
There is no additional subject information available.
X.1.X 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.X Mailbox Status
Mailbox status indicates that something having to do with the
mailbox has cause this DSN. Mailbox issues are assumed to be
under the general control of the recipient.
Vaudreuil Standards Track PAGE 3
RFC 1893 Mail System Status Codes January 1996
X.3.X 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.X 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.X 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. Mail
delivery protocol issues may be controlled by many parties
including the originating system, destination system, or
intermediate system administrators.
X.6.X 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 whom must support a common set of supported
content-types.
X.7.X 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.
Vaudreuil Standards Track PAGE 4
RFC 1893 Mail System Status Codes January 1996
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.
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 codes 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.
Vaudreuil Standards Track PAGE 5
RFC 1893 Mail System Status Codes January 1996
X.1.5 Destination address valid
This mailbox address as specified was valid. This status
code should be used for positive delivery reports.
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.
Vaudreuil Standards Track PAGE 6
RFC 1893 Mail System Status Codes January 1996
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.
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 permanent 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.
Vaudreuil Standards Track PAGE 7
RFC 1893 Mail System Status Codes January 1996
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 which will permit
it to accept this message.
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, either
because the remote system was busy, or otherwise 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 otherwise
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.
Vaudreuil Standards Track PAGE 8
RFC 1893 Mail System Status Codes January 1996
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.
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. This is useful only as a persistent
transient error.
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.
Vaudreuil Standards Track PAGE 9
RFC 1893 Mail System Status Codes January 1996
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.
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 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
Vaudreuil Standards Track PAGE 10
RFC 1893 Mail System Status Codes January 1996
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 permanant
error if the sender has indicated that conversion with loss
is prohibited for the message.
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.
Vaudreuil Standards Track PAGE 11
RFC 1893 Mail System Status Codes January 1996
X.7.4 Security features not supported
A message contained security features such as secure
authentication which 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.
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. References
[SMTP] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
USC/Information Sciences Institute, August 1982.
[DSN] Moore, K., and G. Vaudreuil, "An Extensible Message Format for
Delivery Status Notifications", RFC 1894, University of
Tennessee, Octel Network Services, January 1996.
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.
6. Acknowledgments
The author wishes to offer special thanks to Harald Alvestrand, Marko
Kaittola, and Keith Moore for their extensive review and constructive
suggestions.
Vaudreuil Standards Track PAGE 12
RFC 1893 Mail System Status Codes January 1996
7. Author's Address
Gregory M. Vaudreuil
Octel Network Services
17060 Dallas Parkway
Suite 214
Dallas, TX 75248-1905
Voice/Fax: +1-214-733-2722
EMail: Greg.Vaudreuil@Octel.com
Vaudreuil Standards Track PAGE 13
RFC 1893 Mail System Status Codes January 1996
8. Appendix - 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 1893 Mail System Status Codes January 1996
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
Vaudreuil Standards Track PAGE 15
Enhanced Mail System Status Codes
RFC TOTAL SIZE: 28218 bytes
PUBLICATION DATE: Tuesday, January 9th, 1996
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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