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rfc:rfc1893

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]

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