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



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   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.




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   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.






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       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.





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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.



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       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.







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       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.








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       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.



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          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.




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       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



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          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.







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       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.




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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









































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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





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       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











































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