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Network Working Group                                        G. VaudreuilRequest for Comments: 1893                         Octel Network ServicesCategory: Standards Track                                    January 1996Enhanced Mail System Status CodesStatus 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 numberVaudreuil                   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 19963.   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 toVaudreuil                   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 19967.   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.comVaudreuil                   Standards Track                    [Page 13]

RFC 1893                Mail System Status Codes            January 19968.   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 failedVaudreuil                   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 failureVaudreuil                   Standards Track                    [Page 15]