RFC 3463 (rfc3463) - Page 2 of 16

Enhanced Mail System Status Codes

RFC 3463           Enhanced Mail System Status Codes        January 2003


1. Overview

   There is a need for a standard mechanism for the reporting of mail
   system errors richer than the limited set offered by SMTP and the
   system specific text descriptions sent in mail messages.  There is a
   pressing need for a rich machine-readable, human language independent
   status code for use in delivery status notifications [DSN].  This
   document proposes a new set of status codes for this purpose.

   SMTP [SMTP] error codes have historically been used for reporting
   mail system errors.  Because of limitations in the SMTP code design,
   these are not suitable for use in delivery status notifications.
   SMTP provides about 12 useful codes for delivery reports.  The
   majority of the codes are protocol specific response codes such as
   the 354 response to the SMTP data command.  Each of the 12 useful
   codes are overloaded to indicate several error conditions.  SMTP
   suffers some scars from history, most notably the unfortunate damage
   to the reply code extension mechanism by uncontrolled use.  This
   proposal facilitates future extensibility by requiring the client to
   interpret unknown error codes according to the theory of codes while
   requiring servers to register new response codes.

   The SMTP theory of reply codes are partitioned in the number space in
   such a manner that the remaining available codes will not provide the
   space needed.  The most critical example is the existence of only 5
   remaining codes for mail system errors.  The mail system
   classification includes both host and mailbox error conditions.  The
   remaining third digit space would be completely consumed as needed to
   indicate MIME and media conversion errors and security system errors.

   A revision to the SMTP theory of reply codes to better distribute the
   error conditions in the number space will necessarily be incompatible
   with SMTP.  Further, consumption of the remaining reply-code number
   space for delivery notification reporting will reduce the available
   codes for new ESMTP extensions.

   The following status code set is based on the SMTP theory of reply
   codes.  It adopts the success, permanent error, and transient error
   semantics of the first value, with a further description and
   classification in the second.  This proposal re-distributes the
   classifications to better distribute the error conditions, such as
   separating mailbox from host errors.

   Document Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14 [RFC 2119].



Vaudreuil                   Standards Track