RFC 3463 (rfc3463) - Page 2 of 16
Enhanced Mail System Status Codes
Alternative Format: Original Text Document
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