Keith Moore wrote:
It's not at all clear that the Usenet notion of "expires"
is right for email. I'd look at X.400's expiry date as a
starting point - at least it was designed with email in mind.
Apparently RFC 2156 defines "Expires:" - replacing an older
"Expiry-Date:" defined in RFC 1327. Both about X.400, with
a reference in RFC.ietf-usefor-usefor-12 to RFC 2156.
The IANA registry for the mail "Expires:" and "Expiry-Date"
header fields still points to the initial registry RFC 4021.
RFC 4021 has references to RFC 1327 and 2156, noting that
RFC 1327 "Expiry-Date:" was replaced by RFC 2156 "Expires:".
The initial registry (4021) states that "Expires:" is "not
for general use", but for MIXER (2156).
Apparently one point of this procedural exercise is to get
rid of the "not for general use" note, and explain how an
"Expires:" header field with the known semantics "message
expiry time" can be generally useful (in mail).
The explanation in RFC.ietf-usefor-usefor covers the intent.
It doesn't go into details how this could be implemented by
a news server (that is the job of another memo), let alone
by UAs.
Frank