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.
Keith
Then grab the equivalent NetNews header field syntax without
changing a single comma in it (for interoperability):
<http://tools.ietf.org/html/draft-ietf-usefor-usefor>
That is an approved RFC waiting for its number. It is also
the source for the IANA registration of the Expires: header
field for NetNews. (See section 3.2.5 and chapter 6)