Keith Moore wrote:
How about this: "The sender believes this message will be irrelevant
after the indicated date/time."
"This field is primarily intended for use between senders and recipients
and their agents, rather than by message transport. It is suggested
that the default behavior of an MUA with respect to the expires header
field should be to display such a message in a distinguished way. For
example, the message could be displayed with gray text rather than
black, or in a different font than normal, or (in summaries) with an
image of an hourglass with the sand at the bottom."
"It is also suggested that MUAs allow setting of an expiration date as
an option when composing messages"
"MUAs, Message Stores, and MTAs, MUST NOT delete a message on the basis
of an Expired header field unless given explicit instructions to do so
by the recipient."
"Mail Filters MUST NOT consider an expired header field as criteria to
be considered for deleting the message unless given explicit
instructions to do so by the recipient"
+1 (or rather +5)
behavior for mail-to-NetNews gateways should also be specified.
Transparent, "Expires:" can be kept as is (modulo magic SP in news)