Tony Finch writes:
The maximum time that mail stays undelivered on peoples queues before
being bounced. Theoretically unbounced but in practice a week or so.
Some large ISPs who deliver email to their users over SMTP keep messages
on their queues for up to a month, to allow for users who dial up
infrequently.
For IIM, the expiry on the signature is intended to put a
limit on retransmission time -- we didn't want immortal
messages in circulation. That is, something within the
validity time can be forwarded along and the receiver can
then know whether the signer considers it to be legitimately
in transit. Outside that time, you can still know that the
signer signed it and what their current opinion of that key
is, but beyond that it was really our intention that the
IIM-VERIFY header be the distillation of what the opinion on
that signature was while valid (ie, in transit).
Which is to say that I don't think that dialup users and
their ilk really affect this...
Mike