I disagree that a vacation autoresponder is a user agent function. It is a
function performed for the user, yes.
But the function should be performed by the user's MTA (or mail storage agent,
whichever term you prefer). If the vacation notice is sent by the user's MUA,
and the user's MUA is on a PC, it is likely that the user's PC won't be turned
on while the user is "on vacation". It seems the function has to be done by
the MTA.
whether this is a user agent function depends on whether you think of
the message store as part of the user agent or part of the mail delivery
system or a separate piece. and what you think about this is probably
influenced by the architecture of the mail systems with which you are
most familiar. if you deliver to an MS that has its own access protocols,
you might well think of it as part of an MTS; if you deliver mail to a
file or directory that uses normal file-system access protocols and
implement vacation via an on-delivery-hook (similar to .forward) then
you probably think of it as a UA function.
but for the purpose of this discussion I don't think it matters whether
the message store is part of the UA or part of the MTS - in either case
you want vacation notices to go to the same place as other kinds of
notifications (DSNs and automatically-generated MDNs)
Keith