It wouldn't hurt to have a dummy's guide for writing vacation programs
that spells out the obvious, such as the fact that an out-of-office
notification is an unsolicted, ad hoc DSN.
Having thought about this again as a result of the current discussion,
I would probably argue that it's closer to an unsolicited, ad hoc,
automatically-generated, MDN (in contrast with an MDN that is manually
generated on a per-message basis). The reason I would argue this is that
vacation notice isn't generated by the mail delivery system, it's
generated by a user agent (or at least, something acting on behalf
of the user); and that it's an indication of recipient action rather
than of any action by the message delivery system.
However I would still argue that in the absense of explicit direction on
the part of the sender, that a vacation notice (i.e. a notice that the
recipient's reading of the message will be delayed) should be returned
to the person who sent the message (Sender or Return-path field) rather
than to the person or persons on whose behalf the message was sent (From).
Using Return-path would also be consistent with RFC 2298 recommendations
for automatically-generated MDNs.
Keith