Out Of Office? Don't Advertise It
Out of office replies, often termed "OOF" for Out Of Facility or "vacation auto-responders" are those automated replies to email when the recipient is away for some period of time.
The returned message typically states that a) they're away from their office/home/whatever and won't be checking email, b) the dates for which that will be the case, and hopefully c) an alternate point of contact for the duration of their absence.
It seems like such a good idea. It can even be quite helpful if you're attempting to contact that person while they're away.
Unfortunately the problems with out of office replies have come to far outweigh the benefits.

