(UN*X systems:)
A program for replying to mail automatically (i.e. an autoresponder). The idea is you set your .forward file to pipe all incoming mail to the vacation program. vacation replies to each correspondent with a precomposed message (which, typically, says that you're on vacation), typically in .vacation.msg in your home directory.

Some effort is made to prevent mail loops, by adding special headers to outgoing mail, and not responding to messages flagged as bulk.

A database of people already replied-to is kept, so normally each will receive notification only once.