.forward

From Grokdoc

the .forward file is the UNIX method of specifying that you want your mail to be processed in a non-standard manner. It lets you say what to do when mail comes in, either by putting a new address to receive the mail (forwarding it on) or a program to run.

typically its used to run procmail or the vacation program (to send notices you are on holiday to people, but only once per sender)