.forward
A text file in your home directory telling the mail server to send your mail somewhere else instead.
Sendmail, and most Unix mail servers after it, read the file before delivering anything to the local mailbox. Each line was a destination: another address, a file to append the mail to, or a program after a pipe symbol, such as |/usr/bin/vacation. A line reading \username kept a local copy without sending the mail round in a loop. For students with accounts on three machines it was the only way to read all their mail in one place.
The pipe turned the file into a small programming interface: auto-replies, filtering with procmail, sorting into folders. It also made it dangerous, since the mail ran as the user. Sendmail accordingly learned to refuse files that anyone else could write to — after somebody first demonstrated why.