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Vacation returns a message to the sender of a message announcing that you are currently not able to read the message.
Setting up "vacation" requires three steps, the initialization of vacation and and the creation two files in your home directory. We assume, your login name is "euler.

1) First type
vacation -i 
to initialize the vacation database file.
2) Edit the file ".vacation.msg" contains the message which is sent to the people, who send you mail:

I'm currently out of town and will read
my email later. 
L. Euler

3) Edit the ".forward" file in your homedirectory. contains the line

euler, "|/usr/bin/vacation euler"
It has the effect that your mail will be pipied additionally through the vacation program.

If you use procmail to filter your mail, the .forward file is not needed. Instead, add
|/usr/bin/vacation euler

at the end of your .procmailrc file. Its has the effect that a copy of the email is piped through the vacation program. It is important to put these lines below the other procmail filters. Otherwise, spammers will obtain feedback and your address will remain on their list of active e-mail addresses - resulting in more spam.

Simplicity, Clarity, Generality B.W. Kernighan, R. Pike, in "The Practice of Programming".