From man forward: As with the aliases(4) file, mail messages can be forwarded to another host or given to programs for further processing. The following is an example of the vacation program. Assuming that the user's name is myra, create a .forward file and add the following line:

       \myra, "|/usr/bin/vacation myra"
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The previous example forwards mail to myra (the backslash prevents an accidental aliasing loop), and also sends a copy of the message to the vacation program.