error(s) in forwarding or filtering

This is an automatically generated message. An error has
been found in your .forward file. Details of the error are
reported below. While this error persists, you will receive
a copy of this message for every message that is addressed
to you. If your .forward file is a filter file, or if it is
a non-filter file containing no valid forwarding addresses,
a copy of each incoming message will be put in your normal
mailbox. If a non-filter file contains at least one valid
forwarding address, forwarding to the valid addresses will
happen, and those will be the only deliveries that occur.

The userforward router encountered the following error(s):

unknown filtering command "|/usr/bin/procmail" near line 4 of filter file

No valid addresses were generated.