Filter is part of the Elm package of tools.  Note that you can use filter
to filter your incoming mail even if you are not using Elm to read your mail.

IMPORTANT NOTE
==============
If your system has both procmail and filter installed then you should use 
procmail which is *much* more robust and powerful than filter.  This 
recommendation is almost universal; even the developers of Elm and Filter
recommend procmail over filter.  IT IS POSSIBLE TO LOSE MAIL MESSAGES
WHEN USING FILTER; this is rare but it has happened.