A general introduction to mail filtering on Unix, with information about Procmail, filter, and mailfilter. (Slightly old and apparently no longer maintained, but still an excellent introduction.)
This article will quickly get you up and running with Procmail. By Nancy McGough, of Filtering FAQ fame. (Contents are partially the same as in the Filtering Mail FAQ, too.)
Contains the mailagent package itself. This is a Unix mail filter (a la filter, vacation, or procmail) written in Perl. Last updated sometime in 1997 ...