Spam filter for email which can be invoked from mail delivery agents

SpamAssassin provides you with a way to reduce if not completely eliminate
Unsolicited Commercial Email (SPAM) from your incoming email.  It can
be invoked by a MDA such as sendmail or postfix, or can be called from
a procmail script, .forward file, etc.  It uses a genetic-algorithm
evolved scoring system to identify messages which look spammy, then
adds headers to the message so they can be filtered by the user's mail
reading software.  This distribution includes the spamd/spamc components
which create a server that considerably speeds processing of mail.

To enable spamassassin, if you are receiving mail locally, simply add
this line to your ~/.procmailrc:
INCLUDERC=/etc/mail/spamassassin/spamassassin-default.rc

To filter spam for all users, add that line to /etc/procmailrc
(creating if necessary).