Spam is one of the most annoyingthings today. Although spam received via e-mail is not the only type of spam, it is the most common. Procmail is not only available for Linux. As part of CYGWIN it is installable on Windows, too.
Procmail filterFiltering with Procmail is the heart of any good anti-spam system.
Bypass spam filter for signed mailsFalse positives are the most significant problem in any automatic anti-spam system. To completely bypass the filter for mails which are signed with PGP or GPG, and can be verified locally, is a good way out.
SpamAssassin for the rest(to be continued)