The Spam Bouncer is a set of procmail recipes, or instructions, which
search the headers and text of your incoming email to see if it meets
one or more of several categories (known addresses, rogue ISPs, bulk
mail software etc etc).

The Spam Bouncer sorts suspected spam into two categories -- mail from
known spam sources which is definitely spam, and other mail which is
probably spam, but might also be legitimate. It then tags this email
with appropriate headers giving the spam classification, and responds
according to the parameters you have set.

WWW: http://www.spambouncer.org/