Mailfilter's a proactive spam blocker that, unlike procmail,
actually avoids bringing spam onto your machine in the first place
by examining your POP mailbox.
"Mailfilter is a very usable tool. It helps you to keep
spam away from your mailbox. Contrary to the filtering with
procmail, mailfilter filters the mail online. This means that
mailfilter deletes unwanted spam directly from your POP3 account
before fetchmail (or your favourite POP3 client) even sees it. This
is different from procmail, which zaps the spam after it's been
downloaded and is about to be put into your mailbox. (Examples of
using procmail are in a recent Linux Gazette article,
http://www.linuxgazette.com/issue62/okopnik.html.)
The benefit from that is that you won't download the spam
anymore. This saves bandwidth and makes you a lot happier
especially if you have a slow Internet link.
Mailfilter is a very good tool, easy to install and easy to
maintain. You can add it to your .fetchmailrc as well, thus
automating the procedure of deleting unwanted spam before
downloading."