I've been using PoPFile (go too www.sourceforge.net and search
on popfile) for several years. I get tens of thousands of emails
every year, and 90% of them are SPAM. PoPFile filters them to
a level of accuracy of more than 99%.

It's a free, open-source program that runs on multiple
systems.

Like other good filtering system, it has to be trained--- everyone's
email patterns are different. But within a week, your accuracy will
be up over 90%.