SpamAssassin Filter Software

This is the big one, and should be the cornerstone of your anti spam filter efforts. SpamAssassin is an open source software spam filter system for assigning a score, or probability of an email being a spam. Scoring is done based on rules - a standard set of several hundred rules is distributed with SpamAssassin and you can always write your own, or download more from any number of sites.

Handy things like checking an email for the letter 'q' without a 'u' following it. In the English language, this sort of artifact would be extremely suspect for obvious reasons. And so we assign a healthy couple of points based on that alone.

Writing your own rules can be complicated if you are not familiar with regular expressions, since regular expressions are the main mechanism used in SpamAssassin for matching rules to an email. There are tons of informational sites devoted to the subject of regular expressions, which are beyond the scope and focus of this site - but there can be a decent learning curve. I will offer this - in order to learn how to use regular expressions, visit some of the dedicated sites, have some examples in mind that you would like to write, and by all means, use a tool like RegExp Coach.

We'll delve more deeply into SpamAssassin rules later on. But for now, you would be best served to stick with the standard rule distribution, and any other rule sets you find that have been thoroughly tested.

As of the time of this writing, there are surprisingly few client side implementations of SpamAssassin, which seems like a good fit. Most client side spam filters are bayesian like in nature, but it is difficult to find client side SpamAssassin. It's not unfeasible - SpamAssassin is written in Perl, which runs natively on Linux OSs, but is also supported on traditional desktop operating systems - although not as well supported.