Spam Filters Explained

What do they do? How do they work? Which one is right for me? By Alan Hearnshaw

Spam is a very real problem that many people have to deal with on a daily basis. For those that have decided to do something about it and start to investigate the options available in spam filtering, this article provides a brief introduction to your options and the types of spam filters available.

Despite the bewildering array of spam filters available today, all claiming to the best one "of its kind" there are really just five filtering methodologies in general use today and all products rely on one, or a combination of these:

Content-Based Filters

"In the beginning, there were content-based filters."

These filters scan the contents of the and look for tell-tale signs that the message is spam. In the early days of spamming it was quite simple to look out for "Kill Words" such as "Lose Weight" and mark a message as spam if it was found.

Very soon though, spammers got wise to this and started resorting to all kinds of tricks to get their message past the filters. The days of "obfuscation" had begun. We started getting messages containing the phrase "L0se Welght" (Notice the zero for "o" and "l" for "i") and even more bizarre - and sometimes quite ingenious - variations.

This rendered basic content-based filters somewhat ineffective, although there are one or two on the market now that are clever enough to "see through" theses attempts and still provide good results.

Bayesian Based Filters

"The Reverend Bayes comes to the rescue"