As spam increased in volume and became more of a problem, anti-spam techniques were developed to counteract it. Tools to block spam were developed by a group of professionals. These tools were not always automated, but when used by system administrators of large sites, they could successfully filter spam for a large number of users. In response, spammers evolved their techniques to increase the number of spams delivered by working around and through the filters. As spam filters improved, spammers designed other methods of bypassing the filters and the cycle repeated. This resulted in the development of both spam and anti-spam techniques and tools over a number of years. This evolutionary process continues today.

Anti-spam tools use a wide range of techniques to reduce the volume of spam received by a user. A number of these techniques will be described in this chapter. SpamAssassin is an important Open Source tool that we will examine in the light...