This chapter discussed the building blocks of SpamAssassin—rules. SpamAssassin allows the user to define rules to respond to the spam a site is currently receiving. There are a variety of rule types for processing different parts of the email.
User-defined rules are based on Perl regular expressions. Rule scoring is an important part of SpamAssassin filtering techniques.
Using a corpus and calculating the effectiveness of rules can assist in re-scoring rules to improve filtering. Several custom rulesets can be added to a site, and these should be frequently updated.