SpamAssassin

SpamAssassin is a Perl-based application which is used to filter all incoming mail.

SpamAssassin comes with a large set of rules which are applied to determine whether an email is spam or not. To decide, specific fields within the email header and the email body are typically searched for certain regular expressions, and if these expressions match, the email is assigned a certain score, depending on the test, and several (customizable) headers are added to the mail. The total score resulting from all tests or other criteria can then be used  to set the conditions under which email is moved to a separate spam folder, deleted, flagged etc.

Each test has a label and a description. The label is usually an all upper case identifier separated with underscores, such as "LIMITED_TIME_ONLY", with the description for that label being "Offers a limited time offer". A mail that fails that test (in this case, contains certain variants of the "limited time only" phrase) might be assigned a score of +0.3. With a spam threshold of 5, several other tests would usually have to fail for the mail to be classified as spam. On the other hand, some tests, such as those for invalid message IDs or years, result in a very high score being assigned, where even a single test can almost put a mail "over the edge".

When a mail's total score is higher than the "required_score" setting in SpamAssassin's configuration, the mail is treated as spam and rewritten according to several options. In the default configuration, the content of the mail is appended as a MIME attachment, with a brief excerpt in the message body, and a description of the tests which resulted in the mail being classified as spam. If the score is lower than the defined settings, by default the information about the passed tests and total score is still added to the email headers and can be used in post-processing for less severe actions, such as tagging the mail as suspicious.

SpamAssassin has also been used in many commercial products including:

• Atmail uses the Spamassassin engine, includes custom rulesets, filters and Web interface for users to modify SA runtime preferences

• AntibodyMX incorporates SpamAssassin

• McAfee uses SpamAssassin in its anti-spam tool SpamKiller

• Spamnix is also based on SpamAssassin

• Kerio MailServer uses SpamEliminator, which is based on SpamAssassin, for heuristic spam filtering

• MailLaunder is a hosted spam and virus solution that uses SpamAssassin as part of the filtering process