SpamAssassin is generally regarded as one of the most effective spam filters, especially when used in combination with spam databases. Even simple text-matching alone may, for most users, be sufficient to correctly classify a majority of incoming mail.
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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 by the end user or by the ISP 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 passes 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 (default as of version 2.55), several other tests would usually have to pass 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_hits" 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.
The user can customize these filters using a file "user_prefs" in their home directory. Within this file, they can specify individuals whose emails are never considered spam, or change the scores for certain rules. The user can also define a list of languages which they want to receive mail in, and SpamAssassin then assigns a higher score to all mails that appear to be written in another language. This can be very useful to users receiving a lot of foreign spam but never actually corresponding with people in that language.
People Love It!
"SpamAssassin is an amazing product. When I first started spam filtering in 2000 ... I remember thinking that if I could accurately get rid og 80% of spam that would be good enough. I though that nothing over 80% was theoretically possible. Then I discovered SpamAssassin and now ... I'm getting rid of better than 99% of spam without false positives."
Marc Perkel, www.junkmailfilter.com
"We have been using SpamAssassin for over a year now and we've had great feedback from our customers regarding the drop in spam messages in their inbox. Personally I used to get over 250 spams a day, now I get about five that manage to sneak through - Keep up the good work, thats a great result!"
Mike Glaser - Technical Director - eDIY NZ Website Design and E-mail Hosting
Award Winning Protection
Receiving twice as many votes as the closest contender, SpamAssassin took top honors in the Anti-Spam category of Datamation's Product of the Year 2005.
Testimonials and press release links taken from Spam Assassin's website. Click here to visit Spam Assassin.
