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SpamAssassin is a free rule-based spam filter which also supports Naive Bayesian classification, Realtime Blackhole Listss and similar blacklists, the Distributed Checksum Clearinghouses and Vipul's Razor. SpamAssassin is a Perl-based application (Mail::SpamAssassin in CPAN) which is usually used to filter all incoming mail for one or several users. It can be used as a standalone application or as a client (spamc) in combination with a daemon (spamd) which runs in the background. The latter modus operandi has performance benefits, but potential security downsides.

Typically either variant of the application is set up in a user's .procmailrc file, so that the procmail mail processor pipess all incoming mail through the program, or it is called directly from a graphical mail user agent when new mail arrives. In the latter case, the email client needs to support processing incoming mail through other programs.

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 V2.55), 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".

E-mail recognized as spam by SpamAssassin, here in the Ximian Evolution email client. 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.

Table of contents
1 Bayesian filtering
2 Analysis
3 Usage in commercial products
4 External links

Bayesian filtering

SpamAssassin by default tries to reinforce its own rules through Bayesian filtering, but Bayesian learning is most effective with actual user input. Typically, the user is expected to "feed" example spam mails and example "ham" (useful) mails to the filter, which can then learn the difference between the two. For this purpose, SpamAssassin provides the command-line tool sa-learn, which can be instructed to learn a single mail or an entire mailbox as either ham or spam.

Typically, the user will move unrecognized spam to a separate folder for a while, and then run sa-learn on the folder of non-spam and on the folder of spam separately. Alternatively, if the mail user agent supports it, sa-learn can be called for individual emails. Regardless of the method used to perform the learning, SpamAssassin's Bayesian test will subsequently assign a higher score to e-mails that are similar to previously received spam (or, more accurate, which are similarly different from non-spam as previously received spam e-mails).


SpamAssassin is generally regarded as one of the most effective spam filters, especially when used in combination with spam databases such as the one by Vipul's Razor. Even simple text-matching alone may, for most users, be sufficient to correctly classify a majority of incoming mail.

SpamAssassin is free software, doubly licensed under the Artistic License and the GNU General Public License (GPL).

Usage in commercial products

External links