Editing Apache SpamAssassin (section)

==Operation==
Apache SpamAssassin comes with a large set of rules which are applied to determine whether an email is spam or not. Most rules are based on [[regular expression]]s that are matched against the body or header fields of the message, but Apache SpamAssassin also employs a number of other spam-fighting techniques. The rules are called "tests" in the SpamAssassin documentation.

Each test has a score value that will be assigned to a message if it matches the test's criteria. The scores can be positive or negative, with positive values indicating "spam" and negative "ham" (non-spam messages). A message is matched against all tests and Apache SpamAssassin combines the results into a global score which is assigned to the message. The higher the score, the higher the probability that the message is spam.

Apache SpamAssassin has an internal (configurable) score threshold to classify a message as spam. Usually a message will only be considered as spam if it matches multiple criteria; matching just a single test will not usually be enough to reach the threshold.

If Apache SpamAssassin considers a message to be spam, it can be further rewritten. 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 tests passed 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.

Apache SpamAssassin allows for a per-user configuration of its behavior, even if installed as system-wide service; the configuration can be read from a file or a database.  In their configuration users 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 Apache SpamAssassin then assigns a higher score to all mails that appear to be written in another language.

Apache SpamAssassin is based on heuristics (pattern recognition), and such software exhibits false positives and false negatives.