SpamAssassin is a mail filter which attempts to identify spam using text
analysis and several internet-based realtime blacklists.

Using its rule base, it uses a wide range of heuristic tests on mail
headers and body text to identify "spam", also known as unsolicited
commercial email.

Once identified, the mail can then be optionally tagged as spam for later
filtering using the user's own mail user-agent application.

Additional drop-in rule sets are available at
https://cwiki.apache.org/confluence/display/spamassassin/CustomRulesets