Anti-SPAM
Spam (unsolicited commercial e-mail) accounts for 35% of all messages and is rapidly increasing.

The spam-identification tactics used include:

Header analysis: spammers use a number of tricks to mask their identities, fool you into thinking they've sent a valid mail, or fool you into thinking you must have subscribed at some stage. SpamAssassin tries to spot these.
Text analysis: again, spam mails often have a characteristic style (to put it politely), and some characteristic disclaimers and CYA text. SpamAssassin can spot these, too.
Blacklists: SpamAssassin supports many useful existing blacklists, such as mail-abuse.org, ordb.org or others.
Razor: Vipul's Razor is a collaborative spam-tracking database, which works by taking a signature of spam messages. Since spam typically operates by sending an identical message to hundreds of people, Razor short-circuits this by allowing the first person to receive a spam to add it to the database -- at which point everyone else will automatically block it.

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

SpamAssassin requires very little configuration; you do not need to continually update it with details of your mail accounts, mailing list memberships, etc. It accomplishes filtering without this knowledge, as much as possible.

Features

Reliable: Identifies over 95% of spam using several techniques, including highly advanced heuristics
User Friendly: Spam can be tagged, rejected, discarded, archived or forwarded to other addresses for inspection by administrators
Guards: Pornographic spam can be stripped of all graphical content, protecting users from obscene content
Wide-spectrum: SpamAssassin uses a wide variety of local and network tests to identify spam signatures. This makes it harder for spammers to identify one aspect which they can craft their messages to work around.
Easy to extend: Rules, weights and user-visible text are stored in text configuration files as much as possible, which the user (or sysadmin) can edit to modify or add new rules.
Flexible: SpamAssassin encapsulates its logic in a well-designed, abstract API. As a result, it's not limited to the traditional local-delivery-to-spool case; using the Mail::SpamAssassin classes, it can be used in a wide variety of setups. This means that SpamAssassin support is available for a variety of mail systems -- traditional procmail, a Mail::Audit plugin, qmail, sendmail, Postfix, many others.