SpamAssassin

Use this program to "Unspam" your mailbox(es). The program can be integrated with the mail server to automatically filter all mail for a site. It can also be run by individual users on their own mailbox and integrates with several mail programs. SpamAssassin is highly configurable; if used as a system-wide filter it can still be configured to support per-user preferences.

SpamAssassin was awarded the Linux New Media Award 2006 as the 'Best Linux-based Anti-spam Solution'. Typically either variant of the application is set up in a generic mail filter program, or it is called directly from a mail user agent that supports this, whenever new mail arrives. Mail filter programs such as procmail can be made to pipe all incoming mail through SpamAssassin with an adjustment to user's .procmailrc file. Details see http://en.wikipedia.org/wiki/SpamAssassin.