] SpamAssassin(tm) is a mail filter to identify spam.
]
] 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.

When configured right, this works great. :) It analyzes all the email coming in, and gives each letter a score to make a guess as to whether or not it's spam. Just like it's usually instantly obvious for a human to look at a letter and say, "Ugh, this is just more junk," this program can automatically scan letters *before* they get to your queue, identify the obvious crap, and delete it before it ever gets to your mail queue. Then anything that it's not sure of, it can flag as *probable* spam, but still let you, the recipient, make the final determination.

My company's been using SpamAssassin for awhile now, and it's been awesome at blocking most of the garbage.

Recommended.