Free Spam-Blocking Software
SpamBayes
The SpamBayes project is working on developing a Bayesian anti-spam filter, initially based on the work of Paul Graham. The major difference between this and other, similar projects is the emphasis on testing newer approaches to scoring messages.
K9
K9 is an email filtering application that works in conjunction with your regular POP3 email program and automatically classifies incoming emails as spam (junk email) or non-spam without the need for maintaining dozens of rules or constant updates to be downloaded.
MailWasher
MailWasher is a powerful email checker with effective spam elimination. Discover the safe way to stop unwanted viruses and e-mails before they get to your computer.
Spamihilator
Spamihilator is a program that examines your Emails while downloading from the server and deletes unwanted advertising messages.
SpamPal
Check your mail against the freely-usable DNSBL lists (aka “blocking lists”) of your choice. The list of available DNSBL services is automatically updated, so new DNSBL lists can be used as they appear! Whitelist the email addresses of your friends and family so that mail from them will never be tagged as spam! Don’t agree with a listing in a DNSBL list? You can whitelist entire netblocks so you’ll never be troubled by that listing again!
Spam-Aid
Spam-Aid takes an opposite, more secure, approach than other anti-spam software: rather than checking an email for spam characteristics, it checks an email for non-spam characteristics. Most email users want only mail from persons who know them in some manner: either people already in their address book, old friends who happened to find their email address, or customers wanting information regarding their product. Spam-Aid allows you to determine whose messages are accepted. EmC
EmC combines comprehensive spam email detection and blocking with a basic antivirus scan engine to protect you from malicious file attachments at the same time.