Challenge / Response Spam Filtering Software and Services

(STEVEN) blocks a lot of spam by making sure only mail from authorized senders with a valid From: address gets through. (Windows)

0Spam.com is a very convenient spam filtering service that cleans your email of all mail from non-approved senders before you download it.

Affini takes care of spam by requiring senders to either be authorized or prepared to pay per message delivered to an Affini member.

Bluebottle offers email accounts that are practically free of spam (using challenge/response filtering) and can be accessed on the web or using POP with any email program.

eTrust Anti-Spam allows only mail from approved senders to your Inbox, checks sender credentials to warn about phishing attempts and lets you search your emails quickly, too. (Windows)

MailFrontier Desktop combines multiple anti-spam techniques to get the best possible spam protection. (Windows)

Merak Mail Server is a complete, capable and secure mail server that can adapt to all mailing needs gracefully and integrates server-side Bayesian spam filtering as well as instant messaging. (Windows)

Lyttlesoft Spamologist takes care of the spam problem by filtering out obvious spam altogether and challenging new senders to verify their identity. (Windows)

Spam Bully is a great and efficient anti-spam tool. If you train it well, Spam Bully can rid your Inbox of unwanted emails almost completely. (Windows)

Spam Interceptor is an elegant, easy to use and flexible, but most of all effective spam filtering solution.

Spam Sleuth is a comprehensive anti-spam tool, and thanks to its Bayesian filtering it can also be very effective against junk email.

Tonsho lets you send big files (up to 100 MB) by attaching them to emails as usual no matter the email size supported by the recipient; challenging senders to one POP account to prove they're not spammers, Tonsho can filter junk mail, too.

Personal vqME Anti-Spam keeps your email Inbox free of unwanted mail by accepting messages only from approved senders or if a cash guarantee — which you can claim if the message is unwelcome — is included.

WinAntiSPAM is a simple challenge-response spam fighting tool that works as a POP proxy. (Windows)

Zaep can help your home or organization get pretty much spam free by allowing only mail from whitelisted senders through. (Windows)