TMDA is an open source anti-spam system and local mail delivery agent written in 100% pure Python. TMDA combines whitelists, blacklists, a challenge/response system, and special-purpose e-mail addresses called "tagged addresses" to thwart spam.
TMDA and TMDA-CGI are great programs. I used TMDA only from around 2,000 - 2003. Unfortunately, over the last 15 years, challenge-response email sytems have fallen out of favor by internet gurus and pundits, such as John C Dvorac;and, misuse of the system, such as allowing them to respond to mailing lists, challenging replies, or that challenges could go to forged headers in the original email, resulting in spam to innocent people. I never experienced any such problems with tmda. tmda effectively removed 100% of the spam in my email. I sure do miss it. Challenge Response is still a viable solution to spam. I can tell you that my email is nearly unusable after I stopped using it in 2003, because my ISP would not allow my home email server to communicate on port 25 anymore. In these recent days of decentralization, we need challenge response more than ever! Essentilly, our email system is broken, and the TBTF companies are rendering email unusable, by the shear number of spams sent. My hope is that there becomes a renewed interest in challenge-response. Properly used and supported, it can evolve to an acceptable protocol. You can buy CR systems today from the likes of SpamArrest; and I think AOL offers the service. But, why do we need to rely on them,when we have this great product already in open source? I recently re-installed TMDA, but the email was not delivered after the challenges were responded to. I see that several people have tried to reserect tmda and tmda-cgi at Github, but I can't seem to get it working now.
TMDA works great and gets my Thumbs Up approval. (Note: Thumb ratings without reviews appear to be counted backwards so a thumbs down without a review is likely a thumbs up.)