TMDA is a pure-python open source anti-spam system and local mail
delivery agent for Unix and Unix-like systems. TMDA fights spam
using a combination of whitelists, blacklists, challenge/response
system, and tagged addresses, which are special-purpose e-mail
addresses such as time-dependent addresses, and addresses which only
accept certain kinds of communication.
For more information on TMDA, including downloading and installation
instructions, please visit one of the following mirror sites:
http://www.us.tmda.net/
http://www.de.tmda.net/
http://www.au.tmda.net/
Enjoy.