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TextDrive Support Forum / TMDA on Textdrive HOWTO.
tmda is a whitelisting anti-spam strategy; only mail that is on a whitelist will get through, rather than the blacklisting strategy used by spamassassin et al. The idea is that one generates a baseline whitelist of people that one is in email contact with (easily done with a script that comes with tmda) and then anyone not on that whitelist will receive a challenge from the tmda subsystem. If they respond to the challenge, their email is passed on and they are optionally added to the whitelist so they do not receive further challenges.