I've asked this question before, but never really got anywhere -- I'd like to set up TMDA or some other Challenge-Response system with Zimbra. Setting up TMDA ought to be easy, since it was designed for Postfix, but I hit some barriers with respect to how Zimbra sets up its mail storage directories. I can't seem to find much out there on the internet, so I was wondering if anyone had ventured into setting this up before or had any ideas.
Oh, and before I start getting flamed for bringing up Challenge Response...
I've tried Spamassassin, all the "extra" anti-spam stuff in the wiki, including greylisting. I have superiors who simply demand the added spam protection that a well-designed CR system (like TMDA) can achieve and that they had been spoiled with in the past. I (and the users) understand the downsides of CR (both for the user and the collateral damage), but the people who make the decisions don't care and insist on CR and its effectively 100% spam protection (even though some would argue it's closer to 110%, which admittedly isn't really a good thing).
Lastly, for what it's worth, I had only barely appeased these picky users for several months with SA and the extra customizations on the wiki, but lately a flurry of pornographic spam has snuck its way in, and marking them all as junk doesn't seem to help much. SA is running and still catching the vast majority of the spam that comes in, but now every day each user receives half a dozen blatantly XXX rated spam (it's not even trying to obfuscate the "nasty" words and it's still getting through!), and they won't put up with it any longer. I'm running 4.5.8, but if the suggestion is to upgrade, I can't possibly be expected to go through the upgrade process every couple of months to keep ahead of the spam curve!