Is really working for blog spam?

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Last month I found a post advising to setup Zen from spamhaus on Movable Type to filter spam. Since then, I've had an alarming number of false positives, i.e. legitimate comments that are junked on the basis that the commenter's IP is listed in one of the Zen databases. By alarming, I mean that when more than half the junked comments are in fact legitimate, it's time to junk the spam filter instead. Looks like it's not that smart anyway.

Has someone else seen the same problem?