[_urga]

Have you tried using a quorum of DNSBL (e.g. barracudacentral.org, cbl.abuseat.org, truncate.gbudb.net) to reduce false positives?

In other words, if at least two DNSBL queries agree, then reject, or feed this information to the rest of the spam pipeline?

[thatsaguy]

This is pretty common, and systems like SA do this for you by batching responses and calculating a score.

I found this to be pretty much worthless if you already have greylisting, even for high-quality curated lists such as spamhaus SBL/XBL.