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I'l vote for this. This service is available with many RBL's but not a free one would be good if zimbra maintains its own RBL.. This way better control as well for false positives.
I'd rather have a good recipe on how to feed the originating IPs of rejected mails into a DB.
From there, it should be no problem to export it to an RBL.
I'm pretty sure there are open source reputation/rbl implementations out there.
This is one example: ThreatNet:ATN2004 - search.cpan.org
from an ex-flatmate of mine.
Joining zimbra servers together to better handle threats would be a very interesting extension.