DNSBL

Spamhaus Zen gets my vote

They get my vote as well, although I'm not presently doing any blacklisting (SpamAssassin is keeping on top of it fine on its own). I did use them in the past, however.

We currently outsource all email filtering and I'm very happy about doing that after expending a lot of effort doing it in-house.

We've more recently used the small Spamhaus DROP list with the ConfigServer firewall:

http://www.configserver.com/cp/csf.html

http://www.spamhaus.org/drop/index.lasso

If I was filtering mail locally, I'd probably use Spamhaus.

Whatever you do, just stay away from SORBS. They are horrible – no set listing parameters, delays in de-listings, blocking huge subnets and suspicions of retaliatory listings and delays abound online.

NOT a reliable or beneficial RBL.

+1 for Spamhaus Zen ( http://www.spamhaus.org/zen/ ) from me -- is the best.