Here's a test on my last 4 days of ham and spam. The T_DNS_FROM* rules are using the envelope sender (that is, the MAIL FROM added by my MTA to the message headers). They have a reasonably good hit rate (6.43% of spam hit one of tested SURBL zones) and a 0% FP rate in this test. Only 7 out of 103 of those did not hit one of the URIBL rules, but they did do it with zero FPs (I get FPs mostly because people discuss spam domains in some of the ham tested here. That's another issue, though.)
Maybe it would be worthwhile factoring this into future development. That is, also list known spammer envelope senders domains, maybe get SpamCop to provide lists for that too, I suspect there's some overlap in the other direction as well.
OVERALL% SPAM% HAM% S/O RANK SCORE NAME 2598 1602 996 0.617 0.00 0.00 (all messages) 100.000 61.6628 38.3372 0.617 0.00 0.00 (all messages as %) 3.580 5.8052 0.0000 1.000 1.00 0.01 T_DNS_FROM_SURBL_WS 0.885 1.4357 0.0000 1.000 0.67 0.01 T_DNS_FROM_SURBL_SC 9.161 14.5443 0.5020 0.967 0.56 1.00 URIBL_BE_SURBL 36.066 57.5531 1.5060 0.974 0.44 1.00 URIBL_WS_SURBL 0.423 0.6866 0.0000 1.000 0.33 0.01 T_DNS_FROM_SURBL_BE 29.908 47.5655 1.5060 0.969 0.11 1.00 URIBL_SC_SURBL
How's the multi roll-out going? It would definitely be handy for this test (the code to support it already exists).
Daniel