What Sven suggested is pretty good. But I'd do all the local checks first, then hit the RBLs on the 'Net -- it reduces the load on the RBLs and on the local 'Net, and a CPU is faster than a 'Net connection (I'm on a T1, so I'm very aware of bandwidth usage and speed).
Iptables is also useful. You can block entire IP blocks for MailChimp and the like. And entire countries, like China (many address blocks, though), etc. But Iptables goes through a list of rules pretty quickly.
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Glenn English