[Exim] Questin about RBL's

I use an e-mail forwarding service (pobox.com) for much of my mail...

I would like to configure exim to scan the headers of the incoming message,
and /dev/null any messages that come from a server in the many RBL's I have
configured in my exim.conf... Is there an easy way to do this?

I know pobox could block anything with a 10/10 spam score, but they seem to be
getting a false positive on a good number of messages I actually WANT to get.

Thanks for any suggestions.