I recently did a little write up on how to do this. If you run your own smartermail server this will help you cut out 99% of all spam, with no false positives ( at least with my email there are none ). If you have a VPS this should help you alot!!! Let me know your thoughts
http://blog.whitesites.com/Stop-Spam-in-SmarterMail-with-RBL-SBL-and-BL-__633380424443090911_blog.htm

I'm always hesitant on using RBL's in SmarterMail. RBL's usually are unbiased in who they block, as they should be, however when they start blocking GMail/Yahoo/AOL mailservers they interupt too much legitimate email.

http://foo-projects.org/node/27


One thing I generally recommend if using RBL's is to have it only fail the message if it is on 2 RBLs. To do that you need to make the score of each RBL to 10, and on the SMTP Blocking tab where you enable the filtering set it to only block when the score is at 20. This way if there is one overzealous list it won't cause the messages to fail. I also generally try to shy away from RBL's who charge fees to get removed, as many places out of principal will not pay a list to be removed.

Just my thoughts