The only options I have found for Exchange are cost prohibitive. I also do not want to manage the client side options. I am seriously considering moving to VOPmail and taking advanatge of the blacklist features in thier products.
/added I have been testing Vamsofts free trial product but have not noticed any improvement. They block mail from known openrelay and major blacklist sites. Price is not too bad I just don't like buying bolt-ons that product should do out of the box.