Don't do it! (Score 1)

I was in the same situation and found that I just didn't have the time to deal with it effectively. We crossed paths with the folks at mxlogic and they convinced us to give their service a try. They have a small army of people maintaining a defense against spam. It's worked out great for us and only costs about 50 bucks a year. There's no way you can beat that unless you don't really have anything else to do.