You can't really "stop" spam. It's someone else doing something to you, so it won't end until that person stops. SpamAssassin is about the best tool I've found for making it so you don't have to deal with spam though.
I belive that test is a false positive. Some mail servers will say that they accept the mail from the sender, then filter it out later. I believe that's the case on this type. In other words, it's not actually a problem with your server, their test just isn't 100% accurate.