Since switching to a newer version of SpamAssassin, it catches nearly 100% of all spam and has had no false positives, and I get about 40 spams per day. I think it is enough for most people. However, if you want to be 100% certain, just use any of the existing response-based mail systems out there. There are lots of them. Spam is sent by cheesy automated programs. You will not get responses back from spammers. Most of their addresses will bounce.