I once read an article on 'intellignet' email filtering. This program would gradually learn which mails were spam and which were not. Of course, it took a month or so before it really learned well, but supposedly, it works. The program I used to use for this has disappeared from the Internet and I can't remember its name. But the technology exists, so chances are somebody is still developing it. I'm sorry I can't recommend a specifc program, but maybe someone else has heard of something like this.