Mozilla's Thunderbird uses heuristic blocking, which means that their junk mail blocking part of their programs learns what you consider junk by checking which messages you mark as junk, and "learning" from these choices that you make as to what is junk and what isn't. Mailwasher used similar blocking but also can be set to send a nasty message to the URL from which the objectionable mail originated.

I use Thunderbird, but I also check the list of messages they have moved to the junkfolder to see if I recognise either the sender or the subject of the email. After a while as the program gets better at recognizing objectionable email you don't need to do this, but I always check, just to be sure.

Dave