Back* when email spam started becoming a real problem, I was explaining to a friend my 'system' for blocking all spam. I'll never forget his response:

"That's quite neat", he said. "You could probably make good money selling your system to others. Except, how are you going to get the word out? By spamming people?"

*Probably around the same time Adam Back was working on his anti-spam system. And we all know where that got us.