It is possible to turn this effect to your advantage. I wrote a spam filter that takes advantage of signals that would be easy for spammers to spoof (like the

It is possible to turn this effect to your advantage. I wrote a spam filter that takes advantage of signals that would be easy for spammers to spoof (like the list-id header), but no one spoofs them because no one but me uses this approach to spam filtering. So, ironically, if more people used my spam filter, it would probably stop working as well as it does now.