Ok, so you hope to teach your filter that you are not interested in SEO (because you know that "decent" companies rarely advertise themselves by sending spam).

Ok, so you hope to teach your filter that you are not interested in SEO (because you know that "decent" companies rarely advertise themselves by sending spam). What if one day someone you know writes to you personally and offers just friendly to optimize your site because she thinks it doesn't show up in Google results? And let's say there will be some link, too. There's a chance this message will hit the thresholds and you won't see it. Another possibility could be that we are discussing this by email and we use plenty of those "bad" words. Now get it: Viagra, Rolex watches, SEO, efficient email advertising ;) I'm arguing that the only way to stop some really shitty mail is to force senders pay some price. Could be money or maybe some computational resources (say, MD5). The recipient (updated) would be able to see exactly how much time or other resources were spent on the given message. The more resources the more important the message. 1000 iterations of MD5? Ok, that must be someone I know personally. 10 iterations? Looks like an ad sent to many recipients. That's it. But computers become more and more powerful. Fine, in 5 years you will be able to raise your thresholds and require, say, 10,000 MD5 iterations to catch your attention.