I've actually been working on this exact idea combined with a few other features. The main finding so far is that adding a simple contact list to your email tha

I've actually been working on this exact idea combined with a few other features. The main finding so far is that adding a simple contact list to your email that blocks all messages from senders not in your contacts has some far reaching potential. The biggest one is how it can drastically reduce spam: not worrying about whether some website is going to abuse or share your email is a big advantage. The other big win, is that you don't have to be as secretive of your email address if you don't have to worry about being plagued with spam for life. I plan on making this a public service in the next few weeks.