a very good idea for stopping most spam in its tracks. However, it needs the assistance of the ISP...
The thing is this - most spam is sent in such large numbers to recipients that any ISP would be able to see instantly say 50,000 emails from one sender. All they would have to do is to check on the content of just a few of these to see if it is legitimate or not and if not block it or return it. Just think, one sender getting 50,000 returned emails at once!! (That would keep his inbox busy for a little while :-))
One person could "scan" 30 potential spam emails an hour easily. That is approx 250 a day. If each "Spam" was of say 50,000 emails one person would single handedly be able to stop 12,500,000 spam emails reaching their intended targets each day, or 87,500,000 in a week.
Now, would that not speed up the internet a little?