You can also currently block IPs, domains, URLs and specific message content. There are even globally distributed lists of such things. This is already how we

You can also currently block IPs, domains, URLs and specific message content. There are even globally distributed lists of such things. This is already how we manage to block most spam on the edge. What you're proposing is just a small iteration on the existing defences. An expensive one, which wouldn't work unless you managed to get everyone doing it at the same time.