I vaguely recall seeing something like this on the postfix users list ages ago. you can already do something similar (way less automated, though) with postfix's

I vaguely recall seeing something like this on the postfix users list ages ago. you can already do something similar (way less automated, though) with postfix's sender_access map. as for why it's not a thing.. I think it's too many steps for the masses. you can do it on your own server like I mentioned, though. but on a smaller scale, sure. go for it.