Comment by tshaddox

Isn't the problem more about how to treat brand new domains the first time you encounter them? In order to be friendly to small/new email servers, you would presumably need to initially grant new domains a sufficient reputation for them to send mail reliably. But since domains are essentially unlimited, a bad actor can trivially circumvent your reputation system by spinning up endless domains. This seems like a fairly textbook example of a Sybil attack.