I'm not sure I understand the tradeoff here.

It sounds like people are gaining a little bit of extra privacy (by preventing spammers harvesting your email) while sacrificing a ton of privacy (by allowing a a third party MitM to intercept all of their emails to and from that domain).

I actually like the idea a whole lot, but I'd prefer if this could be done in some provably confidential way (where your service has no ability to see the content of messages, only To and From).