> If I get email from [email protected], then I now know it's from example.tld. If the mail is spam, I can ask example.tld to block alice, or I can simply blacklist example.tld.

Is this better than graylist+whitelist? Is it better than content based filtering? I'm not convinced.

Is it better than requiring tls with valid cert for smtp? I can't see how.

Rather than all this mess, I think I'd prefer we start at tls/ssl required for smtp...