[oever]

> By default you only see what people in your contact list have sent you, and anything else is marked "dubious", and it's up to you to read it or not.

Any email client could implement this policy. You could even prioritize mail over who sent it or whether it's a reply to a mail you sent or have already read.

[fluoridation]

Yes, but if the third server down the line didn't propagate the email, there's not much the client can do. That's what sucks about email as a protocol; it's been taken over by a handful of providers who will refuse to play ball with anyone outside their club, or who doesn't have the time to monitor the continuously-updated black lists.