I think a mail 2.0 would be notify and pull based.... you notify a recipient's mail server that there's a message from <address> for them, then that server connects to the MX of record for the domain of <address> and retrieves <message-id> message.

Would this make mass emails and spam harder, absolutely. Would it be a huge burden for actual communications with people, not so much. From there actual white/black listing processes would work all that much better.