Colin Faulkingham recently proposed RPC-MAIL. It's basically using XML and HTTP instead of SMTP. He has very noble goals (eliminating spam and viruses), but the world isn't likely to give up SMTP yet. There's too much infrastructure invested in the current email protocol to make a backwardly incompatible one attractive.
Faulkingham spends a good deal of time thinking about exchanging cryptographic keys, but doesn't seem to address things like routing, attachments, even the format of the mail message. XML parsing and encoding can be expensive in heavily loaded applications. Imagine a spammer trying to send out 100,000 messages/day! Oh wait, maybe that's a feature...
Now using SMTP to transport XML-RPC messages is a bit more interesting, although finding a useful application for this is a bit dubious.