It all depends on the problem one is trying to solve. Since you are addressing the user experience, then indeed, the protocol is not in your way. It is, for the problem I'm trying to solve. Note that the barrier of network effect can be reduced by providing a gateway between the two communication domains (i.e. Facebook). I've subscribed to www.post.fm so I should be informed of your progress.
IMO a new protocol is something that can come later, we've explored that, but only an email service in a leadership position can make those sorts of changes to the standard thus forcing others to implement them.
At the end of the day there can always be a more efficient protocol, but I don't think that's the biggest problem in email.