Couldn't better email clients and e-mail services be made that do? I'm trying to look at this from an idiot's perspective (which may not be all that far from my ordinary perspective, because I don't know a huge amount of detail about how email works).
Theoretically, but that's not part of the model today. And if you're going to innovate on that, it's unclear that you'd want to do it on top of SMTP as a transport, rather than something newer. SMTP itself provides very little in the way of assurances about identity, deliverability, etc., all of which you'd care about as you build something higher-level.
The closest thing to innovation in this space that I know of is the stuff that Gmail is doing with custom actions [1] and contextual gadgets. And even there, they are parsing the HTML-ish body of the email to trigger the behavior, which is pretty far away from the SMTP layer itself.
