Multipart MIME is fairly clever in that it provides a reasonably human-readable fallback for old clients that don't support it. I'd be very wary of throwing th

Multipart MIME is fairly clever in that it provides a reasonably human-readable fallback for old clients that don't support it. I'd be very wary of throwing that out. One of the benefits of plain-text email is its universality. The only way I can see that working out would be if MTAs and MUAs negotiated during handshaking to decide on what message format they preferred, and then the MTA translated the message either to traditional RFC 2822 or some other preferred format, which could be JSON or pure XML or whatever.