Blockchain mail, anyone? - Available anywhere - Guaranteed notarized sending, guaranteed total ordering of messages - Privacy and security built into the pro

Blockchain mail, anyone? - Available anywhere - Guaranteed notarized sending, guaranteed total ordering of messages - Privacy and security built into the protocol. The incentives aren't there -- what would miners get in this scenario? Also doing it in a way that retains perfect forward secrecy will be tricky. Could be a startup idea.