I wish the DIME (former Dark Mail) protocol was out already, too, so we can compare. There's also Adam Langley's Pond , but sounds like it's too complex, and it only works over Tor. And TextSecure/Axolotl could probably be used as a secure e-mail protocol, too, if you add a proper e-mail like interface. I hope the team behind End-to-End is looking at all of them.
 - https://pond.imperialviolet.org/