I'm not sure of the relevance of the protocol IF the service remains store-and-forward.
Silent Circle and Lavabit seem to think it's better not to use email at all. An open Skype-like system is probably pretty good. Keys are ephemeral, and transferring encrypted files in audio-video sessions makes it hard to MITM.
Email is pretty bad, but its not going anywhere so we should try to fix it.
The service would remain store-and-forward but the content would be encrypted throughout, something which is not currently possible without both parties knowing how to setup and use PGP.
Silent Circle and Lavabit seem to think it's better not to use email at all. An open Skype-like system is probably pretty good. Keys are ephemeral, and transferring encrypted files in audio-video sessions makes it hard to MITM.