> No, I'm asking how a new open protocol can be built on top of email in a way that maintains strong backwards compatibility while offering strong security guar

> No, I'm asking how a new open protocol can be built on top of email in a way that maintains strong backwards compatibility while offering strong security guarantees, like end-to-end encryption. I don't think it's possible. That's exactly what Autocrypt is doing. They're still at level 1, ie opportunistic encryption: try to encrypt if possible, default to plain-text if not. That's 100% backwards-compatible. What we all want is a system where we are sure that messages can't be sent in plaintext. The only way this can happen is if MTAs actually read messages and check if encryption is applied; if not, reject it. That's something that can happen but by definition it can't be backwards-compatible.