I honestly think we shouldn't be writing a new protocol to replace email but repurposing an existing, safe protocol like Signal's protocol and writing an app that looks like an email app.

Not only would it liberate Signal from centralization and make it federated, it would provide all the benefits of the protocol and whatever benefits people believe email have. Signal staff is already working on getting rid on removing the phone number requirement.

Another candidate could be the matrix protocol. I would really appreciate getting rid of email altogether. It's clunky and old.