You type your email into the app, which is just a word processor. When you send it - it saves an encrypted attachment, attaches it to the message and sends via email.
The other party will need the client to read the attachment, and their client will need to connect to a secure central ID entity to confirm they are the recipient client which can open the message.
Wait, that's so wrong. Why don't we just fix our laws. ...
Wait, we will always have bad actors or regimes, do we just design for that?
We need a giant public "mailstore" where everybody (or at least a large pool of people) put the encrypted messages with encrypted recipient information. Everybody who uses this mailstore gets a copy of every message posted to it, but unless they are the intended recipient with the right private key they can't make heads or tails of it.
Maybe usenet could be drafted into handling it. Usenet already handles terabytes of encrypted data on a regular basis nowadays.