I don’t think so. The mail is delivered to the Proton mail server via unencrypted SMTP and then sent on to the client encrypted. The secure layer is between the client and the server. It might also be stored on the server encrypted.

Email is unsecure as hell in any event, but I think the idea is that once it’s delivered, it’s secured.

Isn’t Proton supposed to be end to end encryption between crient and server? And wouldn’t that rule out IMAP?