content: I'm having trouble understanding the nuances of the technology, but it sounds like the reason why POP3 and IMAP won't work is because neither protocol has a way to decrypt the mail they'd be receiving.  Remember that Protonmail only stores encrypted messages that are decrypted on your device, whether in the browser or mobile app.    Protonmail Bridge (unreleased) is supposed to provide a way to run a local relay on your PC that would decrypt mail locally and serve it via IMAP on your PC only; you'd connect your preferred IMAP client to that.