I actually don't use protonmail but you should be able to access your mail with IMAP (both iCloud and Gmail support it so proton should as well).

If by encryption you mean PGP etc, that's handled in roundcube I believe.

mbsync mirrors your protonmail inbox to disk, dovecot shares that mail folder through IMAP, and roundcube or some other frontend connects to dovecot.

For gmail/icloud IMAP, you can't use your regular account password, you have to create an "app password" and use that password for mbsync.

Yeah they have integrated end to end encryption, this made me go digging, and I found they have a bridge app that handles it.

https://proton.me/support/bridge-for-linux

Excited to set this up, I had assumed it wasn't a possibility.

I'm mostly consume proton via proton-bridge, which is running in a dedicated VM. Then all my IMAP clients connect to that single instance.

Also, for email sending I use smtp2go free tier - primarily because I use random emails on my own domains to see who's leaking, and I don't want to set them up separately proton-side.

In that setup, proton itself is kinda unnecessary - I keep that path mostly as an insurance, just in case of acatastrophic homelab failure.