As far as I know they use openPGP and that works automatically between proton users but can be set up to work from and to anyone. The other partner just needs to use a client that supports it (like the objectively best client, Thunderbird ;) ) unfortunately pretty much no one uses openPGP so emails will very seldom be E2E encrypted.
Apparently the Proton clients support web key exchanges so you wouldn’t have to import the key of users that use OpenPGP (if they have imported the key to the exchange) so in theory that would make it better. I have yet to use that functionality in Thunderbird though, since again, pretty much no one uses openPGP.
I have sent one legit openPGP email and that was to my country’s financial inspection asking for an internship. Unfortunately they replied unencrypted and included my email in the reply, lol. It’s fair enough though, since I used a feature that’s probably intended to report fraud and crime.
It’s often more comprehensive than what opensubtitles have to offer. I often struggle to get tv episodes with subs in my native language, but all the streaming services obviously has it.