For some reason I have it in the back of my mind that they were at one point accused of being a honeypot for US intelligence because of their association with MIT. Probably complete BS, but maybe not. Are they as open source as they claim to be? Looks like they’re on github. F-Droid seems to think they have some Google libraries or whatever that they use.
ProtonMail users, how do you like/dislike it?
I wouldn’t really call the BE service “core” to what they provide. All the truly interesting code is in the clients. The server just an email service that stores the email in an encrypted format and talks their custom API.
An open source ProtonMail back-end won’t help you in any way unless you’re trying to host PM yourself I guess?