Earlier quoted context omitted.
Protonmail uses PGP and not "home-brewn encryption" and encryption is on by default between Protonmail users.
False. Even when it does use PGP, it is meaningless, explanation: I just created a spare protonmail account. It asked me to pick a username and password, and my account is created. Next, I send there a message from my other account. Yes, on the receiving end it does write "End-to-end encrypted and signed message", but encrypted and signed by what exactly? I have never created a PGP keys and loaded the public key to P…
> Your ProtonMail private key is generated in your browser. Before sending the private key to the server for storage, we encrypt it with your password (or mailbox password if you use two-password mode). This ensures that you and only you can use your private key.
So the only remaining question is whether ProtonMail has access to your password. If they do, they can decrypt your private key and then decrypt your emails. Often, passwords are sent in plaintext to a server for authentication. But ProtonMail uses the Secure Remote Password (SRP) protocol so they never see your password: https://en.wikipedia.org/wiki/Secure_Remote_Password_protoco.... (source: https://protonmail.com/blog/encrypted_email_authentication/)
Of course, there are other threats to worry about, such as ProtonMail changing their client-side JavaScript to exfiltrate your password. But the system as they've documented it does not appear to have any way to decrypt your email server-side short of guessing your password.