I don’t trust Proton enough to use it exclusively. Personally I use their free email tier as a secondary mailbox.
End-to-end encryption Proton Mail is a private email service that uses open source, independently audited end-to-end encryption and zero-access encryption to secure your communications. This protects against data breaches and ensures no one (not even Proton) can access your inbox. Only you can read your messages.
Which I see as deceptive: end-to-end encryption is working without user involvement only for emails between Proton mailboxes. In other cases user needs to establish PGP encryption on their own. Inbox may be not accessible by Proton (we actually have no clue because server side code is closed source), but unencrypted incoming messages can be easily intercepted by Proton relays.
I’m not saying that Proton does all this nefarious stuff, but their marketing is questionable.
When you send an encrypted email to a non-Proton user, you click on the lock icon to encrypt the email and assign it a password, which you need to get to your user. The recipient then receives an email with a link. They click on the link, enter the password and and can then view your email, which to my understanding is decrypted client-side.