Earlier quoted context omitted.
Hi! PGP is open source, and therefore Proton Mail's encryption is interoperable with anyone who uses PGP to encrypt their emails: https://proton.me/blog/what-is-pgp-encryption . Proton Mail also allows you to send password-protected emails to anyone: https://proton.me/support/open-password-protected-emails .
PGP is its own protocol though, right? Meaning it is an optional addition to email and not part of the email spec at all?
If you want end to end encryption, both ends can just run their own email servers.