@islamicaudiobooks@pixelcode I wasn't talking about ProtonMail's exclusive PM-to-PM E2EE option (or with a password, as you described), like Tutanota also has its own.
ProtonMail, in contrast to Tutanota, also supports OpenPGP, which is an open standard, which makes it interoperable with any email client that supports it (e.g. Thunderbird). So Thunderbird users can send encrypted emails to ProtonMail users.