Before anyone jumps ship on ProtonMail because they didn't read past the headline, by "Rewrites Your Emails, the author means that ProtonMail doesn't support user-applied PGP signatures because of the way they've decided to architect their automatic signing and encryption system. ProtonMail isn't rewriting outbound emails in the sense that the average person is probably thinking. If you're part of the 99.9% who doesn't sign their emails or verify signatures, this issue is unlikely to actually matter to you.
That said, Proton's response to this issue is joke.
That said, Proton's response to this issue is joke.
https://github.com/ProtonMail/proton-bridge/issues/26