ProtonMail mangles emails in a way that they are impossible to download as they were delivered to their servers, meaning MIME signatures cannot be verified, because the emails are altered.

Old issue, affects also web UI not only bridge:

https://web.archive.org/web/20240221213617/https://github.co...

Would you use a secure email service that breaks signature verification on received emails? There were a lot of smaller and bigger things along the years, but realizing I could not verify authenticity of signed emails for apparently no reason pushed me over threshold to leave them.