It was happening on the NYC subway some years back, people were poking people with syringes contaminated with HIV

Austria lol. Mozart, not kangaroos.

They can block the domain at the national level though. People that use their own domains with proton’s service might be fine and pass through, but it would likely be pretty easy to simply filter out all of proton’s own domains (@proton.me, @protonmail.com, @pm.me, @proton mail.ch) instead of trying to block the underlying protocols.