At my employer, some users register with a pm.me email address. When users contact us via email, they use a different protonmail address and never the pm.me address. I know you can never rely on the sender address, but from a support perspective this is still strange and extremely troublesome (for example it is tedious to assign the user to an account). I assume that Protonmail users can only receive emails with pm.me, but not send them. For me actually a reason to block pm.me or to handle it like a throwaway address.