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It ia very much an English speaking world thing, but I wouldn’t say US-centric as we have basically the same greeting in the UK.
Yeah, that wouldn’t fly as much here either - if somebody asked how my day was, I’d be inclined to think they want a summary at least.
I’ve been using protonmail basically since its inception for money-related stuff (due to it being secure), and the one time I’ve had a fraud flag appear while using it was due to being on a VPN at the same time.
… But I’ve had that also happen when I used to daily drive Gmail, so I can’t imagine the Proton part made the difference.
Obviously anecdotes aren’t very good evidence, and maybe in your experience it was your email - but if that is the case, I’d be weary of any provider that automatically flags non-“big tech” addresses as fraudulent. That likely means they’re rather lazy about their cybersecurity.