I was wondering what the norms are regarding whether to use people's first or last name. This is a mix between me not knowing what the norms are in English-speaking countries and not knowing what the norm is inside the EA community. I get the impression that first name is generally the way to go, but I'm very unsure about this. Here's an example of what I mean. 
Let's say I wanted to write an email to Rob Wiblin. I've never interacted with him (except maybe twitter replies), but I've listened to all the 80,000 hours episodes and feel like I know him. He also seems pretty down to Earth and approachable. Is "Hi Rob" appropriate? How about anyone else on the 80,000 hours team? "Hi Michelle" or "Hi Dr. Hutchinson"? 
If I were to write to Marc Lipsitch (who's been a guest on the 80k podcast, but is a Professor at Harvard), I would know to be formal (edit: not so sure anymore that this would be the right call), but for all the cases in between, I'm not sure.

Of course this is not about whether those people in particular would mind me calling them by their first name, but rather about what the right policy is in general.

I know that the standard advice is to use the last name and see whether they sign off with their first or something, but often this feels unnecessarily uptight. Any feedback is greatly appreciated!