I'm starting a grad school and I'm used to start my emails to all my professors with "Dear Professor Smith", even though they respond with their first name at the end of their emails ("Best, John").
I noticed that the guys I'm starting the studies with are using "Hi John" in their emails. I don't think they know professors better than I do.
I don't want to be too familiar, but neither be the only who keeps this distance. Should I switch to "Hi John"?