Let Smith be the last name of a U.S.-grown-up professor you not only have taken part in whose class but also ended up becoming a friend of whom in a general sense; I wonder if it still sounds good if in an email to him one uses "Dear Prof. Smith" to address him. I am not really sure, given this condition, if using "Dear Prof." to begin an email to him would somehow become a tone that he does not like? But, on the other hand, I am afraid using, say "Hi Prof. Smith", would sound like one is exalting himself to the place at which Smith stands as a professional scholar.
The title of this question may not be precise (I am not an English native speaker); please feel free to edit it.