Early in my (academic) life, I was told that it is appropriate to address a faculty as Professor only when he/she possesses the full Professorial rank and I would be better off addressing Assistant and Associate Professors as Dr.X, especially in written communication. However, recently I have been corrected that it is appropriate to address as Professor, faculty of all three ranks irrespective of them possessing a doctorate. So, my first question is, whether it is okay to address an assistant (or associate) Professor as Prof.X with regard to American universities.
Second, I have been told that these professorial ranks are often a source of ego and resentment. In previous letters, I have unfortunately written something along the lines of "I am interested in the research work of Prof. X and Dr.Y". I wish to know whether addressing them as such, with the obvious distinction would lead to ego issues, or any problems in general.
Perhaps the second question does not belong on this forum. Still, I would appreciate if someone from academia or otherwise could clear them for me.