I emailed Professor X regarding typos in, and questions on, his book. I've never met and contacted him. My email commenced with this salutation:
Dear Prof. X
It is customary to address professors by their title in full, just as most people would not abbreviate the full names of people whom they do not know well.
His unpleasantness startled me. Is he correct though? If he is, wouldn't we have to write Mr. in full as Mister or even Master?
My response to comments:
I'd rather not impart more particulars on Prof. X, in case he persecutes me! I'll just say that he's Caucasian, and in Australasia, North America or UK. English is definitely his native language.