Let's assume that I've contacted professors in the United States and referred to them appropriately as "Dr. Smith" or "Professor Smith" based on their having attained a Ph.D.
If they respond to me informally, should I continue to address them formally? If so, for how long?
I may be unaccustomed with the customs at US universities, but if someone addresses me "informally", i.e. with my first name or "Hey", (that was the point of the question, right?) I will do the same.
Who expects anything else should quit university and become a high school teacher.