What is the correct way to respond to a professor's greeting in email?

[Camille]

I always start my emails to professors: "Dear Professor X,"

But when they respond, I always get a more laid back, informal "Hi Camille," which is fine because they have the authority to do that.

Here's my question: When I have to respond back to their response, do I stick with my formal and respectful "Dear professor X" greeting, or is it now acceptable to begin "Hi (professor's first name)" since they have already moderated the tone of the email?

 

[harwarda]

No, you cannot start greeting them by their first names, unless they have specifically said, "Call me Chuck". But you could drop the very formal greeting. Email doesn't alway open with a "Dear...." Just say, "Thanks for answer me so quickly. I have another question...."