Hi all.

I've sent an e-mail message to a professor, and he replied me saying "Dear X" and signing his name only (e.g. "Kind regards, Frank"). Now, do you think I can reply to this letter saying "Dear Frank", or am I to reply like "Dear Prof. X"?

Thank you.
I think if you write "Dear Prof. X" to a professor, you're never wrong!

Welcome to English Forums, by the way Emotion: smile
Thanks, Francesca!

I thought it could be too formal or inappropriate, though, to reply him "Dear Prof. X", as he closed his letter signing his name only.
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If you feel you must ask, then use his title. You obviously are not comfortable with being informal.