Etiquette in writing emails/letters/ addressing people as a whole.?

[gym]

Right, I had written to someone (Dr. John Smith) I didnt know regarding further information about something so I addressed them as Dr. Smith , they replied back to me saying:

Sincerely,
John

SO, do I write back, Dear John? Or do I continue writing Dear Dr. Smith?
WHY do i use his correct title if he signed off as John? Isnt that what he wants to be known as?? when i sign off a letter i say what i want to be known as...?
oh ok so one shouldnt use titles in signatures, but that doesnt stop him using his last name in a signature to indicate its more formal... and every doctor i know uses Dr. in their signature!! in every letter you sign formally you sign it with your full name!