I am really not sure about when it is appropriate to use someone's first name in business emails. If someone addresses me by my first name, but signs his email officially — full name, plus title, department and contact information —, should I address him by first name or still by "Mr. X"?
I agree that you should address someone in email the same way you would in real life. Imagine if you saw this person at a party. Would you say, "Hey, Ted. Nice to see you," or "Good evening, Mr. Williams. It's nice to see you"?