Calling someone by their first name

[deacsoft]

I was wondering if you receive a letter or e-mail from someone and they sign it using their first name only if that makes it okay for you to call them by their first name?

Hypothetical example: You write a letter to John Doe. In your letter you refer to him as "Mr. Doe". He writes back and signs it John.

If I were to write back again would it be cool for me to just call him John, as opposed to Mr. Doe?