<p>he signed off as his first name, so do i start my letter with that?</p>
<p>Is this e-mail?
Have you communicated back and forth?
A guy who interviewed me repeatedly signed his e-mails with his first name.
When I met him for the first time, I called him "Mr. ..."
He insisted, "Please, call me ..."</p>
<p>So, yes, it's a good idea to call your interviewer whatever he or she wants to be called.</p>
<p>Always start letters, emails with Mr., Ms., or Mrs., until instructed by him/her to use the familiar first name. Same with face to face meetings. It is a sign of respect and etiquette.</p>