How to address a user in an email?

We would like to send users of our service an email after they have availed our service. We get the bookings through online website or through offline medium. We have the users name ( just first name or full name) and the email. But, we would like to know how to address a user in the email like "Dear Bill" or "Dear Mr Clinton". So, we are designing a form to get user's input. Have you seen a UI for such a scenario or can give some suggestions ? Is there any best practice or research you can share?