Email Etiquette

Customer service situation: Person emails me to change their email address and I am responding. The name on the account says Billy. The name on the email header says Bill. He did not make a salutation in the email request. What name do I use in the greeting of my email?

In retrospect, I could have gone for the formal last name. However, in this circumstance, I chose Bill, since that is the name that the email referred to.