My T is an LCSW, and I call her by her first name. Though I've also been seeing her for 4 years, so there's familiarity there. In the very beginning, I referred to her as Ms. [last name]. With my marriage counselor, he's a PhD, so I generally call him Dr. [last name] in e-mails, though occasionally I've used his first name (and then felt all insecure about that!). But if I'm talking to T about him, lately I've just been using his first name. I haven't e-mailed p-doc, but I refer to her as Dr. [last name], partly because I've only been seeing her for 6 months and don't have the same relationship.