Q&A: Invitations: Addressing One to a Professor?
How do you address a person who is a Ph.D.? Is it Dr. Mark Donovan or Mr. Mark Donovan, Ph.D.? I haven't seen this question answered in any books or magazines.
For the outer envelope, you can use Mark Donovan, Ph.D. (no Mr.), Doctor Mark Donovan (though this title is usually used only by medical doctors), or Professor Mark Donovan. For the inner, use Dr. Donovan or Professor Donovan.