The formalities of addressing people in letters can become quite confusing, especially if you frequently send letters to different people. While some degrees, such as a doctorate degree, mandate that a title is added before the person's name, that isn't always the case if you're addressing someone with a master's degree. A master's degree by itself doesn't require a special title, but it sometimes does require a post-nominal.
Write the person's full name, followed by the post-nominal "M.A." on the front of the envelope, if you are placing the letter in an envelope. For example, to address John Doe, you would write "John Doe, M.A." If the master degree is integral to the person's job, such as a master's degree of science, then use the full post-nominal for that degree, such as "John Doe, M.S."
Write "Dear" on the first page of the letter, followed by the person's title and the person's last name. For example, to address professor Jane Lane, you would write "Dear Professor Lane:" or "Dear Professor Lane," and then begin writing the body of the letter.
Add only the person's prefix and full name if the person does not have a title. For example, to address a business worker who holds a master's degree but no title, you would write "Dear Mr. John Doe," or simply "Dear John," if you are addressing the person in a non-formal manner.