Whether meeting a superior in person, calling him on the phone or writing him a letter, knowing how to address someone with authority over you is an important part of business etiquette. While simply knowing the sex of the individual in question and using "Mr." or "Mrs." is generally acceptable, going the extra mile and learning the appropriate protocol for the situation will help you make the best impression possible.
Greet your superior in person or on the phone using "Mr." or "Mrs." and his or her last name until you are explicitly given permission to use the person's first name. If you know a more appropriate honorific, such as "Dr.," use this instead.
Use the same prefix or honorific when greeting your superior in a letter or email. For example, "Dear Mr. Montgomery."
Address a superior on the front of a mailed envelope using the prefix or honorific and her full name. For example: "Ms. Margaret Montgomery."