Men are easy when it comes to addressing them. Use “Mr.” It doesn’t matter if the man is single, married, divorced, or widowed; use “Mr.”
Addressing women can be more difficult.
“Ms.” is the feminine equivalent of “Mr.” It is accurate if the woman is single, married, divorced, or widowed. It can always be used.
“Miss” refers to an unmarried woman or a girl. Be careful of using this, especially in a professional setting. It can sound condescending or patronizing to address a woman as if she were a girl (not an adult).
“Mrs.” refers to a married woman. “Mrs.” is not appropriate for all married women, however. If the woman has replaced her last name with her husband’s, then “Mrs.” is accurate. If, however, she has kept her last name, then “Ms.” is the accurate address.
For example, when I married I kept my name. (It was the first thing I received when I was born, why should I give it up?) It drives me crazy when people call me Mrs. Farrell when they know my husband’s last name is not Farrell. No, Mrs. Farrell would be my mother. Or a sister-in-law. Or an aunt. To call me “Mrs. Farrell” implies that I married my father, a brother, or an uncle. Yuck!
It can be tricky if the woman is divorced. Did she keep her ex-husband’s name? Did she change it? Did she remarry? What name does she use? If she is a widow, it’s easier. If she took her husband’s name, then she is still “Mrs. xxx.” If she remarries, then it can become tricky again depending upon what name she uses.
When in doubt about how to address a woman, use “Ms.” At least you will be accurate. If she prefers that you call her something else, she’ll tell you.