According to The American Heritage Book of English Usage,

Using Ms. obviates the need for the guesswork involved in figuring out whether to address someone as Mrs. or Miss: you can’t go wrong with Ms. Whether the woman you are addressing is married or unmarried, has changed her name or not, Ms. is always correct. And the beauty of Ms. is that this information becomes irrelevant, as it should be — and as is has always been for men.

According to The American Heritage Book of English Usage,

Using Ms. obviates the need for the guesswork involved in figuring out whether to address someone as Mrs. or Miss: you can’t go wrong with Ms. Whether the woman you are addressing is married or unmarried, has changed her name or not, Ms. is always correct. And the beauty of Ms. is that this information becomes irrelevant, as it should be — and as it has always been for men.