I agree with Anna, Andrew and Stefano.
It is best to ask how the person prefers to be addressed, and then just go with that. In writing, "Ms." is acceptable unless you find out they prefer something else.
Depending on the region in the US, there are different expectations in both written and spoken English. For example, in the Midwest and Northeast, you might address a woman as "Ms. Smith" initially until you find our her preference of how she wishes to be addressed. She may tell you to call her "Mrs. Smith", "Ms. Smith", or "Jane".
In the Southeast, when asked, an older woman might prefer you call her "Miss Jane" over the other choices. You would probably WRITE Ms. Jane, but the pronunciation is "Miss". I am regularly called "Miss Verna". This is an established way to show respect in this region. I use this to show respect to older women that I meet also. If we become friends or if they insist, I drop the "Ms" and call them by their first name.