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I always use Ms. I don't differentiate the treatment of men depending on if they are married or not, so I do the same with women. In Spain, to call the equivalent to Mrs to a woman is something quite old-fashioned -usually it comes from seniors- and considered offensive. But could be acceptable for a teenager.
Traditionally (in American English, at least), "Mrs." implies married, and "Miss" implies unmarried. "Ms." (pronounced more like "Mizz") avoids that distinction. It is an appropriate professional address for a female regardless of marital status. Marital status is irrelevant.
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Unfortunately, there's no gender-neutral address, which is a problem when one can't tell from the name whether someone is a "Mr." or a "Ms." -- e.g., native English speakers trying to decode Chinese or Indian names. In those cases, I usually just avoid the "Mr." or "Ms." pronoun and simply use the person's full name. "Dear <insert full name here>, ..."