Unmarried Madam

Solution 1:

In US English, when, say, a store clerk addresses the next person at the counter, he might say "May I help you, ma'am?" That is, "ma'am" is the female equivalent of "sir" and serves as a stand-in for a person's name when the name is not known. A more formal term, especially if the woman is noticeably older or of higher status than the speaker, is "madam" (though the term carries other connotations and could be taken as an insult if perceived to be used sarcastically). Neither term carries an implication as to marital status.

If the female person is much younger than the speaker, and especially if less than about 18 years old, the term "miss" may be used instead of "ma'am". However, note that it carries a weak implication of being unmarried.

I suspect that both terms (and "sir") are slowly falling into disuse.

Of course, if addressed by one's last name, Miss/Mrs./Ms. would be used, much as it is in written address form: "John, this is Ms. Smith. I told her you could help her with her tax problems."