Miss", "Ms" or "Mrs"

Here is the rule for titles. "Miss" and "Mrs." bear reference to that woman's marital status. If she is not married, then she is "Miss Smith". If she is married, they you would refer to her as "Mrs. Smith". If you don"t know the marital status of your new client, the best would be to use "Ms. Smith", which does not refer to her marital status at all.