Using Ms. instead of Miss or Mrs. has become the norm in business. I've always thought we use it not only because we don't know a woman's marriage status, but also because ultimately it doesn't matter. At some point, it becomes a little weird and maybe even insulting to call an adult woman "miss."
Lo and behold, it turns out that Ms. originated not just to prevent the awkwardness of assuming a woman is married, but also to prevent the egregious insult of addressing married women with the "inferior title Miss." Visual Thesaurus traced the term's origin back to 1901 when it made its debut in Massachusetts's Springfield Sunday Republican, which argued for its use with this logic:
There is a void in the English language, which with some diffidence, we undertake to fill. Every one has been put in an embarrassing position by ignorance of the status of some women. To call a maiden Mrs. is only a shade worse than to insult a matron with the inferior title Miss. . . . Now, clearly, what is needed is a more comprehensive term which does homage to the sex without expressing any views to their domestic situation and what could be simpler or more logical than the retention of what the two doubtful terms have in common.