Miss: This title indicates an unmarried woman.
Although “Miss” can refer to an unmarried woman of any age–Miss Bennet could be age 3 or age 93–many single women dislike the title because it emphasizes their unmarried status. Therefore, “Miss” is the least useful title for adult women, especially in a business context.
Mrs.: This title identifies a married woman. Pronounce it “MISS-uz.”
Ms.: This title, pronounced “MIZZ” and invented in the 20th century, does not specify a woman’s marital status.
You can use “Ms.” for any adult woman–married or not. “Ms.” is useful if you don’t know if a woman is married, or if you do not want to call attention to her marital status.
Ms. is the best title to use for all women, especially in a professional setting.
Use all titles with a woman’s last/family name, not her first/given name (Ms. Holmes, not Ms. Elizabeth).
By the way, “Mr.” is always appropriate for adult men, no matter their marital status.
Most Americans don’t care for titles at all! Many consider them to be formal, pretentious and unnecessary. So once they are friends with Ms. Elizabeth Holmes, they will probably soon be calling her Elizabeth–or even Beth, Betty, Liz or Lizzie–or some other nickname that she prefers.