Tradition held that a married woman should use the title Mrs. only in conjunction with her husband's name, not her own—"Mrs. Arthur Reynolds" rather than "Mrs. Susan Reynolds." A divorced woman used Mrs. followed by her maiden name and former husband's last name: "Mrs. Hughes Reynolds."
But societal changes gradually made this practice seem a relic from another time. Today it is acceptable for both married and divorced women to be referred to by their first names after the title Mrs., as in "Mrs. Susan Reynolds."
A married woman can choose to be addressed as either "Mrs. Susan Reynolds" or "Mrs. Arthur Reynolds."
In the case of a divorced woman, "Mrs. Arthur Reynolds" is no longer an option. If she retains her former husband's last name (and many women do so that their surname will be the same as their children's) then Mrs. [or Ms.] Susan Reynolds is correct. If she reverts to her maiden name, Ms. is the correct title, as in "Ms. Susan Hughes."
For more information please see Addressing Correspondence.