When addressing an envelope to someone widowed, it is most common to use Mrs. followed by the husband's full name, unless the widow has indicated another preference. The writer could address the envelope with Mrs. or Ms., followed by the woman's first and last names. This latter form of addressing is common for women who used their maiden names during marriage and women who are younger.
On the other hand, a person would not address a divorced woman with her husband's name. Instead, Mrs. or Ms. followed by the women's first and last name, married or maiden, would be appropriate.