Addressing Sympathy Card

Dear Limo Princess,
When sending a sympathy card to the wife whose husband has died, how do you address the envelop? Mrs. John Smith? Mrs. Mary Smith? Ms. Mary Smith? or some other??

Dear Martha,
Even though her husband has passed away, she is still Mrs. John Smith--unless she remarries, she will be Mrs. John Smith for the rest of her life. A divorced woman would be Mrs. Mary Smith. I assume she and her husband were friends of yours, my condolences.