How do you address a new widow when sending a note of sympathy?

Formality be damned-- I would address it Mrs. ____, unless I have already established a long-time first name basis relationship with her, or I am already aware that she prefers not using Mrs. as an address. The time of sending a sympathy card is NOT the time to be ripping away a recent widow's sense of identity. Many continue using 'Mrs.' throughout the rest of their lives even if they remain unmarried. There is nothing at all wrong with that.