When addressing an envelope to a widow, you will want to maintain proper etiquette while affirming the woman's status as the beloved wife of a deceased husband. Some widows will not feel offended if the manner in which the envelope addresses them does not take their deceased husband into account, but others, especially new widows or elderly widows, might. If you have occasion to address a letter to a wife whose husband has departed, stay aware of possible sensitivities by preparing the envelope according to standard etiquette.
Address an envelope to a widow by continuing to refer to her as she has been addressed in the past -- for example, Mrs. Jack Grishom.
Note the manner in which the widow signs her name as time passes. If she begins to use her own name, for example: Mrs. Susan Grishom, follow suit when you address an envelope to her.
Continue to follow the standard manner of addressing the woman as you did before her husband died regarding use of "Mrs." or "Ms." If the woman always preferred the salutation of "Ms.," continue to use that form of address. If she preferred the title of "Mrs.," address future correspondence in that manner.