Whether you're sending a sympathy card to a woman who's recently lost her husband, are inviting a longtime widow to an event such as a wedding or just writing a letter, it's important to follow the proper etiquette when you address the envelope. When writing the envelope, you can use a traditional method of address or alter your approach based on the widow's preference.
Traditional Method
According to the Emily Post Institute, a leading etiquette organization, the traditional and customary way to address a widow's envelope when sending her something via mail is to use the prefix "Mrs." followed by her deceased husband's first and last name. For example, for a woman named Mary Jones whose deceased husband was named Robert Jones, you'd address the envelope with "Mrs. Robert Jones."
Other Options
The Emily Post Institute reports that there are a couple alternate ways of addressing an envelope to a widow. Suitable options are to use "Mrs." or "Ms." and them write the widow's first and last name. For example, write "Mrs. Mary Jones" or "Ms. Mary Jones" on the envelope.
Check Her Preference
It's always a best practice to contact the widow before you address the envelope and ask how she wishes to be addressed. For example, soon after her husband's death, she may prefer to use "Mrs. Robert Jones," but in the years that follow, especially if she's begun dating, she might find that "Ms. Mary Jones" feels best. If it's not appropriate to ask the widow, check with a family member to see if she has a preference.
Informal Writing
If you're writing to the widow informally, it's acceptable to simply list her name on the envelope without the use of any prefix. For example, write "Mary Jones" on the top line of the envelope. This method, notes The Protocol School of Washington, is suitable for informal correspondence.