I'm writing a thank you note to a Jane Doe that I know pretty well. She's married to John Doe. They are probably in their early 70's. If I know the couple pretty well, especially the woman, I adhere to the old (maybe only Southern) way by addressing the letter to only the woman. When I see this woman in public, I call her "Mrs. Jane" so that's how I start the letter (Dear Mrs. Jane,). However, I'm unsure of how to address the envelope. I'm afraid that "Jane Doe" is too informal, but "Mrs. John Doe" seems too formal. The other option, "Mrs. Jane Doe", seems to be reserved for a widow or divorced woman. Keeping in mind that I'm addressing a very, very Southern old-school woman, can someone help to make me feel comfortable about using "Jane Doe" on the envelope (since I'm pretty casual in the note itself) or tell me their opinion of the difference between "Mrs. Jane Doe" and "Mrs. John Doe".