Mail sender chosen unpredictably

MacOS Mail seems to choose the sender unpredictably. My wife and I have one address each on our ISPs email service. Both addresses appear in our Mail window. This morning I sent a message that ended up going from my wife's address. The ? in the Composing window in Mail Preferences says that it's automatically selecting the best account based on the email address of the first recipient in the message (a Yahoo address) as well as the currently selected mailbox (mine) and message (one of mine). How could it possibly select my wife's address?

Is there a solution to this? My wife has the same thing happen–messages going out under my address–nearly every day, and it's happening too often for me.

MacOS 10.13.3