My gmail and wife's gmail have gotten mixed up again. Can you tell me how to fix?
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In regard to your gmail issue: The mix up could be happening by a: not understanding the difference between the "Inbox" and the separate mailboxes in Mail. When Mail.app has multiple email accounts, there is a risk of a person accidentally sending an email from the wrong address. When sending an email, always check the outgoing email address in the email prior to hitting send.

I prefer to never click on the Inbox at the top. Doing so means that you're looking at all email accounts in one box. I prefer to select the mailboxes below individually: