Check if your sender's email is in your "Junk" folder. Then, right click on the email and select "Mark As Not Junk". Click "Address Book". Make sure that the "Personal Address Book" is highlighted. Click "New Card", the "New Card" window will display. Under the "Contact" tab, copy your sender's "From" address and paste it into the email dialog box.
Google old usernames and email addresses. Check Facebook, Google, and other apps for any connected accounts.; Search saved logins in Chrome (or your preferred browser or password manager).