Old sent email repeatedly appears in Mail app on iMac

I am having issues on the iMac where an old sent email keeps popping up. I am using Yahoo on the Mail app, the email has been deleted on both Mail and the Yahoo mail website. The email itself has been deleted from sent and bin and is not longer able to be found, but it keeps popping up and then disappearing. It has been nearly a year, I have restarted the Mac, rebooted, closed everything and still happens.