Domain/Exchange user mismatch

I ran across an interesting issue today with a user that was unable to send e-mail after having changed their password. I ran through the normal troubleshooting steps, nothing significant appeared in their Event Viewer logs for Outlook, and their other e-mail account worked just fine. I started by checking connectivity to the Exchange server (which was good), flushed their dns cache with ipconfig /flushdns, but that wasn’t the issue. They were several months behind on updates and were missing a feature update to Windows 10 ver 1909, but after rebooting and deploying the updates nothing changed. I ran scanpst.exe on their PST file, no issues. I renamed/moved the PST file to force it to be rebuilt and that’s when we lost the ability to authenticate their old account entirely. At this point I removed and re-added their entire Outlook Profile, ran a program-repair on Outlook (and the rest of Office 2016), again, same issue, BUT their other e-mail account could reach the Exchange server if setup independently without any problems. I than restarted the Exchange Server entirely, reviewed what I could in the Exchange Management Console, and was about to give up when I noticed that their domain user name (which we will say is “springfield\lsimpson”) was different from their e-mail address (lisa@springfield.com), and the Outlook e-mail account authentication was trying to authenticate with the other e-mail/delegated e-mail account user name when it NEEDED the domain user name and password. Just goes to show: always question the obvious.