Office apps (Outlook) suggests the wrong username when logging in - change AD UPN
Date Added: 5/26/2021

When you install Microsoft Office and run Outlook for the first time, and start to add a new email account, Outlook will automatically prepopulate the email address for the user.  That email address comes from the "Email" field on their user account in Active Directory.  Then, Outlook will ask for their username and password.  That username prepopulates from their "userPrincipalName" field on their user account in Active Directory.  If your UPN does not match your email account's username (typically your email address), then the user will have to manually edit that username field (when creating their Outlook account) to change it from their UPN to their correct username.

We can fix this so the username prepopulates with their email address.

Disclaimer:
Before following these instructions to change your user accounts' UPN values, make sure no other programs in your domain are using that UPN value, or else those programs could malfunction when the UPN changes.  Make sure you understand what you are doing.  These notes are recorded for my own use, and I am not encouraging anyone else to try them.

Steps:
1) See if the correct UPN domain (your email address's domain) is already available in the dropdown in the Account tab on the user account in AD.  If it is, skip to step 4.

2) Open Active Directory Domains and Trusts, right click on the root element, and choose Properties.

3) Add the correct UPN (your email address's domain) to the list, and click OK.

4) In Active Directory Users and Computers, open the user account you want to edit, go to the Account tab, and change the "User logon name" dropdown to the correct UPN that matches the user's email address.  Then click OK.

5) Repeat step 4 for each user account you want to edit.

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