We have a working hybrid Exchange Online setup which is working fine for all users with our original email address. We recently purchased and new email domain and want to move two users to that domain. The problem is, after changing the user's UPN to the new domain for sign in, the user is constantly getting prompted for user credentials in Outlook.
What has been done:
New email domain has been properly setup in Azure
Email domain added as an additional UPN in Active Directory
Users UPN has been changed to the new UPN
User default email address has been changed to the new domain
Appropriate MX and DNS records have been created
Is there anything else in Active Directory that I should be doing that I'm missing?