Hello, @mpls,
Welcome to the Microsoft Q&A platform!
You mention the user has been assigned a valid Exchange Online license, it is recommended for you to run the following PowerShell command to check the user's authentication status:
Get-MsolUser -UserPrincipalName <UserPrincipalName> | fl ValidationStatus,UsageLocation,*error*
If the ValidationStatus
is not healthy, there might be sync errors in the synchronization process.
After checking that the user's authentication status is correct, as Amit Singh mentioned, mailbox configuration can take up to 24 hours, especially in mixed environments. During this time you can also use the diagnostic tools in the Microsoft 365 Admin Center to identify and resolve issues related to mailbox configuration according to Delays in provisioning a user or mailbox - Exchange | Microsoft Learn.
Should you need more help on this, you can feel free to post back.
If the answer is helpful, please click on “Accept answer” as it could help other members of the Microsoft Q&A community who have similar questions and are looking for solutions.
Thank you for your support and understanding.
Best Wishes,
Alex Zhang