Hi @Hugh O'Keeffe , can you manually update the UPN attribute on AAD? If that still doesn't work follow these troubleshooting steps:
- Verify that the UserPrincipalName (UPN) attribute you fixed on-premises meets the validation requirements of Azure AD.
- Ensure that the on-premises user object has been synchronized to Azure AD after the UPN attribute change. You can force a synchronization using Azure AD Connect.
- Check the Synchronization Service Manager to see if there are any descriptive errors on the object.
- If the issue persists, consider reviewing other attributes that might be causing the validation failure, such as the MailNickName attribute.
Please let me know if you have any questions and I can help you further.
If this answer helps you please mark "Accept Answer" so other users can reference it.
Thank you,
James