You need to resolve the Active Directory issue.

What should you do?

A. Run the IdFix tool then use the Update actions.

B. From Active Directory Domains and Trusts, modify the list of UPN suffixes.
C. From Azure AD Connect, modify the outbound synchronization rule.
D. From Active Directory Users and Computers, select the user accounts and then modify the UPN suffix value.

Explanation:
IdFix is used to perform discovery and remediation of identity objects and their attributes in an on-premises Active Directory environment in preparation for migration to Azure Active Directory. IdFix is intended for the Active Directory administrators responsible for directory synchronization with Azure Active Directory.

Scenario: Active Directory Issue
Several users in humongousinsurance.com have UPNs that contain special characters.
You suspect that some of the characters are unsupported in Azure AD.

References:
https://www.microsoft.com/en-us/download/details.aspx?id=36832