Your network contains an Active Directory forest named adatum.local. The forest contains 500 users and uses adatum.com as a UPN suffix.
You deploy a Microsoft 365 tenant.
You implement directory synchronization and sync only 50 support users.
You discover that five of the synchronized users have usernames that use a UPN suffix of onmicrosoft.com.
You need to ensure that all synchronized identities retain the UPN set in their on-premises user account.
What should you do?
A . From the Microsoft 365 admin center, add adatum.com as a custom domain name.
B . From Windows PowerShell, run the Set-ADDomain CAllowedDNSSuffixes adatum.com command.
C . From Active Directory Users and Computers, modify the UPN suffix of the five user accounts.
D . From the Microsoft 365 admin center, add adatum.local as a custom domain name.
Answer: C
Explanation:
The question states that only five of the synchronized users have usernames that use a UPN suffix of onmicrosoft.com. Therefore the other 45 users have the correct UPN suffix. This tells us that the adatum.com domain has already been added to Office 365 as a custom domain.
The forest is named adatum.local and uses adatum.com as a UPN suffix. User accounts in the domain will have adatum.local as their default UPN suffix. To use adatum.com as the UPN suffix, each user account will need to be configured to use adatum.com as the UPN suffix.
Any synchronized user account that has adatum.local as a UPN suffix will be configured to use a UPN suffix of onmicrosoft.com because adatum.local cannot be added to Office 365 as a custom domain. Therefore, the reason that the five synchronized users have usernames with a UPN suffix of onmicrosoft.com is because their accounts were not configured to use the UPN suffix of contoso.com.
Reference: https://docs.microsoft.com/en-us/office365/enterprise/prepare-a-non-routable-domain-for-directory-synchronization