Adding an alternative UPN suffix is really important part for preparing Active Directory for Office 365. Don’t underestimate this setting if you don’t want to have a trouble.
In order to fix it, login to domain controller
- Open Active Directory Domains and Trusts
- Right click on AD Domains and Trusts then properties
- Select UPN Suffixes tab and type additional UPN suffix for the forest and click Add button. Foer example, my internal system is sonat.int however my external domain is sonatyaylali.com and I will use sonatyaylali.com for office 365. So, this alternative UPN will be sonatyaylali.com
Matching AD UPN suffix and Office 365 UPN suffix
If you have not yet set up Active Directory synchronization, you can skip this task and continue with the next section.
If you have already set up Active Directory synchronization, the user’s UPN for Office 365 may not match the user’s on-premises UPN defined in Active Directory. This can occur when a user was assigned a license before the domain was verified.