In order to integrate your domain with Office 365 you need to make sure that the domain can be verified externally. If for example you have a local domain, for example in my lab scenario its 365demolab.local, this wont be able to be verified by Azure Active Directory so therefore wont be able to be used in Office 365.
One of the ways we can combat this is to add a UPN (User Principle Name) for the domain name that is going to be verified, so in my lab it will 365demolab.com.
We can complete this is a c couple of easy steps.
- On a machine that has the Active Directory Domain servers tools installed, open the “Active Directory Domains and Trusts”.
2. Right Click on “Active Directory Domains and Trusts” and select “Properties”.
3. On the UPN Suffixes tab, type your domain to be verified, for example 365demolab.com and click “Add” the “Apply” and close the window.
To summarise, we have successfully added the domain suffix 365demolab.com as an alternative UPN suffix to Active Directory. The next step is to change the UPN for your users.