@Jim B When you create a tenant in Azure AD you get a default address like contoso.onmicrosoft.com
As per business need, the organizations are required to verify their corporate domain under this tenant.
For example if your corp domain is tailspintoys.com you need verify this under your default tenant and at the same time you need to add this tailspintoys.com as a UPN Suffix to your on-prem environment.
Post that you need to make sure to change all the user's UPN to tailspintoys.com in On-prem so that when they sync they can still have that upn associated with them.
Important thing to note is : If you either don’t verify the tailspintoys.com domain on cloud or do not change the UPN to this at on-prem, the users will get contoso.onmicrosoft.com
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