The default UPN is contained in the Canonical Name attribute on the Partitions container object in the configuration naming context. The default UPN suffix identifies the domain in which the user account is contained. When you create a user account in Active Directory, the default UPN suffix is the DNS name of the first domain in your domain tree.
If you create user accounts by using Active Directory Users and Computers, every user must have a UPN.
Here is a simple command to list the UPN Suffixes of an AD Forest:
Get-adforest | select UPNSuffixes -ExpandProperty UPNSuffixes