Some or all identity references could not be translated.
+ CategoryInfo : NotSpecified: (:) [Set-DatabaseAvailabilityGroup], IdentityNotMappedException
+ FullyQualifiedErrorId : System.Security.Principal.IdentityNotMappedException,Microsoft.Exchange.Management.Syste
About disjoint namespace
In most domain topologies, the primary DNS suffix of the computers in the domain is the same as the DNS domain name. In some cases, you may require these namespaces to be different. This is known as a disjoint namespace.