Configuring cloud directory lookup priority
If this list is empty or a UPN suffix doesn't match any suffixes in this list, the service looks for the UPN in Active Directory first; if the user isn't in Acitve Directory, the service then looks for the UPN in the cloud directory.
For example, if the affected uses are alex@acme.com and joe@foo.com, then you would enter "acme.com, foo.com" in this field.