Symptoms
Group metrics generation doesn't finish in a multi-domain environment. Therefore, the msExchGroupMemberCount and msExchGroupExternalMemberCount attributes are not updated in some Distribution Lists (DLs). This could create a security vulnerability because these attributes are used to determine which DLs have external users.
Resolution
To fix this issue, install Cumulative Update 13 or a later cumulative update for Exchange Server 2019.
