Email forwarding might be a security issue for your domain and may not be allowed.
An admin will be able to view all accounts on their domain, where ‘Forwarding’ is enabled.
In GAT+ , select Users then Email info.
The user has an auto-forwarding address set in their Gmail Settings and/or they have filters created that forward specific emails.
If you would like to take an action and remove the forwarding, an Admin can request Delegated access to the persons email and remove the automatic forwarding from the user account.
Note: Delegate access to Gmail can be found here
Removal and adding of forwarding address can be achieved from the GMail settings: