A problem many companies face is users who automatically forward all their e-mails to external addresses. The easiest way to prevent this is to use the Set-RemoteDomain cmdlet which is used to configure a managed connection for a remote domain. In this case, we will configure the Default Remote Domain (with an address space of “*”) which is the remote domain related to the Internet (if you changed its name or created a different one, please update the code):
Set-RemoteDomain Default –AutoForwardEnabled $False
Please note, however, that this will block any messages that are auto-forwarded by client e-mail programs in your organization from being sent to the internet, for everyone!