Restrict users from forwarding mail to external address

I would like to restrict our users from being able to forward mail to an external address using mail rules. but they still need to be able to forward mail to internal addresses. Is there anyway to do this?

Subject: Server configuration document

You can achieve this using the Server Configuration document. I think it’s under Router/SMTP - Restrictions tab or somewhere along those lines. You can restrict the incoming and outgoing email separately.

Hope this helps.

Sidi