You can use the redirect file to stop users sending mail to any specific external domain. To do this go to the Incoming > Redirect page and add the following rule to the redirect file.
|Remote IP Address||1.2.3.*|
|Local IP Address||*|
|Action||Protocol Refuse with…|
|Parameter||500 Mail to external.com disallowed|
In the above example the Remote IP Address is the same IP range that you have specified under Security > Local IP
The domain that you would like to stop your local users emailing is external.com
Keywords:Stop, Filter, Redirect, Send, sending, mail outgoing