Answer
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.
| MAIL Clause | * |
| Remote IP Address | 1.2.3.* |
| RCPT Clause | external.com |
| 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