To avoid certain user from creating a rule to automatically forward messages to their external email address is by removing the 'send copy to|B' field from the action field in the mail template.
This can be done by editing the mail template in the designer client.
1. Make a copy of the mail template (mail85.ntf) and change the name as mail85_test.ntf
2. Open the mail template(mail85_test.ntf) in designer Client
3. Form folder >> Open the 'RulesDlg' form
4. Double click on action field
5. Click on second tab 'Control'
6. Under the choices, delete the option 'send copy to|B'
7. After deleting the option, an empty space still shows. Press delete button again to remove blank space.
8. Press Esc on Keyboard and click on Save button to save the changes.
Open the user mail file in notes client and replace the design with Mail Template (mail85_test.ntf). Once the design replace is completed, user will not be able to see "send copy to" action field.
Therefore , we can prevent users from forwarding mails using a send copy to by making changes in mail template.