Hi Hernan. If you'd like to restrict outbound access to certain employees, then do the following:
1. Log in as the Sys Admin, select the domain and click the edit button. Click the Features tab and make sure that the "Control of Service Access" feature is enabled.
2. Click the Manage button for the domain, and go to Settings >> Domain Settings >> Users.
3. Edit the user in question, and select the Service Access tab.
4. Uncheck the "Enable outgoing SMTP access" then click Save.
This user will now only be able to send email locally. All external outbound mail attempts will bounce back to the user with a 550 not allowed. I hope this helps.