You should be able to accomplish this by using the Content Filtering rules. Login as domain administrator, click on Filtering, then Content Filtering. Create a new rule that looks at the To: address field. On the next screen click the drop down to set the condition as "Does not match" and then enter your domain in the text box. On the next screen set the response that you want if someone tries to send a mail to any domain other than the one you want to allow...Delete, Bounce, etc.
This rule should do what you want, but test it out well if possible before rolling it into production. Hope this helps.