This can be achieved by using restriction classes. This allows us to define certain addresses that can only send to certain domains/receipients whilst allowing everyone else to send to any legitemate destination.
Below is a quote from the Postfix website that outlines how to achieve the setup you require:
smtpd_restriction_classes = local_only
check_recipient_access hash:/etc/postfix/local_domains, reject
this.domain OK matches this.domain and subdomains
that.domain OK matches that.domain and subdomains
Let me know if you have any issues or questions with this setup or if it proves ineffective/inappropriate.
**EDIT** Sorry forgot to add the original Postfix page to offer some background on the subject http://www.postfix.org/RESTRICTION_CLASS_README.html