The ability to restrict email delivery is available in Google Apps for Business and Education Edition.
By default, users with Gmail accounts at your domain can send mail to and receive mail from any other email address. In some cases, you want to restrict who your users can exchange mail with. For example, a school might want to allow its students to exchange mail with the faculty and other students, but not with people outside of the school.
For More Details & Instructions check the below link:
http://www.google.com/support/a/bin/answer.py?answer=177482&hl=en