I am new to CPanel and recently configured it on a RedHat 5 Server at Rackspace. Everything is working great. I have a customer who hosts his website on this. He recently added a subdomain for test purposes. The subdomain shares the same static IP address as the production site. The website uses PHP scripts to send e-mail. He would like to block the test website from sending outbound e-mail. What is the easiest way to accomplish this?

www.domain.com - allowed to send e-mail through PHP script
test.domain.com - not allowed to send e-mail through PHP script

Incoming e-mail for their domain is hosted on a different server on a different network.

Jim