Restricting Users from sending external emails

We just recently upgraded our mail server from Exchange 5.5 to Exchange 2003. It works just fine except that unlike in Exchange 5.5, SMTP is enabled by default in Exchange 2003 — w/c automatically gives my users permission to send emails to contacts outside my network.

In Exch5.5, if I want to restrict my users in sending external emails, I’ll just have to remove their SMTP, but now in 2003 I cannot do so bec. it’s configured by default.

How do I restrict them?

Please help.