Question : Prevent users from sending external mail on Exchange 2003

I have MS Server 2003 SP2 with exchange 2003 and AD 2003

I want to create a policy to prevent all users from sending external mail and allow them send only internal mail with in themselves.

and I need to create a group (a white list) of users who are allowed to send external mail such as managers and executives

Answer : Prevent users from sending external mail on Exchange 2003

Refer this article same as above but with images:
http://www.msexchange.org/tutorials/MF009.html