Routing email thru pop3 on exchange clients

Let me explain my exchange setup then I will get to my question. I have Exchange 2003 setup to handle internal messaging, shared calendars and contact lists, etc. However, internet email is handled by pop3 on the Outlook clients. So what I have is a pop3 acct on each client to handle internet email and an exchange acct. to handle the internal calendars and such. All clients use the exchange mailbox so that everything is stored on the server and gets backed up each night. The reason for this was to avoid relaying and the issues that go with it. All AD user accts are set to use the internet email addresses rather than the internal domain addresses so that the display and reply addresses are correct when they send internet email. When they send internet email it is routed through the pop3 connection unless it is to someone on the local network whose email address is setup in an AD user acct, in which case it is routed thru the exchange server locally. This is where my problem lies, I need all email to route thru the pop3 acct. Is there a way to stop exchange from routing any email without affecting the shared calenders, contacts and such?