Outlook configured with Exchange and POP3 accounts

I have an Exchange server running on a local server that we no longer want to send and receive external email but still host .pst files for Outlook Web Access. We have an external POP3 to send/receive external mail. The problem is when an address is picked from the global address list (exchange), Outlook sends the outgoing mail to the POP and the address in not valid and fails. any suggestions?