SMTP with Exchange Server 5.5

I have an Exchange 5.5 server running on my LAN, all users were sending/receiving mail thru this. We know are using an internet mail account to receive mail and would also like to send mail thru this service. Is there a way to for a user to send mail using just their internet mail account while still having the exchange service still installed on their machine. I would like to utilize the global address list for exchange but don’t want to use it to send or receive mail.