Exchange 2K w/ int & ext domains

My company uses a outside email server, using the same domain name as are internal AD domain. Only a hand full of people are now going to use Exchange for there mail. I have set up outlook to download there pop mail (ext) and there exchange mail. The problem is when a Exchange client mail to ext emails with the same doamin name the exchange server kicks back the mail w/ “Unknown User”. I know why this happens but Im looking for a solution so exchange users can email other personal in the company that are not using exchange.