Exchange Server 5.5

Hi,
Remote users dialin to thier ISP and pick up their office mail. In the Outlook properties, we set it using our server name for incoming mail and did the same for outgoing. The problem is users are only able to send out to other users in thesame domain(our company) and not able to send out to other users with AOL, compuserve, earthlink, etc..Is there a configuration in Exchange to solve this problem?