exchange server mail box for users from other domain

Am having a mail server(not exchange) out side my office. all users are having mail id “username@domain.com”. I installed exchange server 2003 in my office.it is integrated with ADS and to log in to mail box all users should use “sername@domain.local” I used popbeamer to download mails from the mail server and put in the mailbox of exchange server. mails are routed to the mail box. but to access the mail i need to log in using username@domain.local. How can i log in using the mailID ?