[otrs] Ms Exchange SMTP

Hi, I've configured my otrs 2.04 like this : One debian server with otrs and postfix as mail system. All users have windows and use MS exchange 2000. To make work email-creating tickets, I've set mail system (Exchange) to check our dns first. I've created a dns zone and a mx record for my otrs server. It worked fine. But today, some users can't send mail to some users. We must deliver directly to the smtp server of our provider. Email creating ticket doesn't work anymore because smtp and dns provider can't resolve my otrs domain. Can you tell how can I setup mail system (postfix) to collaborate with exchange, goal is users send a mail to a mailbox (or a contact) and a ticket is created ? Thanks You ! Olivier