How to setup a relay with Exchange 2003?

I have exchange server 2003 setup at present which is fine, however I wish to integrate several users external mail accounts (Gmail etc.) into exchange. I already have a POP3 connector which connects to the external server and retrieves the mail on a predefined interval and that works fine, however I wish to have it so that when the users send E-Mail from their account and it is using the external E-Mail address that instead of sending the E-Mail direct to the end user from the exchange server, it relays through the external providers SMTP host so as to avoid problems with recieving servers running a reverse DNS lookup and then rejecting the E-Mails or flagging them as spam.How to setup a relay with Exchange 2003?
Try Barracuda's mail relay server from Barracuda Networks.

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