Exchange server 2003 currently supports Domino, Exchange 5.5 and I think Lotus note migration only.
What exactly would you like to migrate?
If it is only the mails, then you can use POP3 connector (if you’re running SBS, if not you may get others like: http://exchangepop3.com and so on.
Then all you need do is pull all mails from your Redhat mail to exchange using POP3.
Before you commence though, you need to create users and their respective e-mails using your Exchange server Active Directory mmc. You’ll find it under programs-exchange-active directory.
Just an advice, why not try ZImbra Mail. It works on Linux, and has virtually all features that Exchange has. It also includes IM, a feature that is missing in exchange 2003.