Exchange 2003 OWA Referral through IIS to Exchange 2007 OWA

Okay, I've got a tricky one for you Experts. Background:

I have a front end Exchange 2003 Server running on Server 2003. It's running IIS serving up a forward facing, password protected website acting as a portal page for customers. When you login to this page (using Basic Auth), you have a link on the "Intranet" to connect to your Exchange OWA account sitting on the same box. Follow me so far?

We have recently introduced an Exchange 2007 on a 2008 SP2 server into the network, and are planning to migrate to it. Right now it's living as the backend server as we slowly migrate the customers mailboxes onto it.

So I have noticed that when I go to the "Intranet" internally (behind the network firewall) and login using a mailbox account that has been moved to the new Exchange server, when I click on the email link (which is really just www.mycompany.com/exhange) - it will refer me to the 2007 OWA. It asks me twice to reenter my user / pass, but then it gives me my OWA box. The logging in twice is fine, but the weird thing is that when I hit this same exact page externally (Outside the corporate firewall) it won't refer me to the new page. The page just come's up not found.

I have setup external DNS to reflect the correct names, and I can hit OWA on that same Exchange 2007 server fine if I use the direct link to it, but I would like to be able to have customers be redirected to the new OWA during the transition over. I believe this problem is related to DNS, but I can't be sure.

Does anyone have any ideas?

Thanks!