Cannot send mails from user in Exchange 2010 to user in Exchange 2003 using Outlook 2007 (it works with Outlook 2010 beta)

We currently have a mixed environment with one Exchange 2010 and one Exchange 2003.  When I use Outlook 2007 signed in as a user in Exchange 2010, I cannot send emails to a user in Exchange 2003 server.  Exchange 2003 responds back as "Undeliverable".  I use the Message Tracking Center in Exchange 2003, and I can see the message goes all the way to: SMTP:Message Delivered Locally to <user email address>, then the next line it says: SMTP:Non-Delivered Report (NDR) Generated.  With the same user sign-in Exchange 2010 using Outlook 2010 beta or owa of the CAS 2010 , I can send emails to a user in Exchange 2003 without problem.  I examine the Message Tracking Center for this mail flow, and I can see SMTP Store Driver: Message Delivered Locally to Store to <user name> in stead of NDR.  What cause the problem between Outlook 2007 and Outlook 2010? and how to fix this issue?

Thanks,

KV