Exchange 2007 - 2003 Routing

We have a single Ex2003 server that has served our organization for several years.  We just added a more robust Ex2007 Cluster with two CAS/Hub servers (NLB) and two Mailbox servers (CCR).  The two systems are part of the same Exchange Organization and will be coexisting for the next several months as we transition out the 2003 server.  Lets call the 2007 servers in question CAS1, CAS2 & MB.  The 2003 server will be referred to as 2003. Here is the situation:

Mail from 2003 & 2007 flows to SMTP outbound normally.
Mail from SMTP inbound flows to 2003 & 2007 norrmally.
Mail from 2003 flows to 2007 normally.
Mail from 2007 flows to 2003 normally only when handled by CAS2.  Additionally all inbound mail is sent from the 2003 server to 2007 on CAS2.  

A bidirectional Routing Group Connector (Interop RGC) has been created for [2003] <-> [2007.CAS1, 2007.CAS2]
A call to get-routinggroupconnector returns six connectors, 2003 -> CAS1; 2003 -> CAS2; CAS1 -> 2003; CAS2 -> 2003; Interop RGC; Interop RGC.

The undelivered messages  hang in the Queue (no error)

Any ideas whay mail sent from a 2007 mailbox, handled by CAS/HUB1 is not delivered to the 2003 server, whereas mail handled by CAS/HUB2 is delivered?
PSBITAsked:
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abdulzisCommented:
Any errors in application log on HUB1?
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PSBITAuthor Commented:
One error that I'm not familiar with that could have a bearing.  An unknown error thrown by EXPROX.
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PSBITAuthor Commented:
EXPROX is not the cause and probably not a symptom either.  Just an error thrown by the CAS proxying the 2003 OWA.
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PSBITAuthor Commented:
I removed and recreated all RGCs.  I added a registry to key to Ex2003 to suppress state changes (HKLM\System\CurrentControlSet\Services\RESvc\Parameters; SuppressStateChanges = 0x1).  I ran EXBPA a dozen times from different servers to try to identify any problems.  I demoted a consistently sketchy DC.  I swore and threatened the entire cluster.  Some part of that made it work because after compelteing all those actions and staring menacingly at it RDP session for 30 minutes, the queue suddenly fired up and began delivering mail.  
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PSBITAuthor Commented:
Found a definitve solution after the problem popped up two more times.
The issue seems to be caused by the an inability for the CAS/Hub server to query DNS.  Instead of having the serve automatically select DNS form the NIC, I manually set Internal and External DNS servers (in both cases my corporate DNS servers) and this seems to have fixed the problem immediately.
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