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  <-> [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?