Took me a while to figure this one out, so I thought I would help anyone out who is having a similar issue. The symptom is an inability to send mail from Exchange 2003 to Exchange 2007 mailboxes, with properly operating routing group connectors. Mail will flow from Exchange 2007 to Exchange 2003 with no problem, and from Exchange 2003 and 2007 to the Internet The best I can figure out as the cause of this issue, is when I used the instructions to move a Exchange server role from one physical machine to another, while maintaining the same computer name. Everything appeared to go normally. I did the setup.com /recoverserver to restore the server. I don't know if this was the cause as we didn't have the system live in the previous environment and had not tested mail flow. The server moved was a Mailbox server only. The Hub Transport and Client Access roles were housed on another server, which was also moved similiarly, however it did not have this problem. When I turned up logging on the routing in Exchange 2003, the following event was present: Event Type: WarningEvent Source: MSExchangeTransportEvent Category: Routing Engine/Service Event ID: 960Date: 11/22/2007Time: 5:22:54 PMUser: N/AComputer: EXCHANGE1Description:When trying to obtain routing related info about the following server, the server doesn't appear to be a member of any routing group. Legitimately, this could happen if the server is non-mail related server (for example an IM server). Otherwise, check your DS configuration for any corruption in routing group membership attribute. <CN=EXCHANGE2,CN=Servers,CN=Exchange Administrative Group (FYDIBOHF23SPDLT),CN=Administrative Groups,CN=Domain,CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=domain,DC=com>For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp. After a couple of weeks of digging, I found the attribute in ADSI edit under the Exchange 2007 routing group for msExchRoutingGroupMembersBL. I noticed that it did not have the Exchange2 server added into it. I incorrectly attempted to modify this here and add the Exchange 2007 Mailbox server. This popped back an error stating that it was a system owned attribute and could not be modified. After further digging I found a technet article stating to add the DN of the Routing Group to the msExchHomeRoutingGroup attribute on the Exchange2 server and wait for replication. This is what resolved the issue. TechNet article for reference: http://technet.microsoft.com/en-us/library/bb267020.aspx I believe this is a bug with the RecoverServer function of the Exchange 2007 setup, but am not sure as I don't have the resources to attempt to replicate on my end.