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I recently installed Exchange 2007 into an organization with multiple Exchange 2003 servers. The new Exchange 2007 server is a member server, not a DC, and was setup with the master Exchange 2003 server as it’s local Bridgehead.

Here is the problem I encountered following setup:

1. I could successfully send email from a mailbox on the Exchange 2007 server to a mailbox on the Exchange 2003 server.
2. I could not send email from a Exchange 2003 server mailbox to a mailbox on the Exchange 2007 server. Messages that are sent sit in the “Messages with an unreachable destination”queue on the Exchange 2003 server.
3. External email destined for a mailbox on the Exchange 2007 server also did not arrive (SMTP is still routed through the Exchange 2003 server).
4. I could telnet to the Exchange 2007 server successfully and send an email to a mailbox on the server without any problem.
5. I could not send a message from a mailbox on the Exchange 2003 server to an address outside of the organization.

Troubleshooting the issue with the message tracking and flow tools included with Exchange 2007 did not provide any real insight though the NDR created once the queues was manually emptied did provide a bit more insight.

From:   System Administrator  Sent:  Wed 6/18/2008 11:29 AM
To:   Administrator
Subject:   Undeliverable: Test
Attachments:   Test‎(648B)‎
Your message did not reach some or all of the intended recipients.

Subject: Test
Sent: 6/18/2008 11:04 AM

The following recipient(s) could not be reached:

Test User on 6/18/2008 11:29 AM
This message was rejected due to the current administrative policy by the destination server. Please retry at a later time. If that fails, contact your system administrator.
<server.domain.com #4.3.2>

In order to resolve the issue I ended up looking at the routing group connector that is automatically created during the Exchange 2007 setup process. After deleting and manually recreating the connector using the Exchange PowerShell all is working as it should be.

The command to recreate the connector is as follows:

New-RoutingGroupConnector -Name “Interop RGC” -SourceTransportServers “Ex2007Hub1.contoso.com” -TargetTransportServers “Ex2003BH1.contoso.com” -Cost 100 -Bidirectional $true -PublicFolderReferralsEnabled $true

For more information see the Microsoft Exchange Server Tech Center.