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Just to set the scene, I'm running Exchange 2010 in coexistence with Exchange 2003 while we migrate across. All outgoing email for the "*" SMTP namespace goes out through an SMTP connector and through a smart host (Messagelabs)

Due to the fact that I can't upgrade the above SMTP connector until I've decommissioned our Exchange 2003 servers, mail flow is currently going through the Exchange 2003 server in each site, and then out to Messagelabs.

Our setup in this site consists of a Mailbox server by itself, and two CAS/HUB servers in an array.

I'm running software on each CAS/HUB server that stamps outgoing emails with a signature. Most outgoing emails take the following route:

  1. MAPI Client -> MBX Server
  2. MBX Server -> CAS/HUB Server (one of two in the array)
  3. CAS/HUB Server -> Exchange 2003 Server
  4. Exchange 2003 Server -> Messagelabs

The problem is that some emails are flowing as follows:

  1. MAPI Client -> MBX Server
  2. MBX Server -> CAS/HUB Server 2
  3. CAS/HUB Server 2 -> CAS/HUB Server 1
  4. CAS/HUB Server 1 -> Exchange 2003 Server
  5. Exchange 2003 Server -> Messagelabs

While this isn't an issue for mailflow, it causes issues because the signature software stamps the email twice; each time it passes through each CAS/HUB server.

Is it normal for emails to pass between two CAS servers within the same AD site on their way out? I would have thought that the CAS servers would figure out that the next "hop" was to send though the Exchange 2003 server.

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Depending on your server setup, that could be normal. You don't mention how many servers you have and what Exchange roles each has installed, but there are certain situations this might be intended behaviour.

If for example you have a Hub Transport Server which is also a Mailbox Server that is a DAG member, Exchange will route the message via another Hub Transport server to ensure the message is contained in the transport dumpster of another Hub server. This is to ensure that if the server fails which is both a Hub server and Mailbox DAG member, the message is not lost in transit.

This feature is called Shadow Redundancy, and to be honest TechNet has an awful lot more to say about it in much greater detail than I do, so you might want to check that out if it's relevant to your situation.

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I should note that the CAS/HUB roles are separate to the Mailbox role in this site, and we haven't yet set up a DAG. –  Daniel.S Oct 18 '11 at 23:01
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