Shadow Redundancy in Exchange Server 2010

Exchange Server 2010 has introduced a new feature, Shadow Redundancy, which provides redundancy for the entire message while it is in transit. This feature delay the message deletion from the message queue (mail.queue) on Hub Transport or Edge Transport Server until Exchange server has verified that message has completed all the hops and only then message will be deleted. Message will be resubmitted if any of the count fails.

Let us try to understand how Shadow Redundancy actually works:-

1.  Mike wants to send a message to John.

2. Mike messages is first delivered to its hub transport server, Server-A.

3. Server-A sends that message to the next hub transport server, which is Server-B and retains a shadow copy of that message.

4. Server-B sends that message to the next hub transport server, which is Server-C and queue a discard status for the Server-A.

5. Server-A queries the Server-B for discard status and upon receiving the discard notification from Server-B, it can remove the shadow copy of the message from its database.