Understanding Exchange 2010 Shadow Redundancy
" Exchange 2007 introduced the Transport Dumpster feature for the Hub server role. An Exchange 2007 hub server maintains a queue of messages delivered recently to recipients whose mailboxes are on a clustered mailbox server. When a failover is experienced, the clustered mailbox server automatically requests every Hub server in the Active Directory site to resubmit mail from the transport dumpster queue. This prevents mail from being lost during the time taken for the cluster to fail over. While this does provide a basic level of transport redundancy, it is only available for message delivery in a Cluster Continuous Replication environment and does not address potential message loss when messages are in transit between hub and edge servers.
Exchange 2010 introduces the Shadow Redundancy feature to provide redundancy for messages for the entire time they are in transit. The solution involves a technique similar to the Transport Dumpster. With Shadow Redundancy, the deletion of a message from the transport databases is delayed until the transport server verifies that all of the next hops for that message have completed delivery. If any of the next hops fail before reporting back successful delivery, the message is resubmitted for delivery to that next hop. "
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Understanding Shadow Redundancy: Exchange 2010 Help