Now that we understand how the transport pipeline works in Exchange 2013 and how e-mails are routed, let us look at how high availability is achieved to ensure no e-mails are lost while in transit. This is done by keeping redundant copies of e-mails both before and after they are successfully delivered. Transport dumpster was introduced in Exchange 2007, and shadow redundancy in Exchange 2010. Exchange 2013 took these two features a step further.
As a brief summary, the following are the main improvements in transport high availability:
Shadow redundancy generates a redundant copy of an e-mail on a different server before it is accepted. If shadow redundancy is not supported by the sending server, this is not a problem, as we will see shortly.
Shadow redundancy uses both DAGs and AD sites as boundaries for transport high availability, which eliminates unnecessary redundant e-mail traffic across DAGs or AD sites.
The transport dumpster feature, now called...