One part of the new Exchange 2013 transport improvement is the use of delivery groups, and Active Directory sites are considered simply one of the types wherein. Other types include DAGs (database availability groups) as boundaries. If a group of mailbox servers are part of a DAG but exist in different sites, the Mailbox server will not consider the Active Directory site and site-link costs when routing a message; instead, it will focus on other DAG members in the delivery group to route the message. It does so even if the mailbox server is in another site, regardless of whether another Mailbox server is outside the DAG in the same site.
Along with the Routable DAG Delivery Group and Active Directory Site Delivery Group, there are also Version Routing Delivery Groups (based on servers with the same Exchange version), Connector Source Server Delivery Groups (based on servers of different types that are coped as source servers for a send connector), and Distribution Group Expansion Server Delivery Groups (based on expansion servers for a distribution group).
Another transport change includes the use of a SafetyNet. This feature existed in Exchange 2007 and 2010 as the transport dumpster, a fail-safe mechanism to capture email that might have been lost during a failover or switchover. In Exchange 2013, this feature is taken to the next level: Messages are held in a queue database on each mailbox server, which by default holds these messages for two days. If a failover doesn't go well, the mailbox server checks the queue database of the SafetyNet for any messages that need to be restored.
For more detail on the services, ports, and architectural adjustments, there's a good TechNet article you should read. It has a couple of detailed charts illustrating how mail flow works in Exchange 2013 that really help pull these new concepts together.
This story, "Understanding Exchange 2013: New transport features," was originally published at InfoWorld.com. Read more of J. Peter Bruzzese's Enterprise Windows blog and follow the latest developments in Windows at InfoWorld.com. For the latest business technology news, follow InfoWorld.com on Twitter.