Nice one 🙂
When the Hub Transport server is co-located with a Mailbox server that's a member of a DAG, there are changes in routing behavior to ensure that the resiliency features in both server roles will provide the necessary protection for messages sent to and received by users on that server. The Hub Transport server role was modified so that it now attempts to reroute a message for a local Mailbox server to another Hub Transport server in the same site if the Hub Transport server is also a DAG member and it has a copy of the mailbox database mounted locally. This extra hop was added to put the message in the transport dumpster on a different Hub Transport server.
For example, EX1 hosts the Hub Transport server role and Mailbox server role and is a member of a DAG. When a message arrives in transport for EX1 destined for a recipient whose mailbox is also on EX1, transport will reroute the message to another Hub Transport server in the site (for example, EX2), and that server will deliver the message to the mailbox on EX1.
There's a second, similar behavior change related to the Microsoft Exchange Mail Submission service. This service was modified so that it would not submit messages to a local Hub Transport server role when the Mailbox server or Hub Transport server is a member of a DAG. In this scenario, the behavior of transport is to load balance submission requests across other Hub Transport servers in the same Active Directory site, and fall back to a local Hub Transport server if there are no other available Hub Transport servers in the same site.
Nice one 🙂