In Exchange 2010, the way emails get routed changes when you have a DAG member that hosts both the Mailbox and HUB transport role. This change was brought in to ensure the protection of messages while it uses Dumpster 2.0
For example, let’s say that I have a server named EXCH1 which hosts both Mailbox and HUB role. Emails that are sent to a mailbox in EXCH1 will be handled by the HUB transport role present on the same server. This is the default behaviour.
Now, let’s say that we have a DAG with servers (EXCH1, EXCH2 etc) hosting all the three roles. When a message is sent to a mailbox located on EXCH1, the transport re-routes the email to a different hub transport server in the same AD site (say EXCH2) and that server delivers the email to the mailbox in EXCH1. This extra hop was added to put the message in the transport dumpster on a different HUB transport server.
In short, the HUB transport server role was modified so that it reroutes a message for a local mailbox server to another HUB transport server in the same site, if both HUB and Mailbox roles are on the same DAG member.
The Exchange Mail Submission service was also modified so that it would not submit messages to a local HUB transport server when the Mailbox & HUB transport role co-exist on a DAG member. Instead, it will try to load balance submission requests across other HUB transport servers in the same AD site and fall back to a local HUB server if there are no other available HUB servers in the same site.