What must be deployed in each Active Directory site where a Mailbox server role is installed?

The deployment of the Hub Transport server role in each Active Directory site where a Mailbox server role is installed is critical for ensuring the proper functioning of the Exchange messaging system. The Hub Transport server role is responsible for the routing of mail within the organization and the delivery of messages to and from Mailbox servers.

In a Microsoft Exchange environment, the Hub Transport server manages the flow of email messages across the Exchange organization by using a set of transport queues. It facilitates the transportation of messages between Mailbox servers, ensuring messages are routed effectively. This role relies heavily on Active Directory to access information about users and settings related to message routing.

If the Hub Transport server role is not deployed in a site with a Mailbox server, there would be no local mechanism to route messages efficiently to and from those Mailbox servers, leading to delays or failures in message delivery.

The other roles—Edge Transport, Unified Messaging, and Client Access server—all serve different functions within the Exchange architecture. The Edge Transport role, for instance, is typically deployed in perimeter networks to handle incoming and outgoing email with the internet but does not manage internal mail flow. Unified Messaging provides voicemail and call integration capabilities, while the Client Access server role handles client connections to the Exchange environment but doesn’t