Exchange Server 2016 Mail Flow
Routing in the Mailbox Transport Service
Similar to the Transport service, the Mailbox Transport service builds the routing table based on information from the AD DS. The Mailbox Transport service also uses delivery groups for message routing.
The Mailbox Transport service always belongs to the same delivery group as the Mailbox server, and that group is called the local delivery group. This service also does not automatically send messages to the Hub Transport service in its local delivery group. The Mailbox Transport service only communicates with the Hub Transport service on Mailbox servers and with mailbox databases on the local Mailbox server. It never communicates with mailbox databases on other Mailbox servers.
When a message is sent from the user’s mailbox, the Transport Submission component in the Mailbox Transport service resolves the message recipient to the appropriate mailbox database, and then the Transport Submission component looks for the routing information for each mailbox database.
The delivery groups used by the Mailbox Transport Submission service are:
- Routable DAG
- Mailbox delivery group
- Active Directory site
Depending on the number and the type of message recipients, the Mailbox Transport Submission service performs one of the following actions:
- If the message has a single recipient with a mailbox, the Mailbox Transport service selects a Mailbox server in the target delivery group. If the target delivery group spans multiple sites, the Front End Transport service gives preference to the Mailbox server based on the proximity of the AD DS site.
- If the message has multiple mailbox recipients, the Mailbox Transport service uses the first 20 recipients to select a Mailbox server in the closest delivery group.
- If there are no mailbox recipients in the message, the Mailbox Transport service selects a Mailbox server in the local delivery group.
The Mailbox Transport service communicates with the Transport service. The message can be accepted or rejected for delivery to the local mailbox database when the message is sent from the Transport service to the Mailbox Transport service. The message is accepted for delivery if the recipient resides in an active copy of a local mailbox database. However, if the recipient is not in the active copy of the local mailbox database, the Mailbox Transport service provides a non-delivery response to the Transport service.
A non-delivery response occurs when an active copy of the local mailbox database is moved to another mailbox server, but the Transport service still does not have the updated information. In this case, the Mailbox Transport service issues an NDR to the Transport service, with instructions to retry delivery, generate an NDR, or reroute the message.