The Transport service never communicates directly with a mailbox database. For Mailbox servers that don't belong to DAGs and for interoperability with previous versions of Exchange, Active Directory site membership is still used in routing decisions.

Routing is fully aware of database availability groups (DAGs), and is able to use DAG membership in routing decisions, even when the DAG members are in different Active Directory sites. These are the notable changes to routing compared to Exchange 2010: Routing in Exchange 2016 and Exchange 2019 is virtually unchanged from Exchange 2013. The categorizer is a component of the Transport service that processes all incoming messages and determines what to do with the message based on information about their destinations. Routing decisions are made during message categorization. The primary task of the Transport service that exists on Mailbox servers in your Exchange organization is to route messages received from users and external sources to their ultimate destinations.

Nearest mailbox