Which feature allows Exchange to designate transport encryption for messages?

Transport Layer Security (TLS) is the feature that allows Exchange to designate transport encryption for messages. TLS is a cryptographic protocol designed to provide secure communication over a computer network. In the context of Microsoft Exchange, TLS encrypts the connection between mail servers, ensuring that email data is protected while it is being transmitted across potentially untrusted networks. This encryption safeguards messages from being intercepted or tampered with during transit.

While S/MIME also offers encryption, it does so at the message level, providing end-to-end encryption for the content of emails rather than securing the transport layer itself. Active Directory Protection focuses on securing directory services and identity information rather than directly addressing transport encryption. Mail Flow Control generally manages how messages are routed and delivered but does not specifically address encryption measures during transmission. Thus, TLS is unequivocally the primary method by which Exchange secures email in transit through transport encryption.