The correct answer is D. 25 . SMTP is the standard protocol used for transferring email between mail servers, and TCP port 25 is the traditional and most common port used for SMTP relay and server-to-server email transport. Proofpoint’s SMTP relay reference aligns with this standard mail-flow model, where SMTP is the protocol responsible for message transfer between mail systems.
The other ports listed are associated with different services. Port 22 is commonly used for SSH, port 443 for HTTPS, and port 80 for HTTP. Those are important network ports, but they are not the standard answer for SMTP connectivity in the context of mail flow and Proofpoint administration. In the Threat Protection Administrator course, understanding SMTP basics is essential because route configuration, TLS behavior, queue handling, and delivery troubleshooting all rely on knowing how SMTP sessions operate at the transport level.
Although modern mail submission can also involve other ports in certain client scenarios, this question asks for a common SMTP connectivity port, and the course-level expected answer is the standard server-to-server SMTP port. For mail transfer in the context of Proofpoint and SMTP routing, that port is 25 . Therefore, the verified answer is D .
