Mail servers at two companies hand messages to each other. Which port carries that server-to-server transfer?
Select an answer to reveal the explanation.
Short Explanation
You use TCP port 25 for the handoff when one mail server transfers a message to another. TCP ports 110 and 143 are for a user to retrieve mail, while TCP port 443 is for HTTPS—not this server-to-server transfer.
Full Explanation
SMTP transfers mail between servers, and that transfer uses TCP port 25. The two companies' servers are handing off a message, so TCP port 25 is the port to use. TCP port 110 is POP3, which a client uses to retrieve mail; it is not how two servers hand messages to each other. TCP port 143 is IMAP, another client retrieval protocol that leaves mail on the server for the user, not the server-to-server transfer. TCP port 443 is HTTPS. Web encryption is not the mail-transfer port in this exchange. Exam caveat: many providers also accept mail submission from users on a different port, and they may block port 25 from a residential network. The transfer between the two companies' servers is still the SMTP case. Operational check: permit TCP 25 between the mail servers, send a test message, and confirm it arrives. Send a test message from the other company as well so the path works in both directions. Confirm the message is accepted by the far server, not only that the port is open, and note the permitted port on the mail change.