Which of the following ports is used to transfer data between mail exchange servers?

The correct answer is 25, which is the default port used by SMTP (Simple Mail Transfer Protocol) for communication between mail servers. According to CompTIA Network+ (N10-009) objectives under common port numbers and protocols, SMTP operates over TCP port 25 for server-to-server email transmission.

When an email is sent from one domain to another, the sending mail server queries DNS for the recipient domain’s MX (Mail Exchange) record and then establishes an SMTP session over TCP port 25 to deliver the message. This port is specifically designated for mail relay between mail transfer agents (MTAs).

Port 21 is used for FTP (File Transfer Protocol), 53 is used for DNS (Domain Name System), and 69 is used for TFTP (Trivial File Transfer Protocol). None of these are responsible for transferring email between mail servers.

While SMTP can also use ports 587 (submission) and 465 (SMTPS) for client-to-server communication, port 25 remains the standard for mail server-to-mail server communication.

Therefore, TCP port 25 is the correct answer.