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

The correct answer is B. Port 25. In CompTIA Network+ N10-009, candidates are expected to know common networking protocols, their purposes, and their associated port numbers. SMTP, or Simple Mail Transfer Protocol, uses TCP port 25 and is primarily responsible for sending email messages between mail servers. When one organization’s mail server needs to deliver a message to another organization’s mail server, SMTP commonly performs that server-to-server transfer using port 25.

The other options are associated with different services. Port 21 is used by FTP for command/control communication during file transfers. Port 53 is used by DNS for name resolution, translating domain names into IP addresses. Port 69 is used by TFTP, a simplified file transfer protocol often associated with lightweight network device file transfers, such as firmware or configuration files.

For Network+ exam purposes, it is important to distinguish SMTP from email retrieval protocols. POP3 uses port 110, and IMAP uses port 143 for client mail retrieval, while SMTP is used for sending and relaying email. Therefore, the port used to transfer data between mail exchange servers is TCP 25.