TCP port 25 is used by?

TCP port 25 is used by?

• A. Domain Name System (DNS)
• B. Simple Mail Transfer Protocol (SMTP)
• C. Post Office Protocol (POP3)
• D. IMAP

Answer: (B)

Port 25 is the standard channel for SMTP, the protocol used to transfer email between mail servers and to relay messages across the Internet. That’s why this is the correct association. DNS operates on port 53, which is for domain name resolution, not mail transfer. POP3 uses port 110 (and 995 for POP3S) and is for retrieving mail from a server, not sending it between servers. IMAP uses port 143 (and 993 for IMAPS) for accessing mail on a server, also not for mail relaying. In practice, client submission often uses port 587 with encryption, but the traditional role of port 25 is SMTP relaying between mail servers.

Port 25 is the standard channel for SMTP, the protocol used to transfer email between mail servers and to relay messages across the Internet. That’s why this is the correct association. DNS operates on port 53, which is for domain name resolution, not mail transfer. POP3 uses port 110 (and 995 for POP3S) and is for retrieving mail from a server, not sending it between servers. IMAP uses port 143 (and 993 for IMAPS) for accessing mail on a server, also not for mail relaying. In practice, client submission often uses port 587 with encryption, but the traditional role of port 25 is SMTP relaying between mail servers.