Which TCP port is traditionally assigned as the standard default port for…
2023
Which TCP port is traditionally assigned as the standard default port for unencrypted SMTP communication between mail servers?
- A.
80
- B.
25
- C.
110
- D.
143
- E.
465
Attempted by 98 students.
Show answer & explanation
Correct answer: B
Concept
Well-known TCP/UDP port numbers (0–1023) are reserved by IANA so that a client always knows where to reach a given service by default. Each application-layer protocol is bound to a fixed standard port: a server listens on that port and a client connects to it without any prior negotiation.
Application
The protocol that transfers email between mail servers (and from a client to a server using the classic relay model) is SMTP — the Simple Mail Transfer Protocol. For plain, unencrypted SMTP message transfer, IANA and RFC 5321 assign the well-known port 25. A mail server therefore listens on TCP 25 to accept incoming mail from other servers by default.
Contrast
80 — the well-known port for HTTP (web traffic), not mail transfer.
110 — POP3, used by a client to download mail from a server, not to relay it between servers.
143 — IMAP, used by a client to access mailboxes on a server, again a retrieval protocol rather than transfer.
465 — historically SMTPS / implicit-TLS submission; it carries SMTP over an encrypted channel, so it is not the unencrypted default the question asks for.
Result
The standard default port for unencrypted SMTP communication between mail servers is TCP port 25.