Email protocols: Which TCP/IP protocol is used to transfer electronic mail from one mail server to another across the Internet?

Difficulty: Easy

Correct Answer: SMTP

Explanation:


Introduction / Context:
Email delivery across the Internet relies on a standard application-layer protocol to relay messages between mail transfer agents (MTAs). Recognizing this protocol helps in troubleshooting mail flow and configuring servers.



Given Data / Assumptions:

  • We are focusing on server-to-server message transfer (relay), not local client retrieval.
  • Common TCP/IP protocols are listed as distractors.
  • Terminology: MTA = Mail Transfer Agent (e.g., Postfix, Sendmail, Exim).


Concept / Approach:
SMTP (Simple Mail Transfer Protocol) moves mail between MTAs over TCP, typically on port 25 (and 587/Submission for client submission). Retrieval by clients uses POP3 or IMAP, which are not listed here. FTP is for file transfers, SNMP for network management, and RPC for generic remote procedure calls.



Step-by-Step Solution:

Identify the protocol that relays mail between servers → SMTP.Map ports and roles: SMTP on 25 (server relay) and 587 (submission).Choose the matching option.


Verification / Alternative check:
Examine mail logs showing SMTP transactions (HELO/EHLO, MAIL FROM, RCPT TO, DATA). Network captures reveal SMTP commands over TCP.



Why Other Options Are Wrong:
FTP: File transfer, not email. SNMP: Device monitoring/management. RPC: Generic remote calls, not mail relay. None of the above: Incorrect because SMTP is correct.



Common Pitfalls:
Confusing SMTP (server relay) with client mailbox access (POP3/IMAP); assuming SMTP provides message encryption by default (STARTTLS is an extension).



Final Answer:
SMTP

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