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Short note: SMTP and POP

Short answer

SMTP is a push protocol on TCP port 25 that sends mail from sender to receiver server, while POP3 is a pull protocol on TCP port 110 that downloads mail to the receiver's user agent. The path is sender UA, SMTP servers, receiver server, then POP3 to receiver UA.

Write a short note on SMTP and POP.

SMTP (Simple Mail Transfer Protocol)

SMTP (RFC 5321) is a text-based push protocol that transfers mail from the sender's user agent to its mail server and between mail servers. It uses TCP port 25 (587 for submission). It sends only; it cannot be used to read mail.

Commands: HELO/EHLO, MAIL FROM, RCPT TO, DATA, QUIT.

C: HELO a.com        S: 250 OK
C: MAIL FROM:<a@a.com>   S: 250 OK
C: RCPT TO:<b@b.com>     S: 250 OK
C: DATA   ... message ... "."  S: 250 queued
C: QUIT

POP (Post Office Protocol, POP3)

POP3 (RFC 1939) is a pull protocol used by the receiver's user agent to download mail from its mailbox on the mail server. It uses TCP port 110.

  • Phases: authorisation (USER, PASS), transaction (STAT, LIST, RETR, DELE), update (QUIT).
  • Mail is usually deleted from the server after download ("download and delete"), or kept ("download and keep").

Together

Sender UA -SMTP-> Sender server -SMTP-> Receiver server -POP3-> Receiver UA

✓ Answer checked by Claude Opus 5.5 (high effort)

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