---
title: "Short note: SMTP and POP"
subject: "Computer Networks and Security"
chapter: 6
key: "semester_7/computer_networks_and_security:smtp-pop-note"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/smtp-pop-note
site: BCT Engineering
---

# Short note: SMTP and POP

Computer Networks and Security · Chapter 6: Application Layer

- Asked in: 2074 Asoj, 4 marks
- Video: [SMTP, POP, and IMAP: Essential Email Protocols in Computer Networks](https://www.youtube.com/watch?v=-56DpCEbDvk) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/smtp-pop-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-6

## Question

Write a short note on 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.

## Answer

#### 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.

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

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. SMTP is a:
   A. Pull protocol for reading mail
   B. Push protocol that sends mail
   C. File transfer protocol
   D. Name lookup protocol
   Answer: B. SMTP pushes mail; it cannot read mail. (Key point: SMTP push)

2. POP3 phases are:
   A. Authorisation, transaction, update
   B. HELO, DATA, QUIT
   C. Discover, offer, request
   D. SYN, ACK, FIN
   Answer: A. POP3: authorisation, transaction, update. (Key point: POP3 phases)

3. Which SMTP command specifies the recipient?
   A. MAIL FROM
   B. RCPT TO
   C. DATA
   D. USER
   Answer: B. RCPT TO names the recipient. (Key point: Commands)

## Related in other subjects

- [Browser features; POP, SMTP, IMAP comparison](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Apop-smtp-imap-compare) — Internet and Intranet, chapter 3, asked 2×