---
title: "Email server and its protocols"
subject: "Computer Networks and Security"
chapter: 6
key: "semester_7/computer_networks_and_security:email-protocols-server"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/email-protocols-server
site: BCT Engineering
---

# Email server and its protocols

Computer Networks and Security · Chapter 6: Application Layer

- Asked in: 2074 Chaitra, 5 marks; 2072 Kartik, 2+6 marks; 2070 Chaitra, 2+6 marks (asked 3 times)
- Video: [Lec-85 : SMTP vs POP3 vs IMAP with real life example | All in 1 | Application layer Protocols](https://www.youtube.com/watch?v=cjPVlxmf_Vk) — Gate Smashers
- 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/email-protocols-server
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-6

## Question

What do you mean by an email server (components)? Which protocols are used in sending and receiving email? Illustrate with necessary figure.

## Short answer

Email system comprises user agents (compose/read), mail servers with MTAs, mailboxes, and queues, and mail access agents. SMTP (port 25/587) sends mail between servers and from user agents; POP3 (110) or IMAP (143) retrieves mail; HTTP/HTTPS enables webmail. Mail flows: UA→SMTP→sender's server→SMTP→recipient's server→POP3/IMAP→UA.

## Answer

#### Email server and its components
An **email system** lets users send messages that are stored and forwarded through servers. It has three main components:

1. **User agent (UA):** the program used to compose, read and reply to mail (Outlook, Thunderbird, Gmail app).
2. **Mail server:** contains the **mailbox** (stores incoming messages of each user), a **message queue** (outgoing mail waiting to be sent) and the **MTA (Mail Transfer Agent)** that talks SMTP to other servers.
3. **Mail access agent / protocol:** retrieves mail from the server (POP3, IMAP, HTTP).

```text
Alice UA --SMTP--> [Alice's mail server] --SMTP--> [Bob's mail server]
                        (MTA, queue)                 (MTA, mailbox)
                                                           |
                                                POP3/IMAP/HTTP
                                                           v
                                                        Bob UA
```

#### Protocols
| Protocol | Role | Port |
|---|---|---|
| SMTP | Sending: UA to server and server to server (push) | 25, 587 |
| POP3 | Retrieval: downloads mail from server to the client | 110 |
| IMAP4 | Retrieval: mail stays on the server, folders managed there | 143 |
| HTTP/HTTPS | Webmail access | 80/443 |
| MIME | Extension of message format for non-ASCII and attachments | - |

#### Process
1. Alice composes mail and her UA sends it to her mail server with SMTP.
2. Her server looks up the MX record of Bob's domain in DNS, opens a TCP connection to Bob's mail server and sends the message using SMTP.
3. The message is stored in Bob's mailbox.
4. Bob's UA retrieves it using POP3 or IMAP.

## Check yourself

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

1. Which are the three main components of an email system?
   A. User agent, mail server, mail access protocol
   B. Router, switch, DNS
   C. Browser, proxy, firewall
   D. MTA, DHCP, FTP
   Answer: A. UA, mail server (mailbox, queue, MTA) and mail access agent/protocol. (Key point: Components)

2. What is SMTP used for?
   A. Retrieving mail from the mailbox
   B. Sending mail from UA to server and between servers
   C. Resolving MX records
   D. Encoding attachments
   Answer: B. SMTP is the push protocol for sending; POP3/IMAP retrieve. (Key point: SMTP role)

3. How does Alice's mail server find Bob's mail server?
   A. By broadcasting
   B. By looking up the MX record of Bob's domain in DNS
   C. By using POP3
   D. By asking the DHCP server
   Answer: B. The MX record in DNS gives the mail server of the domain. (Key point: Finding Bob's server)

4. Which retrieval protocol keeps mail on the server and manages folders there?
   A. SMTP
   B. POP3
   C. IMAP4
   D. MIME
   Answer: C. IMAP4 keeps mail on the server; POP3 downloads it. (Key point: POP3 vs IMAP)

## Related in Computer Networks and Security

- [Web and file server communication](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aweb-file-server-communication) — Computer Networks and Security, chapter 6
- [Why two transport protocols exist](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Awhy-two-transport-protocols) — Computer Networks and Security, chapter 5
- [Switching and why necessary](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aswitching-meaning-necessity) — Computer Networks and Security, chapter 2
- [Sending images over 7-bit ASCII SMTP](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Asmtp-images-mime) — Computer Networks and Security, chapter 6

## Related in other subjects

- [Email security with diagram and protocols](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aemail-security-diagram) — Internet and Intranet, chapter 3
- [Receiving and sending end powers from ABCD, 132/134 kV](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aabcd-power-132kv-134kv) — Power System Analysis I, chapter 6
- [Power output with 46.85 kV sending, 33 kV receiving](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Apower-output-46-85kv-33kv) — Power System Analysis I, chapter 6
- [Sending and receiving power, 140∠10° and 132 kV](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Apower-140kv-132kv-a099) — Power System Analysis I, chapter 6