--- title: "Web-mail to POP3 message transfer protocols" subject: "Internet and Intranet" chapter: 3 key: "semester_8/internet_and_intranet:alice-bob-email-protocols" source: https://www.bctengineer.com/q/semester_8/internet_and_intranet/alice-bob-email-protocols site: BCT Engineering --- # Web-mail to POP3 message transfer protocols Internet and Intranet · Chapter 3: Protocols and Client/Server Applications - Asked in: 2069 Bhadra (old course), 10 marks - Video: [What are POP3, IMAP, and SMTP?](https://www.youtube.com/watch?v=gLNOVbcyWbI) — Ask Leo! - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/internet_and_intranet/alice-bob-email-protocols - Whole chapter: https://www.bctengineer.com/md/semester_8/internet_and_intranet/chapter-3 ## Question Suppose Alice with a web-based email account sends a message to Bob, who accesses his mail from his mail server using POP3. Discuss how the message is transferred from Alice to Bob. Be sure to list the series of application layer protocols that are used to move the message between the hosts. ## Short answer Alice's browser sends the message to her webmail server via HTTP POST. After DNS MX lookup, that server pushes it to Bob's server using SMTP. Bob's client retrieves it with POP3. Application layer protocols: HTTP, DNS, SMTP, POP3. ## Answer **Setup:** Alice uses web-mail (for example Gmail/Yahoo), so her "user agent" is a web browser. Bob has a mailbox on his own mail server and reads it with a POP3 client. Assume Alice's mail server is `mail.alice-isp.com` and Bob's is `mail.bob.com`. ```text Alice's browser Alice's mail server Bob's mail server Bob's PC | HTTP | | | |---------------->| | | | |------ SMTP -------->| | | | |<---- POP3 ----| ``` #### Steps of transfer 1. **Alice to her mail server (HTTP).** Alice logs in through the browser, composes the message and clicks Send. The browser sends the message with an **HTTP POST** to her web-mail server. Unlike with a normal mail client, no SMTP is used on this hop. 2. **Queue.** The web-mail server places the message in its outgoing queue. 3. **DNS lookup.** The server queries DNS for the MX record of `bob.com` and then the A record of `mail.bob.com`. This uses another application layer protocol: **DNS**. 4. **Alice's server to Bob's server (SMTP).** The server opens a TCP connection to port 25 of Bob's mail server and sends the message using **SMTP** (HELO, MAIL FROM, RCPT TO, DATA, QUIT). If Bob's server is down, the message stays in the queue and is retried. 5. **Storage.** Bob's mail server puts the message in Bob's mailbox. 6. **Bob retrieves the message (POP3).** Bob's mail client connects to port 110 of his server and runs **POP3**: authorization phase (USER, PASS), transaction phase (LIST, RETR, DELE), and update phase (QUIT). The message is downloaded to Bob's PC and, in the usual "download and delete" mode, removed from the server. #### Protocol series | Hop | Protocol | |---|---| | Alice's browser -> her web-mail server | HTTP (POST) | | Name lookup of Bob's mail server | DNS | | Alice's mail server -> Bob's mail server | SMTP | | Bob's mail server -> Bob's PC | POP3 | HTTP, SMTP and POP3 run over TCP; DNS normally runs over UDP. The key point: SMTP is a **push** protocol used here only between the mail servers; POP3 is a **pull** protocol used by the receiver; and HTTP replaces SMTP on the sender's side because Alice uses web-mail. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Alice uses web-mail. Which protocol carries the message from her browser to her mail server? A. SMTP B. POP3 C. HTTP (POST) D. IMAP Answer: C. With web-mail the browser is the user agent, so the message goes to her server by HTTP POST, not SMTP. (Key point: First hop uses HTTP) 2. Which protocol moves the message from Alice's mail server to Bob's mail server? A. HTTP B. SMTP over TCP port 25 C. POP3 over port 110 D. DNS Answer: B. Mail servers exchange mail with SMTP, a push protocol, over a TCP connection to port 25. (Key point: Server-to-server SMTP) 3. Why is DNS part of the protocol series? A. It encrypts the message B. It downloads the message for Bob C. It supplies the MX/A records so Alice's server can find Bob's mail server D. It replaces SMTP when the server is down Answer: C. Alice's server queries DNS for the MX record of bob.com and the IP of mail.bob.com before opening the SMTP connection. (Key point: DNS role) 4. Which is the correct sequence of POP3 phases when Bob retrieves mail? A. Transaction, authorization, update B. Authorization, transaction, update C. Update, authorization, transaction D. Authorization, update, transaction Answer: B. POP3 runs authorization (USER, PASS), transaction (LIST, RETR, DELE) and finally update (QUIT). (Key point: POP3 phases) ## Related in Internet and Intranet - [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 - [E-mail components and message delivery path](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aemail-components-delivery-path) — Internet and Intranet, chapter 3 - [HTTP request methods; web server and virtual hosting](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Ahttp-methods-config-virtual-hosting) — Internet and Intranet, chapter 4 - [Intranet vs Internet and intranet protocols](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aintranet-vs-internet-protocols) — Internet and Intranet, chapter 1 - [Virtual hosting types; name-based steps](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Avirtual-hosting-types-named-steps) — Internet and Intranet, chapter 4, asked 2× - [XML and JavaScript for client-side web apps](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Axml-javascript-client-side) — Internet and Intranet, chapter 6 ## Related in other subjects - [Email server and its protocols](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aemail-protocols-server) — Computer Networks and Security, chapter 6, asked 3× - [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 - [Short note: Web server](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aweb-server-note) — Computer Networks and Security, chapter 6 - [SS7 message transfer part levels](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-mtp-levels) — Telecommunications, chapter 5