--- title: "E-mail components and message delivery path" subject: "Internet and Intranet" chapter: 3 key: "semester_8/internet_and_intranet:email-components-delivery-path" source: https://www.bctengineer.com/q/semester_8/internet_and_intranet/email-components-delivery-path site: BCT Engineering --- # E-mail components and message delivery path Internet and Intranet · Chapter 3: Protocols and Client/Server Applications - Asked in: 2070 Magh, 10 marks - Video: [How Email Works](https://www.youtube.com/watch?v=ELioaGg4WOY) — PowerCert Animated Videos - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/internet_and_intranet/email-components-delivery-path - Whole chapter: https://www.bctengineer.com/md/semester_8/internet_and_intranet/chapter-3 ## Question Discuss the e-mail system components with its functions and the path of e-mail messages to be delivered from source domain to destination domain. ## Short answer Key components: user agent (MUA), mail server (MTA/MDA), and protocols SMTP (push), POP3/IMAP (pull). A message travels: sender's UA to source MTA via SMTP/HTTP, DNS MX lookup, SMTP to destination MTA, then retrieval by receiver via POP3/IMAP. ## Answer #### Components of the e-mail system Internet e-mail has three major parts: user agents, mail servers and the mail protocols (SMTP, POP3, IMAP). In more detail, the components are: 1. **User Agent (UA / MUA)**: the program the user works with (Outlook, Thunderbird, Gmail web interface). It composes, edits, reads, replies to, forwards and stores messages locally. 2. **Mail server (MTA + mailboxes)**: holds each user's **mailbox** (incoming messages) and an outgoing **message queue**. It acts as an SMTP server when receiving and as an SMTP client when forwarding. If delivery fails, it retries (typically every 30 min for several days) and then sends a failure notice. 3. **Message Transfer Agent (MTA)**: moves mail between servers using **SMTP** (port 25; submission port 587). 4. **Mail Delivery Agent (MDA)**: puts the message into the receiver's mailbox. 5. **Mail access protocols**: **POP3** (port 110, download and delete), **IMAP** (port 143, keeps mail on the server and manages folders) or HTTP for web-mail. SMTP is a push protocol; POP3/IMAP are pull protocols. 6. **DNS**: the MX record of the destination domain tells the sending server which mail server accepts mail for that domain. #### Path of a message from source to destination domain Example: alice@source.com sends to bob@dest.com. ```text Alice's UA Source mail Dest mail Bob's UA (source.com) server (MTA) server (MTA) (dest.com) | 1.SMTP | | | |---------------->| 2.queue | | | | 3. DNS: MX of | | | | dest.com | | | | 4. TCP+SMTP | | | |------------------>| 5. MDA -> Bob's| | | | mailbox | | | | 6. POP3/IMAP | | | |<---------------| ``` 1. Alice composes the message in her UA and clicks Send. The UA sends it to her own mail server using SMTP (or HTTP for web-mail). 2. The source server places the message in its outgoing queue. 3. The server asks DNS for the **MX record** of dest.com and then for the IP address of that mail server. 4. The source server opens a TCP connection (port 25) to the destination mail server and runs the SMTP dialogue: HELO, MAIL FROM, RCPT TO, DATA, QUIT. SMTP transfer is direct: normally there are no intermediate servers. 5. The destination server accepts the message and its MDA stores it in Bob's mailbox. 6. When Bob opens his UA, it retrieves the message with POP3 or IMAP (or HTTP for web-mail). So the protocol chain is: UA -> SMTP -> source MTA -> SMTP -> destination MTA -> POP3/IMAP -> Bob's UA. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which component moves mail between mail servers, and with which protocol? A. MDA using POP3 B. User agent using IMAP C. MTA using SMTP D. DNS using MX Answer: C. The Message Transfer Agent transfers mail between servers using SMTP (port 25). (Key point: Role of MTA / SMTP) 2. How does the source mail server find where to send mail for dest.com? A. It asks the user agent B. It queries DNS for the MX record of dest.com C. It uses the POP3 server list D. It broadcasts to all servers Answer: B. The MX record of the destination domain names the mail server that accepts its mail; the sender then resolves its IP. (Key point: Role of DNS MX record) 3. Which statement about mail protocols is correct? A. SMTP and POP3 are both push protocols B. SMTP is a push protocol; POP3 and IMAP are pull protocols C. IMAP is used between mail servers D. POP3 keeps mail on the server and manages folders Answer: B. SMTP pushes mail to servers; the receiver pulls mail from his mailbox with POP3 or IMAP. (Key point: Push vs pull protocols) 4. What is the correct protocol chain for Alice at source.com to Bob at dest.com? A. UA -> POP3 -> MTA -> POP3 -> MTA -> SMTP -> UA B. UA -> SMTP -> source MTA -> SMTP -> destination MTA -> POP3/IMAP -> Bob's UA C. UA -> IMAP -> source MTA -> SMTP -> UA D. UA -> SMTP -> DNS -> POP3 -> Bob's UA Answer: B. Alice's UA submits via SMTP, MTAs relay via SMTP directly, and Bob's UA retrieves with POP3 or IMAP. (Key point: Delivery path) ## Related in Internet and Intranet - [Web-mail to POP3 message transfer protocols](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aalice-bob-email-protocols) — Internet and Intranet, chapter 3 - [Short note: E-mail agents and third functions](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Anote-email-agents) — Internet and Intranet, chapter 3 ## Related in other subjects - [Application, system and device domain](https://www.bctengineer.com/question.md?key=semester_8%2Fmultimedia_system%3Aapplication-system-device-domain) — Multimedia System, chapter 1 - [LTI system output in time domain](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Alti-output-time-domain) — Communication Systems, chapter 1 - [Major components of a power station and functions](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Apower-station-components-functions) — Power System Analysis I, chapter 1 - [Rules of inference: e-mail and program](https://www.bctengineer.com/question.md?key=semester_4%2Fdiscrete_structure%3Arules-inference-email-program) — Discrete Structure, chapter 1