Lecture 15 - Email Systems

Email $\approx$ ไปรษณีย์ อาจจะมีส่งง่าย ๆ ในตึกเดียวกัน หรือซับซ้อน ข้ามประเทศเลย

Email Architecture Overview

Four Scenarios of Email Architecture

• Email architecture can be explained through four scenarios
• Each scenario increases in complexity
• The fourth scenario is the most common in modern email exchanges

Analogy: Think of these scenarios like different ways to send a physical letter - from handing it directly to someone in the same building (simplest) to using multiple postal services for international delivery (most complex)

Key Components of Email Systems

ส่วนประกอบหลัก ๆ ของ Email

1. Mail User Agent (MUA)

ก็แอปที่เราใช้เปิดอ่าน ส่งอีเมลทั่วไปเนี่ยแหละ

• Definition: Client software of the mail system
• Functions:
  • Provides user interface for reading email
  • Sends email
  • Receives email
• Examples: Outlook, Gmail app, Apple Mail, Thunderbird

Your usage: Gmail and iCloud Mail are web-based MUAs that you access through browsers or apps

2. Mail Transfer Agent (MTA)

เปรียบเหมือนบุรุษไปรษณีย์ เป็น Software ที่คอยรับ Mail จากเรา แล้วก็ส่งต่อกันไปตาม Server จนกว่าจะถึงปลายทาง

• Definition: Software responsible for message storage and forwarding
• Functions:
  • Monitors user agent requests
  • Finds corresponding email addresses of targets
  • Transmits messages between servers
  • Manages message buffers
• Client-Server Model: Works in pairs (MTA Client and MTA Server)
• Protocol: Uses SMTP (Simple Mail Transfer Protocol)

3. Mail Access Agent (MAA)

เป็นตัวกลางที่จะไปดึง Email ที่คนอื่นส่งมาให้เรามาอ่าน

• Definition: Connects users to the mail system for retrieving messages
• Protocols Used:
  • POP (Post Office Protocol)
  • IMAP (Internet Message Access Protocol)
• Purpose: Allows users to pull messages from the server to their device

4. Mail Spool

พื้นที่พักข้อมูลชั่วคราวบน Server: ใช้เก็บ Email ที่รอกดส่ง หรือ Email ที่ตีกลับมา

• Definition: Storage area for emails
• Contents:
  • Emails waiting to be delivered
  • Emails that have returned with errors

Four Email Delivery Scenarios

Scenario 1: Same Mail Server

Components needed: 2 User Agents only (MUA สองฝั่ง)

Flow:

1. Alice's UA sends message to mail server boxes
2. Server stores message in Bob's mailbox
3. Bob's UA retrieves message from mail server

When used: Both sender and receiver have accounts on the same mail server

Analogy: Like leaving a note in your roommate's mailbox in your shared apartment building

Scenario 2: Different Mail Servers

Components needed:

• 2 User Agents
• 1 pair of MTAs (Client and Server)

Flow:

1. Alice's UA sends to her mail server's MTA Client
2. MTA Client connects to spool
3. MTA Client sends across Internet using SMTP
4. Recipient's MTA Server receives message
5. Stored in Bob's mailbox (Boxes)
6. Bob's UA retrieves the message

When used: Sender and receiver are on different mail servers

Analogy: Like mailing a letter from one city's post office to another city's post office

Scenario 3: Sender via LAN/WAN

Components needed:

• 2 User Agents
• 2 pairs of MTAs (Client and Server for each side)

Flow:

1. Alice's UA sends to MTA Client over LAN/WAN
2. First MTA Client sends to first MTA Server
3. Message stored in spool
4. Second MTA Client picks up from spool
5. Sends across Internet
6. Recipient's MTA Server receives
7. Stored in Bob's mailbox
8. Bob's UA retrieves message

When used: Sender is connected to mail server via LAN or WAN

Analogy: Like sending a letter from your office's internal mail room, which then forwards it to the city post office

Scenario 4: Both via LAN/WAN (Most Common Today)

Components needed:

• 2 User Agents
• 2 pairs of MTAs (Client and Server)
• 1 pair of MAAs (Client and Server)

Flow:

1. Alice's UA sends to MTA Client over LAN/WAN
2. MTA Client sends to MTA Server
3. Message stored in spool
4. Another MTA Client picks from spool
5. Sends across Internet using SMTP
6. Recipient MTA Server receives
7. Stored in mailbox (Boxes)
8. MAA Server holds the message
9. Bob's MAA Client requests message
10. Bob's UA displays message

When used: Both sender and receiver connect via LAN/WAN (modern email scenario)

This is how Gmail and iCloud Mail work for you! Your browser/app acts as both the UA and MAA Client, connecting to Google's or Apple's servers over the internet.

Push vs Pull Protocols

มาดูภาษาที่มันใช้คุยกันบ้าง (Protocol)

Push Protocol (SMTP)

• Direction: Client → Server
• Usage: Used by MTA Client to push messages to MTA Server
• Stages: Used in first and second stages of mail delivery

Analogy: Like actively delivering a package to someone's doorstep

Pull Protocol (POP3/IMAP)

ดึง Email จาก Server มา MUA เรา

• Direction: Server → Client (upon request)
• Usage: Used by MAA Client to pull messages from MAA Server
• Process:
  1. Client sends request
  2. Server sends response with email data
• Stages: Used in third stage (message retrieval)

Analogy: Like going to the post office to pick up a package that's waiting for you

• POP3 ดึงเอาลงมาเก็บ (Defualt: ลบสำเนาบน Server ทิ้งไปด้วย)
  • อันนี้เหมาะกับคนที่ดูบนเครื่องเดียวเท่านั้นอะ
• IMAP คือจะจัดการบน Server, Email ทุกอย่างจะถูกเก็บไว้บน Server เป็นหลัก
  • ทำให้เข้าดูจากหลายเครื่องได้ เห็นข้อมูลชุดเดียวกัน
  • ส่วนใหญ่ใช้อันนี้นะ

Email Address Format

Structure

[Local part] @ [Domain name]

Components:

• Local part: Mailbox address of the recipient (username)
• @: Separator symbol
• Domain name: The domain name of the mail server

Example:

forouzan@deanza.edu

• Local part: forouzan
• Domain: deanza.edu

Email Message Structure

Standard Email Format

1. Envelope

Contents:

• Mail From: sender's email address
• RCPT To: recipient's email address

2. Header

Common fields:

• From: Sender's name and address
• To: Recipient's name and address
• Date: Date and time sent
• Subject: Subject line

3. Body

Contents:

• Blank line separator (separates header from body)
• Message content
• Signature

Note: The envelope is like the outside of a physical letter (for routing), while the header is like the letterhead (for display to recipient)

Simple Mail Transfer Protocol (SMTP)

Overview

• Purpose: Defines MTA client and server communication in the Internet
• Type: Push protocol
• Usage: Two pairs of MTA client-server programs in most common scenario

SMTP Communication Range

Protocol Coverage:

[Alice's Computer] --SMTP--> [Mail Server] --SMTP--> [Mail Server] --POP3/IMAP--> [Bob's Computer]
                    via LAN/WAN           via Internet            via LAN/WAN

SMTP Commands and Responses

SMTP Commands (Client → Server)

SMTP Response Codes (Server → Client)

Positive Completion Reply (2xx)

Positive Intermediate Reply (3xx)

Transient Negative Completion Reply (4xx)

Code	Description
421	Service not available
450	Mailbox not available
451	Command aborted: local error
452	Command aborted; insufficient storage

Permanent Negative Completion Reply (5xx)

Pattern Recognition:

• 2xx = Success
• 3xx = Need more info
• 4xx = Temporary failure (try again)
• 5xx = Permanent failure (won't work)

SMTP Transaction Examples

Connection Establishment

Flow:

Message Transfer

Flow:

Client → Server: MAIL FROM: forouzan@deanza.edu
Server → Client: 250 OK
Client → Server: RCPT TO: firouz@net.edu
Server → Client: 250 OK
Client → Server: DATA
Server → Client: 354 start mail input
Client → Server: From: Behrouz Forouzan
Client → Server: To: Firouz Mosharraf
Client → Server: Date: 1/6/05
Client → Server: Subject: Network
Client → Server: [blank line]
Client → Server: Dear Mr. Mosharraf
Client → Server: We want to inform you that...
Client → Server: [message content continues]
Client → Server: . (single dot - end of message)
Server → Client: 250 OK

Key Points:

• Envelope section: MAIL FROM and RCPT TO
• Header section: From:, To:, Date:, Subject:
• Blank line separates header from body
• Single dot (.) on a line by itself marks end of message body

Connection Termination

Flow:

Client → Server: QUIT
Server → Client: 221 service closed

Message Access Agents (MAA)

Why MAA is Needed

Problem with SMTP for retrieval:

• SMTP is a push protocol (client pushes to server)
• Third stage needs a pull protocol (client pulls from server)
• Direction of bulk data flow is from server to client

Solution:

• Use MAA with pull protocols: POP3 or IMAP4
• Allows users to retrieve messages from their mailbox

Protocol Usage Stages

Stage 1: [User] --SMTP--> [Mail Server] (Push)
Stage 2: [Mail Server] --SMTP--> [Mail Server] (Push)
Stage 3: [Mail Server] --POP3/IMAP4--> [User] (Pull)

POP3 (Post Office Protocol version 3)

POP3 Transaction Flow

Typical Session:

1. Client → Server: user-name
2. Server → Client: OK
3. Client → Server: password
4. Server → Client: OK
5. Client → Server: list
6. Server → Client: e-mail numbers and their sizes
7. Client → Server: retrieve 1
8. Server → Client: e-mail 1
   ...
n-1. Client → Server: retrieve N
n. Server → Client: e-mail N

Characteristics:

• Simple download protocol
• Typically downloads and deletes from server
• Three states: Authorization, Transaction, Update
• Uses TCP port 110

Analogy: Like going to a post office box, taking all your letters home, and the post office empties your box

IMAP (Internet Message Access Protocol)

IMAP Advantages over POP3

Key Features:

• Messages remain on server
• Can organize messages in folders on server
• Can access same mailbox from multiple devices
• Can download headers only, then fetch specific messages
• Supports searching messages on server
• More complex but more powerful than POP3

Use Cases:

• Multiple device access (phone, tablet, computer)
• Managing large mailboxes
• Slow internet connections (can browse headers first)

Your experience: Gmail and iCloud Mail use IMAP-like protocols, which is why you can see the same emails on your phone, tablet, and computer!

MIME (Multipurpose Internet Mail Extensions)

ตะก่อน Email ส่งได้แต่ Text Eng ล้วน ๆ แล้วอะไรทำให้มันส่งภาษาไทย, รูปภาพ, File ได้ล่ะ?

หน้าที่หลัก ๆ ของ MIME เลยก็คือ แปลงข้อมูลอะไรก็ได้ที่ไม่ใช่ ASCII 7-bit ธรรมดา (รูปภาพ, ภาษาอื่น) ให้ระบบ MTA ดั้งเดิมที่เข้าใจแต่ ASCII 7-bit ได้

Purpose of MIME

Problem:

• Original email system only supports 7-bit NVT ASCII format
• Cannot send non-ASCII data (images, videos, other languages, etc.)

Solution:

• MIME transforms non-ASCII data to NVT ASCII at sender
• MTA sends through Internet
• MIME transforms back to original data at receiver

MIME Process Flow

[Alice's Non-ASCII file] 
     ↓
[MIME encoding to 7-bit ASCII] 
     ↓
[E-mail System transfer]
     ↓
[MIME decoding from 7-bit ASCII]
     ↓
[Bob receives original Non-ASCII file]

Analogy: Like translating a document to a universal language for shipping, then translating it back to the original language when it arrives

MIME Headers

ใน Email มันจะมีการเพิ่มข้อมูลส่วน Header เพื่อบอกรายละเอียดของ MIME เช่น Content-Type บอกว่าข้อมูลที่แนบมาเป็นอะไร, Content-Transfer-Encoding วิธีเข้ารหัสแบบไหน

Email with MIME Structure

┌─────────────────────────────────────────────────────┐
│ E-mail header (From, To, Date, Subject)             │
├─────────────────────────────────────────────────────┤
│ MIME-Version: 1.1                                   │
│ Content-Type: type/subtype                          │
│ Content-Transfer-Encoding: encoding type            │
│ Content-Id: message id                              │
│ Content-Description: textual explanation            │
├─────────────────────────────────────────────────────┤
│ E-mail body                                         │
└─────────────────────────────────────────────────────┘

MIME Header Fields

1. MIME-Version: Indicates MIME version (usually 1.1)
2. Content-Type: Specifies data type and subtype
3. Content-Transfer-Encoding: Specifies encoding method used
4. Content-Id: Unique identifier for this message
5. Content-Description: Human-readable description of non-textual content

MIME Content Types and Subtypes

Data Types in MIME

Real-world example: When you attach a photo to an email in Gmail, it might be encoded as Content-Type: image/jpeg

MIME Content Transfer Encodings

Encoding Types

Base64 Encoding

เหมาะมาก ไฟล์รูปภาพ, ไฟล์โปรแกรม อะไรที่เป็น Binary ล้วน ๆ เลย

How Base64 Works

Process:

1. Take 3 bytes (24 bits) of non-ASCII data
2. Combine and split into four 6-bit groups
3. Convert each 6-bit group to a printable ASCII character
4. Result: 4 ASCII characters for every 3 bytes of data

Example:

Non-ASCII data (3 bytes):
11001100 | 10000001 | 00111001

Combine and split into 6-bit groups:
110011 | 001000 | 000100 | 111001
  51       8        4       57

Convert to Base64 characters:
  z        I        E        5

Efficiency:

• Input: 3 bytes (24 bits)
• Output: 4 characters (32 bits in ASCII)
• Expansion: 33% size increase

Analogy: Like converting a complex instruction manual into simple pictograms that anyone can understand, but it takes more space

Base64 Conversion Table

Special characters:

• + represents value 62
• / represents value 63
• = used for padding at the end

Quoted-Printable Encoding

เหมาะกับข้อมูลที่ส่วนใหญ่เป็นข้อความหรือว่า Text อยู่แล้ว แต่มึตัวอักษรที่ไม่ใช่ ASCII ปนมาเล็กน้อย เช่น Email ภาษาไทยที่มีสระ/วรรยุกษ์

Overview

• Full Name: Quoted-Printable (QP encoding)
• Purpose: Binary-to-text encoding using printable ASCII characters
• Best Used: When data has a small non-ASCII portion

Encoding Rules

For non-ASCII characters:

• Sent as 3 characters
• Format: =XX where XX is hexadecimal representation of the byte

Example:

Mixed ASCII and non-ASCII data:
00100110 | 01001100 | 10011101 | 00111001 | 01001011
    &         L         9D         9          K

After Quoted-printable encoding:
00100110 | 01001100 | 00111101 | 00111001 | 01000100 | 00111001 | 01001011
    &         L          =          9          D          9          K

Result: &L=9D9K

• ASCII characters (&, L, 9, K) remain unchanged
• Non-ASCII byte (10011101 = 9D in hex) becomes =9D

Efficiency:

• More efficient than Base64 when most data is ASCII
• Only expands non-ASCII bytes (3 characters per byte)

Use case: Perfect for emails with mostly English text but containing a few special characters or emoji

Web-Based Email

อันนี้คือจะเป็น Mail ที่อยู่ในเว็บพวก gmail.com ไรงี้ มาดูกันว่ามันต่างกับสิ่งที่มัน Installed มาบนเครื่องอย่าง Apple Mail ยังไงบ้าง

Overview

• Modern websites provide email services to anyone with access
• Common providers: Gmail, Hotmail (Outlook), Yahoo Mail
• Key concept: Uses web browser as the user interface

Case 1: Sender Uses Web-Based, Receiver Uses Traditional

Components:

[Alice] --HTTP--> [Alice's Site] --SMTP--> [Internet] --SMTP--> [Bob's Site] --HTTP--> [Bob]
         Web        SMTP Server                          SMTP Server    
         Browser                                                        Traditional
                                                                        Client

Flow:

1. Alice uses HTTP to send via web browser
2. Alice's site has both HTTP Server and SMTP Client
3. SMTP Client pushes to recipient's SMTP Server via Internet
4. Bob uses HTTP transactions to retrieve from his site

จริง ๆ คือเราคุยกับ Web Server ผ่าน HTTP แทนที่จะคุยกับ Mail Server ด้วย POP/IMAP โดยตรง แต่ว่า Background มันก็ยังใช้ Technology เหล่านั้นอยู่

Your Gmail experience: When you send an email from Gmail's website, this is what's happening behind the scenes!

Case 2: Both Use Web-Based Email

Components:

[Alice] --HTTP--> [Alice's Site] --SMTP--> [Bob's Site] --HTTP--> [Bob]
         Web      HTTP Server              HTTP Server    Web
         Browser  SMTP Client              SMTP Server    Browser

Flow:

1. Alice uses HTTP transactions to submit email
2. HTTP Server receives and passes to SMTP Client
3. SMTP Client sends to Bob's SMTP Server via Internet
4. Bob uses HTTP transactions to retrieve email

Characteristics:

• No traditional email client needed
• All interaction through web browser
• Server handles all SMTP operations
• User only sees HTTP interface

Real-world example: Both you and your friend checking Gmail or iCloud Mail through web browsers - this is Case 2!

Email Security

Security Protocols for Email

1. Pretty Good Privacy (PGP)

มาตรฐานที่ใช้เข้ารหัส Encryption เนื้อหา Email ทำให้คนที่ดักข้อมูลระหว่างทาง อ่านไม่ออก + Digital Signature เป็นการยืนยันว่าส่งมาจากผู้ส่งคนนั้นจริง ๆ (1, 2)

Features:

• Encrypts email content
• Decrypts received emails
• Authenticates messages with digital signatures
• Can encrypt stored files
• Popular program for email encryption

Usage:

• Individual users can implement
• Often used for sensitive communications
• Provides end-to-end encryption

2. Secure MIME (S/MIME)

Features:

• Internet standard for email security
• Digitally signs MIME-based email data
• Uses public key encryption
• Ensures message integrity and authenticity

Encryption Key Types

ทุกคนมีกุญแจอยู่ 2 ดอก ทำงานคู่กัน

1. Public Key - กุญแจสาธารณะ เปิดเผยได้ ใครรู้ก็ได้
2. Private Key - กุญแจส่วนตัว เก็บเป็นความลับสุดยอด ห้ามให้ใครรู้
   สมมติ A จะส่งข้อความลับหา B: A จะต้องเอา Public Key ของ B มาใช้ในการเข้ารหัสข้อความ แล้วส่ง Email ที่เข้ารหัสแล้วไปให้ B พอ B ได้รับ ก็จะใช้ Private Key ของตัวเอง มาถอดรหัส Email นั้น ๆ ก็จะอ่านได้

Public Key

• Availability: Available to everyone
• Storage: Publicly accessible repository or directory
• Purpose: Anyone can use to encrypt messages to you
• Security: Safe to share openly

Private Key

• Availability: Known only to the owner
• Storage: Kept secret and secure
• Purpose:
  • Decrypt messages encrypted with your public key
  • Sign messages to prove they came from you
• Security: Must never be shared

Email Encryption Process

[Sender] → Use recipient's PUBLIC KEY to encrypt → [Encrypted Email] → [Internet]
                                                                           ↓
[Recipient] ← Use own PRIVATE KEY to decrypt ← [Encrypted Email arrives]

Flow:

1. Sender encrypts plaintext with recipient's public key
2. Encrypted email travels through internet
3. Recipient decrypts with their own private key
4. Plaintext is recovered

Analogy: Public key is like a padlock you give to everyone - they can lock messages, but only you have the key to unlock them!

Summary

Key Concepts Review

1. Email Architecture:
   • Four scenarios explain different email delivery methods
   • Fourth scenario (both via LAN/WAN) is most common today
2. User Agent (UA):
   • Provides services for reading, sending, receiving email
   • Two types: command-driven and GUI-based
3. Message Transfer Agent (MTA):
   • Handles email delivery between servers
   • Uses SMTP protocol
   • Works in client-server pairs
4. Email Transfer Phases:
   • Connection establishment (HELO)
   • Message transfer (MAIL FROM, RCPT TO, DATA)
   • Connection termination (QUIT)
5. Message Access Agents (MAAs):
   • POP3: Downloads and typically deletes from server
   • IMAP: Keeps messages on server, allows organization
6. MIME:
   • Transforms non-ASCII data to ASCII format
   • Enables sending images, videos, attachments
   • Multiple content types and encoding methods
   • Base64: For binary data (33% size increase)
   • Quoted-printable: For mostly ASCII with some non-ASCII
7. Web-Based Email:
   • Uses HTTP for user interaction
   • SMTP still used for server-to-server transfer
   • Examples: Gmail, Outlook, Yahoo Mail
8. Email Security:
   • PGP: Popular encryption program
   • S/MIME: Internet standard for secure email
   • Uses public/private key cryptography

Quick Reference

Port Numbers

• SMTP: Port 25 (or 587 for submission)
• POP3: Port 110
• IMAP: Port 143
• HTTP: Port 80
• HTTPS: Port 443

Protocol Summary

• SMTP: Push protocol for sending
• POP3: Pull protocol for retrieval (download & delete)
• IMAP: Pull protocol for retrieval (keep on server)
• HTTP/HTTPS: Web-based email access

Common Response Codes

• 220: Service ready
• 250: Command successful
• 354: Start sending message
• 421: Service unavailable
• 550: Mailbox unavailable

Additional Resources

Terms to Remember

• UA: User Agent (your email client)
• MTA: Message Transfer Agent (email server software)
• MAA: Message Access Agent (retrieval software)
• SMTP: Simple Mail Transfer Protocol (sending)
• POP3: Post Office Protocol (simple retrieval)
• IMAP: Internet Message Access Protocol (advanced retrieval)
• MIME: Multipurpose Internet Mail Extensions (attachments)
• MUA: Mail User Agent (another term for UA)
• NVT: Network Virtual Terminal (ASCII format)