Email

 

Electronic Mail Application in a Network


  • Electronic Mail (E-mail) is one of the most widely used network applications on the Internet.
  • It allows users to send, receive, store, and manage messages across computer networks.
  • Advantages: Fast delivery, low cost, accessibility from anywhere, ability to attach files.
Main components of Email:

1. User Agent (UA)


2. Message Transfer Agent (MTA) – uses SMTP


3. Message Access Agent (MAA) – uses IMAP or POP

Steps :

1. UA → MTA Client

  • UA (User Agent) is the email application (like Outlook, Gmail web interface) used by Alice to compose and send an email.
  • Once Alice hits “send,” the UA hands the email to the MTA Client.

2. MTA Client → MTA Server (Sender Side)

  • The MTA (Message Transfer Agent) Client sends the message to the MTA Server.
  • The MTA server manages outgoing mail and queues it for delivery.

3. MTA Server → Mail Queue (Pool)

  • If the destination server is not immediately reachable, the mail is stored in a mail pool (queue) until it can be sent.

4. Mail Pool → MTA Client (for delivery)

  • The MTA client picks the mail from the queue and prepares it for sending over the network.

5. MTA Client → Internet → MTA Server (Receiver Side)

  • The MTA client sends the email through the Internet using SMTP (Simple Mail Transfer Protocol).
  • On the receiving side, the MTA Server accepts the email.

6. MTA Server → Mailbox (Receiver)

  • The receiver's MTA server stores the email in the recipient’s mailbox until they retrieve it.

7. Mailbox → MAA Server

  • The MAA (Message Access Agent), using protocols like POP3 or IMAP, manages retrieval requests from the receiver’s UA.

8. MAA Server → MAA Client

  • The MAA server sends the stored email to the MAA Client on the receiver’s device.

9. MAA Client → UA (Receiver)

  • The MAA Client delivers the email to the User Agent of the receiver, allowing them to read it.


2. Components of E-mail System

An electronic mail system in a network involves three main components:

a. User Agent (UA)

  • Software application used by the user to compose, send, read, and manage emails.
  • Examples: Microsoft Outlook, Gmail, Thunderbird.
  • Functions:
    • Create new messages.
    • Read incoming messages.
    • Reply, forward, delete messages.
    • Attach files.

b. Message Transfer Agent (MTA)

  • Responsible for transferring messages from sender to recipient’s mail server.
  • Uses SMTP (Simple Mail Transfer Protocol) to send messages.
  • Examples: Sendmail, Postfix, Microsoft Exchange Server.

c. Message Access Agent (MAA)

  • Retrieves and manages messages from the mail server for the user.
  • Uses IMAP or POP protocols.
  • IMAP (Internet Message Access Protocol): Keeps messages on server; allows access from multiple devices.
  • POP (Post Office Protocol): Downloads messages to device; often removes from server.

WORKING OF EMAIL

  • Same System: Only two User Agents are needed, no MTA.
  • Different Systems: Requires two UAs, two pairs of MTAs (client and server), and two MAAs (client and server).

Process:

  1. Sender (Alice) uses UA to create and send message to her Mail Server.
  2. Mail Server stores the message in a spool (queue) until it is sent.
  3. MTA client sends the message to the MTA server at the receiver’s site via the Internet.
  4. Receiver’s UA allows reading of the message.
  5. Receiver (Bob) uses MAA client to retrieve the message from MAA server.

USER AGENT (UA)

  • Software for sending, receiving, and managing emails.
  • Functions: composing, reading, replying, forwarding messages, handling mailbox.

Types:

  • Command-driven: Uses keyboard commands (e.g., mail, pine, elm).
  • GUI-based: Uses icons, menus, and windows (e.g., Eudora, Outlook).

MESSAGE TRANSFER AGENT (MTA)

  • Handles actual mail transfer.
  • Sending mail: client MTA needed.
  • Receiving mail: server MTA needed.
  • Uses Simple Mail Transfer Protocol (SMTP).

MESSAGE ACCESS AGENT (MAA)

  • Retrieves messages from mailbox.
  • Examples: POP3, IMAP4.

ADDRESS FORMAT OF EMAIL

  • Format: userid@domain
    • Local part: address of mailbox on mail server.
    • Domain name: mail server hostname.

MESSAGE FORMAT OF EMAIL

  • Two parts:
    1. Header – contains fields like:
      • From: sender’s identifier.
      • To: recipient’s address.
      • Subject: purpose of message.
      • Date: time of sending.
    2. Body – actual message.
  • Header and body are separated by a blank line.


4. Protocols Used in E-mail Applications

a. SMTP (Simple Mail Transfer Protocol)

  • Purpose: Sending and relaying messages between mail servers.
  • Port Number: 25 (also 587 for client-to-server submission).
  • Operation:
    • Establishes a connection between sending and receiving mail servers.
    • Transfers message data in text format.
    • Uses commands like HELO, MAIL FROM, RCPT TO, DATA, QUIT.

b. MIME (Multipurpose Internet Mail Extensions)

  • Extends SMTP to handle multimedia content like images, audio, and attachments.
  • Converts binary files into text format for safe email transfer.

c. POP (Post Office Protocol)

  • Version: POP3 is most common.
  • Downloads messages from mail server to local device.
  • Port Number: 110 (SSL: 995).
  • Suitable for users who access email from a single device.

d. IMAP (Internet Message Access Protocol)

  • Keeps messages on the server and allows access from multiple devices.
  • Port Number: 143 (SSL: 993).
  • Supports folder organization, message searching, and synchronization.

5. Example of Email Transmission

Scenario: Alice sends an email to Bob.

  1. Alice composes a message in Gmail (User Agent).
  2. Gmail’s outgoing mail server (MTA) uses SMTP to send the message.
  3. The message travels across the network to Bob’s mail server.
  4. Bob’s UA (e.g., Outlook) uses IMAP to fetch the message from his server.
  5. Bob reads the message.

6. Security in E-mail Applications

  • TLS/SSL encryption for SMTP, POP, IMAP to protect message content during transmission.
  • Authentication to prevent unauthorized access.
  • Spam filters to block unwanted mail.
  • Digital signatures to ensure authenticity.

7. Advantages of E-mail in Networking

  • Speed: Instant delivery.
  • Cost-effective: Minimal transmission cost.
  • Accessibility: Available worldwide.
  • Attachments: Send documents, images, videos.
  • Searchability: Easy to find past messages.

8. Limitations

  • Requires Internet access.
  • Spam and phishing threats.
  • May face server downtime or delays.