Electronic Mail (E-mail) - Architecture

 

Electronic Mail (E-mail)

Introduction

Electronic Mail (E-mail) is one of the most widely used Internet applications that enables users to exchange electronic messages over the Internet.

Unlike HTTP or FTP, e-mail is not a simple request–response service. It is primarily a one-way communication system, where the sender sends a message and the receiver may or may not reply. Since users are not always online, e-mail relies on mail servers to temporarily store and forward messages.

E-mail Architecture

The electronic mail system follows a store-and-forward architecture using mail servers as intermediaries between the sender and the receiver.

A typical e-mail system consists of:

  • Sender (Alice)
  • Sender's Mail Server
  • Receiver's Mail Server
  • Receiver (Bob)

The architecture uses three types of software agents:

  1. User Agent (UA)
  2. Mail Transfer Agent (MTA)
  3. Message Access Agent (MAA)

Components of E-mail Architecture

1. User Agent (UA)

The User Agent (UA) is the software used by users to compose, send, receive, reply to, and forward e-mails.

Functions

  • Compose messages
  • Read received messages
  • Reply to messages
  • Forward messages
  • Manage local mailboxes

Types of User Agents

(a) Command-driven UA

  • Operated using keyboard commands
  • Older style interface

Examples:

  • mail
  • pine
  • elm

(b) GUI-based UA

  • Graphical interface
  • Uses windows, menus, icons, mouse, and keyboard

Examples:

  • Microsoft Outlook
  • Eudora

2. Mail Transfer Agent (MTA)

The Mail Transfer Agent (MTA) transfers e-mail messages from one mail server to another.

It follows the client-server paradigm.

Functions

  • Transfers outgoing mail
  • Delivers mail between mail servers
  • Uses a queue (spool) to store outgoing messages
  • Pushes messages to the destination mail server

The MTA client is activated whenever there is a message waiting in the spool.

3. Message Access Agent (MAA)

The Message Access Agent (MAA) allows the receiver to retrieve messages stored in the mailbox.

Unlike MTA, which pushes messages to another server, MAA pulls messages from the receiver's mailbox.

This allows users to receive mail whenever they connect to the Internet.

Working of E-mail Architecture

The complete process involves the following steps:

  1. Alice composes an e-mail using the User Agent (UA).
  2. The UA sends the message to Alice's mail server.
  3. The message is stored temporarily in the spool (queue).
  4. The MTA client transfers the message to Bob's mail server over the Internet.
  5. The MTA server at Bob's mail server receives the message.
  6. The message is stored in Bob's mailbox.
  7. Bob starts his User Agent (UA).
  8. The MAA client requests Bob's mailbox.
  9. The MAA server delivers the stored message to Bob.

Mailbox and Spool

Mailbox

A mailbox is a special file on the mail server where incoming messages for a user are stored.

  • One mailbox is created for each user.
  • Only the mailbox owner can access it.

Spool (Queue)

A spool is a temporary storage area used to hold outgoing messages waiting for transmission.

Messages remain in the spool until the MTA transfers them.

E-mail Address

Every e-mail user has a unique address.

Format

Local-part@Domain-name

Example

binu@mec.ac.in

Components

PartDescription
Local Part    User mailbox name
@    Separator
Domain Name    Mail server domain

Format of an E-mail

An e-mail consists of two major parts:

1. Envelope

Contains routing information.

Examples:

  • Sender address
  • Receiver address

2. Message

The message consists of:

Header

Contains information such as:

  • From
  • To
  • Date
  • Subject

Body

Contains the actual message.



Mailing List (Group List)

A mailing list allows one alias (group name) to represent multiple e-mail addresses.

When an e-mail is sent to the alias:

  • The mail server expands the alias.
  • Separate copies of the message are delivered to all members.

Example:

faculty@mec.ac.in

may automatically deliver the e-mail to all faculty members.

Advantages of E-mail Architecture

  • Users need not be online simultaneously.
  • Messages are stored safely until retrieved.
  • Supports one-to-many communication using mailing lists.
  • Reliable message delivery through mail servers.
  • Easy management using mailboxes and queues.

Summary

ComponentFunction
User Agent (UA)    Composes, sends, receives, and manages e-mails
Mail Transfer Agent (MTA)    Transfers e-mails between mail servers
Message Access Agent (MAA)    Retrieves e-mails from the receiver's mailbox
Mailbox    Stores incoming messages
Spool (Queue)    Stores outgoing messages waiting to be sent
E-mail Address    Identifies the recipient's mailbox and mail server

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