Chapter 9: Email Systems & How They Work

	Intelligentedu.com Home -> Learn About Computers, Software Development & Information Technology -> Table of Contents

Chapter 9:
Email Systems & How They Work

Introduction

Email can be sent internally to members of an organization through their internal computer network or they can be sent externally to anyone in the world by using web based email services. It's very similar to the traditional mail system with post boxes, post offices, envelopes and addresses. The difference is that here messages are sent electronically to other computer users via Internet, taking a fraction of the time to reach the recipient. Also one can send computer documents, graphics, software or anything else that can be turned into a digital form attached to an email message. In order for messages to be sent from one computer to another, the message needs to be converted into a digital form and forwarded to a computer that acts as a mail server or post office. This mail server sorts and directs mails to and from the users. The only way this mail server can direct mail though, to all users is by being connected to a network that all the users are also connected to.

Basic parts of the email system

1. Client Software
Client software is a user interface that allows users to read, address and send messages. It shows the list of all of the messages in mailbox by displaying the message headers that shows who sent the mail, the subject of the mail and may also show the time and date of the message and the message size. Most email clients also let users add attachments to messages. Client Software connects to the Message Store to display the messages kept there. It may also connect to the Management System to display a centralized address book. Most of the current client software keeps a separate local address book for personal contacts. Some well known examples of the client software are Microsoft Outlook, Outlook Express, Eudora and Pegasus.

2. Message Store
A Message Store can exist in more than one place at the same time. Generally, in an organization where workstation computers are all connected to file server computers with a LAN, its best to keep all email in one main Message Store on the server. Client software at workstation connects and views messages from the server but does not make a separate local copy of it. This model is followed in many semi-permanent message stores like Exchange, Groupwise and Notes. There are also transient Message Stores that keep a copy of the message until it can be sent onto the next destination in its journey. This is commonly referred to as "Store-and-Forward."

3. Message Transport
Message Transport Agents (MTAs) are software systems that keep moving messages along to the intended destination. The first MTA exists in the Client Software that connects with its counterpart on the email server. MTAs always exist in pairs and must agree ahead of time on how they will communicate with each other. These pre-defined communication standards or protocols can be open-standards that are widely used by many products or proprietary and are used only between software packages from the same company. Some open-standards protocols are: SMTP (Simple Message Transfer Protocol) for sending and either POP3 (Post Office Protocol) or IMAP (Internet Message Access Protocol) for receiving. Within a single MTA, a message will usually be moved more than once to different directories on the hard drive using file copy facilities built into the operating system of the host computer. These hard drive directories, dedicated to message traffic, are also known as queues because the messages are dispatched again according to the order in which they arrived.

4. Management Systems
Management software is used to create user mailboxes, to maintain address lists and routing information, and to monitor message traffic.

5. Gateways

Gateways are needed only if a message must be translated in order to travel from a proprietary, private email system to another, or to the public Internet mail system.

Email Protocols

1. SMTP

SMTP or Simple Mail Transfer Protocol is a protocol for sending email messages between servers. It is the most common protocol for sending email between two servers of the Internet. These emails can then be retrieved from servers either (Post Office Protocol) POP or (Internet Message Access Protocol) IMAP. SMTP servers are also sometimes referred as outgoing mail servers.

2. POP
POP or Post Office Protocol is a protocol used to retrieve email from a mail server. Almost all email applications use the POP protocol. There are two versions of POP, namely, POP2 (requires SMTP to send messages) and POP3 (can be used with or without SMTP).

3. IMAP
IMAP or Internet Message Access Protocol is a protocol for retrieving email messages from the mail server. The latest version, IMAP4, is similar to POP3 but supports some additional features. For example, with IMAP4, one can search through email messages for keywords while the messages are still on mail server. User can then download chosen messages to the machine.

4. MIME
MIME or Multipurpose Internet Mail Extensions is a protocol used for formatting non-ASCII messages to be sent the Internet. Many of the current email services support MIME, enabling them to send and receive graphics, audio, and video files through the email system. In addition, MIME supports messages in character sets other than ASCII.

Message Paths of Emails

1. The Intra-Office Message
An intra-office message never leaves the secure private LAN of the corporate offices. It is written by one of the users within the organization, is stored on the corporate file server, gets read by another user at a different workstation, but is never copied or stored on the hard drive at either workstation. The contents of the message may be viewed over the LAN many times without ever being copied.

2. To an Individual outside the Corporate LAN
In this message transmission, usually the recipient is an individual at home or other place, rather than in an office behind the corporate firewall. It is possible for an office to receive email for several individuals through a single POP3 mailbox, referred as multi-drop POP. It is also possible for an office email server to collect mail from several POP3 mailboxes to redistribute to people inside the corporate network, referred as multi-POP. After leaving the corporate mail server, email messages travel over the public and unprotected Internet systems.

3. The Internal message accessed from outside the Corporate LAN
In this message path, email message has a dual life. Message is first written and stored inside the corporate LAN. Then the originator and/or the recipient need to connect to the corporate LAN from outside. They use the public Internet just to establish a connection with the office LAN. Using that temporary connection they make copies of their messages to work with later off-line. Message transits the public Internet via open standards protocols of POP3, IMAP or SMTP.

4. From one private LAN to another private LAN
Two separate LANs could be could be connected using leased line, but that would be the same as having the two systems on the same LAN. Generally, two separate LANs use the public Internet for a connection. Message gets written by one of the users and any LAN and it is then stored on the file server of that LAN. The file servers of both LANs connect to each other via mail servers that serve as intermediaries. The message is then copied to the other file server from which it gets read by another user at a different LAN, but is never copied or stored on the hard drive at either workstation. The contents of the message may be viewed over either LAN many times without ever being copied. Temporary copies of the message are made on the hard drive of each of the two mail servers. The message transits both between private LANs and over the public Internet.