Electronic mail in computer network


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Electronic mail is One of the most popular Internet services. The designers of the Internet probably never imagined the popularity of this application program. 

The sender and the receiver of the e-mail are users (or application programs) on the same system; they are directly connected to a shared system. The administrator has created one mailbox for each user where the received messages are stored. A mailbox is part of a local hard drive, a special file with permission restrictions. Only the owner of the mailbox has access to it.

User Agent 

The first component of an electronic mail system is the user-agent (VA). It provides a service to the user to make the process of sending and receiving a message easier.

There are two types of user agents: command-driven and GUI-based.

A command-driven user agent normally accepts a one-character command from the keyboard to perform its task. 

For example, a user can type the character r, at the command prompt, to reply to the sender of the message, or type the character R to reply to the sender and all recipients.

GUI-Based Modem user agents are GUI-based. They contain graphical-user interface (GUI) components that allow the user to interact with the software by using both the keyboard and the mouse. 

They have graphical components such as icons, menu bars, and windows that make the services easy to access. Some examples of GUI-based user agents are Eudora, Microsoft’s Outlook, and Netscape.

Services Provided by a User-Agent


A user agent is a software package (program) that composes, reads, replies to, and forwards messages. It also handles mailboxes.

Composing Messages 

A user agent helps the user compose the e-mail message to be sent out. Most user agents provide a template on the screen to be filled in by the user. Some even have a built-in editor that can do spell checking, grammar checking

Reading Messages 

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The second duty of the user agent is to read the incoming messages. Each e-mail contains the following fields.

  • A number field.
  •  A flag field that shows the status of the mail such as new, already read but not replied to, or read and replied to.
  • The size of the message.
  • The sender.
  • The optional subject field.

Replying to Messages 

After reading a message, a user can use the user agent to reply to a message.

Forwarding Messages 

Replying is defined as sending a message to the sender or recipients of the copy. Forwarding is defined as sending a message to a third party.

Handling Mailboxes

A user agent normally creates two mailboxes: an inbox and an outbox. The inbox keeps all the received e-mails until they are deleted by the user. The outbox keeps all the sent e-mails until the user deletes them.

Local Part 

The local part defines the name of a special file, called the user mailbox, where all the mail received for a user is stored for retrieval by the message access agent.

Domain Name 

The second part of the address is the domain name.


Electronic mail has a simple structure. Its simplicity, however, comes at a price. 

It can send messages only in NVT 7-bit ASCII format. In other words, it has some limitations. For example, it cannot be used for languages that are not supported by 7-bit

ASCII characters (such as French, German, Hebrew, Russian, Chinese, and Japanese). Also, it cannot be used to send binary files or video or audio data. 

Multipurpose Internet Mail Extensions (MIME) is a supplementary protocol that allows non-ASCII data to be sent through e-mail. MIME transforms non-ASCII data at the sender site to NVT ASCII data and delivers them to the client MTA to be sent through the Internet. The message at the receiving side is transformed back to the original data.

  • MIME defines five headers that can be added to the original e-mail header section to define the transformation parameters:
  • MIME-Version This header defines the version of MIME used. The current version is 1.1.
  • Content-Type This header defines the type of data used in the body of the message.
  • Content-Transfer-Encoding This header defines the method used to encode the messages into Os and Is for transport.
  • Content-Description This header defines whether the body is image, audio, or video.

Message Transfer Agent: SMTP 

 The actual mail transfer is done through message transfer agents. 

 To send mail, a system must have the client MTA, and to receive mail, a system must have a server MTA. 

 The formal protocol that defines the MTA client and server on the Internet is called the SimpleMail Transfer Protocol (SMTP).

Responses are sent from the server to the client. A response is a three-digit code that may be followed by additional textual information.

Let us see how we can directly use SMTP to send an �e-mail and simulate the commands and responses we described in this section. We use TELNET to log into port 25 (the well-known port for SMTP). We then use the commands directly to send an e-mail. In this example, forouzanb@adelphia.net is sending an e-mail to himself. The first few lines show TELNET trying to connect to the Adelphia mail server. After connection, we can type the SMTP commands and then receive the responses, as shown on the next slide. Note that we have added, for clarification, some comment lines, designated by the “=” signs. These lines are not part of the e-mail procedure.

Mail Transfer Phases


The process of transferring a mail message occurs in three phases: connection establishment, mail transfer, and connection termination. The first and the second stages of mail delivery use SMTP. However, SMTP is not involved in the third stage.

SMTP is a push protocol: PUSH from client to server.

On the other hand, the third stage needs a pull protocol; the client must pull messages from the server. The third stage uses a message access agent.

Currently, two message access protocols are available: Post Office Protocol, version 3(POP3), and Internet Mail Access Protocol, version 4 (IMAP4).



 Post Office Protocol, version 3 (POP3) is simple and limited in functionality. 

 The client POP3 software is installed on the recipient computer; the server POP3 software is installed on the mail server.

  POP3 has two modes: the delete mode and the keep mode. In the delete mode, the mail is deleted from the mailbox after each retrieval. In the keep mode, the mail remains in the mailbox after retrieval.


Another mail access protocol is Internet Mail Access Protocol, version 4 (IMAP4). IMAP4 is similar to POP3, but it has more features.

POP3 is deficient in several ways. It does not allow the user to organize her mail on the server; the user cannot have different folders on the server. (Of course, the user can create folders on her own computer.) In addition, POP3 does not allow the user to partially check the contents of the mail before downloading.

IMAP4 provides the following extra functions:


  1.  A user can check the e-mail header before downloading.
  2.  A user can search the contents of the e-mail for a specific string of characters before downloading.
  3.  A user can partially download an e-mail. This is especially useful if bandwidth is limited and the e-mail contains multimedia with high bandwidth requirements.
  4.  A user can create, delete, or rename mailboxes on the mail server.
  5.  A user can create a hierarchy of mailboxes in a folder for e-mail storage.

Web-Based Mail

  • E-mail is such a common application that some websites today provide this service to anyone who accesses the site. Two common sites are Hotmail and Yahoo, Gmail.
  • Mail transfer from Alice’s browser to her mail server is done through HTTP.
  • The transfer of the message from the sending mail server to the receiving
  • mail server is still through SMTP. 
  • Finally, the message from the receiving server (the Web server) to Bob’s browser is done through HTTP.


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