Chapter 8: Electronic Mail

Overview

Some people use the Internet for viewing Web pages of dubious content, others for finding worthy adversaries for online games , and many for working and learning. But none of us can live without communication. Despite all of the new technologies invented to make communication easier (IRC, ICQ, etc.), electronic mail, or email for short, has remained one of the main communication means and will always remain so. Email was the impetus to local network development, and it was one of the first Internet services to be offered .

For me, an email client has become the main program I use for corresponding with my readers, friends , coworkers, and so on. The people I work with on my books live in different towns and even different countries . My closest partners are more than 600 miles away, and the publishing house office is 1,000 miles away. I don't know how I would be able to handle this arrangement without email, but with it I can live in the south and work for a company located in the north.

How does email operate ? The following are the main stages of sending an email:

1. A user creates a message using an email client (an email program), specifies the addressee, and sends the message to an email server. Most often, SMTP servers are used for sending email.

2. After receiving the message, the server determines its destination. The email address consists of two parts divided by the at (@) character: the user name and the server name , for example, username@ servername .com . The IP address of the servername.com server is established using DNS.

3. The source mail server sends the message to the server, on which the recipient is registered.

4. After receiving the message, the servername.com server places it into the username user's mailbox.

5. The addressee checks his or her mailbox using a mail client and can download message.

The process just described is similar to how the traditional mail operates. The servers play the role of post offices, which sort the mail by its destination addresses, send it to the addressees, and finally deliver it to their mailboxes.

As was mentioned, for transferring messages, mail servers use SMTP, which was developed at the dawn of the Internet. It has long been considered as lacking functionalities, but it is still used extensively.

Several decades ago, the UNIX-to-UNIX Copy Protocol (UUCP) was employed for working with mail. But it was tied to the specific operating system, and its functionalities were limited; therefore, it did not become commonly used and is rarely employed today.

There exist three protocols for receiving mail. These are the following:

• Post Office Protocol version 3 (POP3) This is the most commonly used protocol for receiving mail today.

• Internet Message Access Protocol version 4 (IMAP4) The capabilities of this protocol are greater than those of POP3.

• Messaging Application Programming Interface (MAPI) This protocol is used in Microsoft networks on Microsoft Exchange servers.

The most commonly used Linux mail package is an old sendmail program. The program possesses great capabilities but is rather difficult to use. Because it was developed so long ago, the sendmail server has UUCP capabilities, which are not that common nowadays.

The operating principle of sendmail is quite simple. After receiving a message from a client, the program determines the recipient and enters the service information necessary to deliver the letter into its header. Further actions depend on the server's configuration. Thus, a letter can be sent immediately or placed into storage to be mailed later. Periodically, the accumulated messages are sent to their addressees.