How Does E-mail Work?
Every Internet mail user has a unique Internet e-mail address. This e-mail address is in the format as:
username@domainname
To send an e-mail to someone, you need to know only the person's e-mail address. You need not know the hardware and software specification of the recipient. Figure 12.7 shows how mail messages move across the Internet.
The figure shows a very simplified model of an e-mail system. Following steps are involved in sending an e-mail message:
Step 1: The sender composes the mail message using his mail client software. A mail client allows a user to compose, edit and send the mail message. There are a number of mail client software available. Netscape Mail, Outlook Express, Eudora, Pine, etc., are examples of mail clients.
Step 2: After composing the mail message, the user sends it to the recipient's
e-mail address. The message propagates across the Internet before it reaches the mail server of the recipient. The domain name in the recipient's e-mail address identifies his mail server and the username identifies the recipient on the server. For example, when you send mail to registrarQajpl.vsnl.net.in the address of the mail server is jpl.vsnl.net.in and the username is registrar.
Step 3: The recipient connects to his e-mail account on his mail server to read the messages sent to him.
The recipient also uses a mail client to receive, save and print mail messages. Figure 12.7 may give you an impression that the mail messages are directly sent to the recipient's mail server. There is no direct link between the sender's computer and the recipient's mail server. A mail message propagates across several networks on the Internet before it reaches its destination.
Internet Mail Protocols
Internet e-mail is based on standards such as Simple Mail Transfer Protocol (SMTP), Post Office Protocol (POP), Internet Message Access Protocol (IMAP) and Multipurpose Internet Mail Extensions (MIME). These standards are known as mail protocols.
The SMTP specifies how messages are sent on the Internet. POP and IMAP define how mail clients can access messages on a mail server, and the MIME standard lets the mail client understand different types of data such as graphics, video, application files and text files.
An e-mail client based on these protocols can exchange messages with any other user on the Internet. These protocols are very important because there are a number of mail clients available in the market. Each of them provides a different set of features. The mail protocols help in standardizing the process of exchanging mail protocols, you need not know which mail client the recipient is using, what operating system he is using or what type of computer he is using. You just have to know his e-mail address to send message to him. The following are the mail protocols:
The simple mail transfer protocol (SMTP)
One of the most popular network services is electronic mail. The TCP/IP protocol that supports electronic mail on the Internet is called Simple Mail Transfer Protocol (SMTP).When a mail transfer program contacts a server on a remote machine, it forms a TCP connection over which it communicates. Once the connection is in place, the two programs follow the SMTP that allows the sender to identify itself, specify a recipient and transfer an e-mail message. SMTP provides for mail exchange between users on the same or different computers and supports:
• Sending a single message to one or more recipients.
• Sending messages that include text, voice or graphics.
• Sending messages to users on networks outside the Internet.
SMTP uses the ASCII character set for composing a message. An Internet mail message has two parts: a header and a body. The header of the message includes the address of the recipient, address of the sender, subject and other information about the message such as date and time when it was sent, type of mailing client the sender is using, etc. The body of the message contains the actual message. Figure shows the format of an e-mail message.
Mail header:
The SMTP protocol is used to transfer a message from the SMTP sender to the SMTP receiver over a TCP connection. SMTP attempts to provide reliable operation but does not guarantee to recover from lost messages. No end-to-end acknowledgement is returned to a message's originator that a message is successfully delivered to the message's recipient; error indications are not guaranteed to be returned either. However, the SMTP-based mail system is generally considered reliable.
SMTP suffers from several drawbacks, such as:
• SMTP can only transmit ASCII characters and thus cannot transmit executable files or other binary objects.
• SMTP does not allow the attachment of files, such as images and audio.
• SMTP can only transmit 7-bit ASCII character, thus it does not support an extended ASCII character set.
Post office protocol (POP)
SMTP expects the destination host, the mail server receiving the mail, to be on-line all the time; otherwise, a TCP connection cannot be established. For this reason, it is not practical to establish an SMTP session with a desktop computer because desktop computers are usually powered down at the end of the day.
In many organizations, mail is received by an SMTP server, that is always online. This SMTP server provides a mail drop service. The server receives the mail on behalf of every host in the organization. Workstations interact with the SMTP host to retrieve messages by using a client-server protocol such as Post Office Protocol (POP). Post Office Protocol requires an additional server to run on the computer with the mailbox. The additional server, of course, uses the POP protocol. A user runs e-mail software that becomes a client of the POP server to access the contents of the mailbox. Figure 12.9 illustrates one way to use POP.
Note that the mail can arrive from the sender's computer or mail gateway. To retrieve messages from the mailbox, a user runs a program that becomes a client of the POP server. As the figure shows, a computer that has a mailbox must run two servers. A conventional mail server accepts incoming e-mail and stores it in the appropriate mailbox. The mail can arrive either directly from the original sender or from a mail gateway. (A computer dedicated to processing electronic mail is often called a mail gateway). A POP server allows a user on a remote machine to access the mailbox.
Although both the e-mail server and POP server communicate across the Internet, there are several differences. First, the mail server uses the SMTP protocol, while the POP server uses the POP protocol. Second, the mail server accepts a message from an arbitrary sender, while the POP server allows a user to access the mailbox only after the user enters authentication information (say, the password) messages. Third, the mail server can transfer only e-mail messages, while a POP server can provide information about the mailbox contents.
Although Figure shows a client accessing the POP server across the Internet, POP is especially popular among users who rely on dialup telephone connections. In such cases a computer with the user's mailbox remains attached to the Internet as shown in the figure. However, the user's computer does not need a permanent Internet connection. Instead, the computer can attach to a modem and use a telephone connection. To receive e-mail, the user forms a dialup connection either to the mailbox computer or to some other computer on the Internet. Once the user connects to a computer on the Internet, the user can run a POP client to contact the server and access e-mail.
The POP was developed for single user computers. There are three versions of this protocol: POP, POP2 and POP3.
Internet message access protocol (IMAP)
Developed at Stanford University in 1986, the Internet Message Access Protocol is for retrieving e-mail messages. The latest version, IMAP4, is similar to POP3 but supports some additional features. For example, with IMAP4, you can search through your e-mail messages for keywords while the messages are still on mail server. You can then choose which messages to download to your machine. Like POP, IMAP uses SMTP for communication between the e-mail client and server.
IMAP4 provides the following extra functions:
• A user can check the e-mail header prior to downloading.
• A user can search the contents of the e-mail for a specific string of characters prior to downloading.
• A user can partially download the e-mail. This is especially useful if bandwidth is limited and the e-mail contains multimedia with high bandwidth requirements.
• A user can create, delete, or rename mailboxes on the mail server.
• A user can create a hierarchy of mailboxes in a folder for e-mail storage.
Multipurpose internet maid extension (MIME)
The SMTP can be used to send only messages that are composed using ASCII character set. This restricts the utility of electronic mail. There is another protocol MIME that can be used to exchange e-mail messages containing non-textual data such as graphics, sound, and other multimedia files.
Multipurpose Internet Mail Extensions (MIME) offer a way to extend Internet standard mail so that users can interchange text in languages with different character sets and multimedia electronic mail among different computer systems that have implemented Internet standard mail.
Whenever you want to send a non-text file such as a spreadsheet, program file, graphics file or a sound file, you can encode this file using MIME. The MIME encodes these files in a textual form, which can be sent using the SMTP. The recipient can then decode the MIME-encoded data to the original non-text file. Most e-mail clients such as Netscape automatically encode and decode the e-mail messages containing non-text data. However, there are some e-mail clients, which require you to encode data using an encoding utility such as Info-X-Fer.
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