Mail Server

When a mail is sent it has to reach its proper destination. In order that the mail is sent to the proper destination, the destination site has to be running a program called a mail server that listens for requests to deliver mail. The mail server does the following:

• Accept the message and store it in the expected mailbox.
• Forward the message somewhere else, usually to a place specified by the owner of the mailbox,but possibly to a mailing list.
• Reject the message as undeliverable, either because the mailbox does not exist or because the mailbox is full or because the server is facing some temporary problems.

There are basically two kinds of mail servers based on the protocol. They are:

• Basic Simple Mail Transfer Protocol (SMTP) delivery. The server translates the mailbox name into a local file name and appends the message to the file.
• Post Office Protocol (POP) delivery. The server still stores messages somewhere, in a place derived from the mailbox name. However, it allows mail-receiving connections from other Internet sites. The mail agent on the recipient's site knows to open an Internet connection to the POP server, request contents of particular messages and (optionally) remove messages from the server's mailbox.

POP service is newer than SMTP service; it has the large advantage that the mail can be accessed from anywhere on the Internet, without logging into the server.

Electronic Mail

Electronic mail or e-mail involves transmission of messages over communication network. The messages can be notes or files. Some electronic-mail systems are confined to a single computer or network, but others have gateways to other computer systems, enabling users to send electronic mail anywhere in the world. Companies that are fully computerized make extensive use of e-mail because it is fast, flexible and reliable.

Electronic communication, because of its speed and broadcasting ability, is fundamentally different from paper-based communication. Because the turnaround time can be so fast, email is more conversational than traditional paper-based media.

Most e-mail systems include a rudimentary text editor for composing and editing messages. A message is sent to the recipient by specifying the recipient's address. An address is a text string of the form mailbox@site. The second part is a string identifying a particular site on the Internet; the first part is a string identifying a particular mailbox at that site. For example, consider a mail id like abc_def@yahoo.com. In this case, abc_def is the username and yahoo.com is the website.

Every Internet site has an Internet Protocol (IP) address, specified as four decimal numbers (each in the range 0-255) separated by dots. The transport service sends the site name string to a Domain Name Server (DNS), which translates the name into an IP address. The transport service then starts up an Internet connection to the given IP address and task the destination site to deliver mail to the given mailbox.

A message can be sent to several users at once. This is called broadcasting. The sent message are stored in electronic mailboxes of the recipient. The recipient has to check the mailbox to see if mail has been received and can decide on whether to save it or remove it off the mailbox.

Different e-mail systems use different formats and there are some emerging standards that make it possible for users on all systems to exchange messages. An important e-mail standards is MAPI. The CCITT standards organization has developed the X.400 standard, which attempts to provide a universal way of addressing messages.