The Mailbox

The postman arrives some time during the day to deliver postal mails. At that time, the recipient may not be at home. Therefore, the postman deposits the letters in the mailbox. We check our mailbox after returning home from work. Therefore, usually there is some gap between the time the mail is actually delivered in the box and the time it is actually opened. That is why this type of communication is called asynchronous, opposed to the synchronous telephonic conversation, where both parties are communicating at the same time. Similarly, when you write an email to somebody, that person may not have switched his computer on. Should this email reach upto that computer only to find that it cannot accept it? To solve this problem, another computer is given the responsibility of storing email messages centrally. This computer, along with the software, is called email server. The email server is dedicated to his task of storing and distributing emails, but can, in theory, also perform other tasks. There is a mailbox (i.e. some disk space),on the email server computer for each client computer connected to it and wanting to use the email facility. That server has to be kept on constantly. When the sender types in his email, it is sent from his computer to the email server of the sender, where it is stored first.

Similarly, all emails received for all the users connected to the email server are received and stored on this server first. The reason is that this email server is always on, even if the user (client) computers are shut off. When the client computer starts and connects to the server computer, the client can pick up the email from his mailbox on the server and either bring it on to his hard disk of the client computer, or just read it without bringing it to its own computer (i.e. download) and retain it or delete it. Thus, the user of the client computer can read all emails one by one and reply to them or delete them or forward them. Therefore, in this regard, emails are similar to postal mails. They can be stored until the recipient wants to have a look at them. However, unlike postal mails, which take days, or even weeks to travel from the sender to the recipient, emails travel very fast in a few minutes. In this aspect, emails are similar to telephone calls.

When a user A wants to write an email to another user P, A creates a message on his PC and sends it. It is first stored on its email server (S 1). From there, it travels through the Internet to the email server of P (i.e. S2). It is stored in the mailbox of P on the hard disk of S2. When P logs on, his PC is connected to his server (S2) and he is notified that there are new messages in his mailbox. P can then read them one by one, redirect them, delete them or transfer them to his local PC (i.e. download).

The email service provided by the Internet differs from other communication mechanisms in one more respect. This feature, called as spooling, allows a user to compose and send an email message even if his network is currently disconnected or the recipient is not currently connected to his end of the network. When an email message is sent, a copy of the email is placed in a storage area on the server's disk, called as spool.

A spool is a queue of messages. The messages in a spool are sent on a first come first served basis. That is, a background process on the email server periodically searches every message in a spool automatically after a specified time interval, and an attempt is made to send it to the intended recipient. For instance, the background process can attempt to send every message in a spool after every 30 seconds. If the message cannot be sent due to any reasons such as too many messages in the queue, the date and time when an attempt was made to send it is recorded. After a specified number of attempts or time interval, the message is removed from the spool and is returned back to the original sender with an appropriate error message. Until that time, the message remains in the spool. In other words, a message can be considered as delivered successfully only when both the client and the server conclude that the recipient has received the email message correctly. Till that time, copies of the email message are retained in both the sending spool and the receiving mailbox.

The postal system worldwide identifies the recipient using his unique postal address usually some combination of city/zip code and street name and numbers, etc. In a similar fashion, an email is sent to a person using the person's email address. An email address is very similar to postal address it helps the email system to uniquely identify a particular recipient. We shall now look at email addresses in more depth.

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