Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 244
Post Office Protocol (POP)

The original mail program RFC 821 (which is the one in use today) was set up to send messages directly to a user logged in to a terminal, as well as store these messages to a mailbox. The commands allowed for the receiver to determine if the user was logged on to a terminal (not a PC), if they were accepting messages, and if they were not, is there a mailbox to deliver some mail to. There were no message attachments and messages were sent and received in 7-bit ASCII (8th bit was set to 0); therefore, this would not allow for binary messages to be sent (i.e., no attachments). In fact, the original message was not to exceed 1000 characters (however, implementations that could go beyond this barrier were strongly encouraged to do so).

So, to operate mail, the host must be operational (able to receive) all the time. Today, terminals do exist, but more commonly, personal computers have taken their place. Therefore, the final recipient will be the personal computer. The personal computer will have both SMTP and POP. Even though a personal computer will retrieve its mail via POP, it will still use the SMTP functions to send its mail. Since SMTP expects to be able to deliver mail immediately, this would mean that all users would have to have their personal computers on 100 percent of the time in order to accept mail. Second, to receive and read your mail, you must log on to a specific host.

To operate a mail server generally requires that the mail server is available for a majority of the time, has the ability to store many mail messages, and is able to fully run SMTP and accept mail from an SMTP sender. While this may have been feasible for situations like terminal-to-host connectivity, it is not feasible for situations that we have today; namely, personal computers and mobile workers. SMTP is a very robust transaction-oriented protocol and requires the statements previously discussed to operate fully.

What we need is the ability for SMTP to operate (drop off the mail, like a PO Box at the post office), and then another protocol to download to our personal computers (we drop by the post office and retrieve our mail from the post office box.). POP is the protocol to allow for this. Mail can be delivered to a drop-off point and POP allows us to log in and retrieve our mail.

When you sign up with an Internet Service Provider, a POP account is assigned to you; for example, mnaugle@POP3.ISP1.com. You use this when configuring your mail program. Also, when sending mail you must give the SMTP server name to the configuration program as well. The protocol of POP3 is not used for sending mail.