What is POP3 ? Post office protocol

 POST OFFICE PROTOCOL (POP3)

·        Post Office Protocol, version 3 is a message access protocol that enables the client to fetch an e-mail from the remote mail server i.e. it enables the users to move mails from the mail server to computers. It is a receive only protocol that is designed to work in off-line access mode.

·        SMTP expects the destination host, the mail server receiving the mail, to be on-line all the time; otherwise TCP connection cannot be established.

·        For this reason, it is not practical to establish an SMTP session with desktop computer because desktop computers are usually powered off at the end of the day.

·        In such a case, the mail is received by an SMTP server that is always on-line. This SMTP server provides mail drop service (see fig.).

·        The server receives the mail on behalf of its clients and when user gets online, POP automatically collects or downloads all the pending mails from the mail server to user's computer.

Mail access by POP3 .

·        The client POP3 software is installed on the receiver's computer the server POP3 software is installed the mail server.

·        POP3 is described in RFC 1939 and it uses well-known TCP port 110.

·        The communication procedure is similar to SMTP and uses ASClI characters.

·        POP3 begins when user starts the mail reader.

·        The mail reader calls up the ISP (or mail server) and establishes a TCP connection with the message transfer agent at port 110.

·        Once the connection has been established, the POP3 protocol goes through three states in sequence:

1.      Authorization

2.      2. Transactions

3.      Update

·        The Authorization state deals with user log in. The client sends its user name and password.

·        The transaction state deals with the user collecting the e-mails and marking them for deletion from the mailbox.

·        The update state causes the e-mails to be deleted.

·        Once the user has logged in, the client can send the LIST command to list the contents of its mailbox. In this case the server displays one message line along with its length. This list ends with a period (see fig).

·        The client can retrieve messages using RETR command and can also mark them for deletion with DELE.

·        When all the messages have been retrieved, the client gives QUIT command to end the transaction state and enter the update state.

·        When the server has deleted all the messages, it sends a reply and breaks the TCP connection.

·        Although POP3 is used to download messages from the server, the SMTP client is still needed on the desktop computers to forward messages from workstation user to its SMTP server.

·        The POP users & require less Internet usage time and consumes minimum server resources as users can download their mails, disconnect from Internet, and read and compose messages offline. When they want to send a reply, they can reconnect.

·        POP protocol's ability to work only in offline mode poses one major Problem. The messages once read are deleted from mail server. So, it is not compatible with access from multiple computer file system.