Post Office Protocols Version
In computing the post office protocol version 3 (POP3) is an application layer internet standard protocols used by local e mail clients to retrieve e mail from a remote server over a TCP/ IP connection. POP3 and IMAP4 ( internet message access protocols ) as the two most prevalent internet standard protocol for e mail retrieval. Virtually all modern mail clients and servers support both.
POP3 begins when the user starts the mail reader. The mail reader calls up the ISP ( unless there is already a connection ) and establishes a TCP connection with the message transfer agent at part 110once the connection has been established the POP 3 protocol goes through three states in sequence.
During the first phase authorization the user agent sends a username and a password to authenticate the user. During the second phase transaction the user agent retrieves message, also during this phase the user agent can mark message for deletion remove deflection marks. And obtain mail statistics. The third phase update occurs after the client has issued the quit command ending the POP3 session at this time the mail server deletes the message that were marked for deflection.
The authorization phase has two principal commands user < user name> and pass to illustrate these two commands we suggest that the telnet is directly applied to a POP3 server using port 110 and issue these commands. Suppose that mail server is the name of your mail server. You will see something like telnet mail server 110.
S : + ok POP 3 serer ready
C : USER Harish
S : + ok user accepted
C : pass vegetables
S : ok login successful
Note : the line marked c are from the client and those marked s are from the server.
In transaction phase, the user agent issues commands and the server responds to each command with a reply. There are two possible responses + ok used by the server to indicate that the previous command was fine and ERR used by the server to indicate that something was wrong with the previous command. A user agent using POP3 can often be configured ( by the user) to download and delete or to download and keep. The sequence of commands issued by a POP3. User agent depends on which of these two mode the user agent is operating in. In the download and delete mode the user agent will issue the LIST RETR and DELE commands. As an example suppose user has two message in his her mailbox.
S: + ok 2 messages
S : 1 120
S : 2 200
C: RETER 1
S: < the POP3 server sends message 1>
C: DELE 1
S: ok message 1 deleted
C: RETR 2
S : + ok
S: < the POP3 server sends message >
S : + ok Dewey POP3 server signing off ( Mail drop empty)
C: < close connection >
After processing the quit command, the POP3 server enters the update phase and removes message 1 and 2 from the mailbox.
A problem with download and delete mode is that the recipient. Harish of group of people want to access his mail message from multiple machines for example his office PC, his home PC and his portable computer. This mode partitions Harish mail message over these three machines if Harish first reads a message on his office PC he will not be able to read the message from his portable at home late in the evening. In the download and keep mode the user agent leaves the message on the mail server after downloading them. In this case Harish can reread messaged from different machines he can access a message from work and access it again again later the week from home.