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.

a. Authorization

b.Transactions

c.Update

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:

 C :

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.

C: LIST

 S: + ok  2 messages

S : 1 120

 S : 2 200

S:

 C: RETER 1

 S: < the POP3 server sends  message 1>

 S: .

 C:  DELE 1

 S: ok  message 1 deleted

 C: RETR 2

 S : + ok

 S: < the  POP3  server sends  message >

 C: Quit

 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.