Post Office Protocol POP3

Initially, the server host starts the POP3 service by listening on TCP port 110. When a client host wishes to make use of the service, it establishes a TCP connection with the server host. When the connection is established, the POP3 server sends a greeting. The client and POP3 server then exchange commands and responses (respectively) until the connection is closed or aborted.

First you start out in the AUTHORIZATION state. In this state you can enter one of 3 commands:

• USER
• PASS
• QUIT

Once your userid and password are recognized the server will access your mailbox and you enter the TRANSACTION state. In this state you can enter one of these commands:

• STAT - returns the number of message and the number of octets
• LIST [msg] - returns the list of waiting messages

    C: LIST
    S: +OK 2 messages (320 octets)
    S: 1 120
    S: 2 200
    S: .
       ...
    C: LIST 2
    S: +OK 2 200
       ...
    C: LIST 3
    S: -ERR no such message, only 2 messages in maildrop
  

• RETR msg types out the given message

    C: RETR 1
    S: +OK 120 octets
    S: <the POP3 server sends the entire message here>
    S: .
  

• DELE msg - deletes the message

    C: DELE 1
    S: +OK message 1 deleted
       ...
    C: DELE 2
    S: -ERR message 2 already deleted
  

• NOOP - do nothing, resets the timeout counter

    C: NOOP
    S: +OK
  

• RSET - unmarks deleted messages (undeletes them)

    C: RSET
    S: +OK maildrop has 2 messages (320 octets)
  

You can also enter the QUIT command which will move you to the UPDATE state. In the UPDATE state the server will remove messages that were deleted. Note that if the connection crashes the server will not enter the update state and, thus, will not delete the messages.

  C: QUIT
  S: +OK dewey POP3 server signing off (maildrop empty)
     ...
  C: QUIT
  S: +OK dewey POP3 server signing off (2 messages left)
     ...

There are some optional POP3 commands:

• TOP msg n - type out ``n'' lines of msg number

    C: TOP 1 10
    S: +OK
    S: <the POP3 server sends the headers of the
        message, a blank line, and the first 10 lines
        of the body of the message>
    S: .
       ...
    C: TOP 100 3
    S: -ERR no such message
  

• UIDL [msg] - unique ID listing numbers

    C: UIDL
    S: +OK
    S: 1 whqtswO00WBw418f9t5JxYwZ
    S: 2 QhdPYR:00WBw1Ph7x7
    S: .
       ...
    C: UIDL 2
    S: +OK 2 QhdPYR:00WBw1Ph7x7
       ...
    C: UIDL 3
    S: -ERR no such message, only 2 messages in maildrop
  

• USER name - identify the name of the mailbox

    C: USER frated
    S: -ERR sorry, no mailbox for frated here
       ...
    C: USER mrose
    S: +OK mrose is a real hoopy frood
  

• PASS string - give a password associated with the mailbox

    C: USER mrose
    S: +OK mrose is a real hoopy frood
    C: PASS secret
    S: -ERR maildrop already locked
       ...
    C: USER mrose
    S: +OK mrose is a real hoopy frood
    C: PASS secret
    S: +OK mrose's maildrop has 2 messages (320 octets)
  

• APOP name digest - a mailbox name and an MD5 digest string

    S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
    C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
    S: +OK maildrop has 1 message (369 octets)
       In this example, the shared  secret  is  the  string  `tanstaaf'.  
       Hence, the MD5 algorithm is applied to the string
          <1896.697170952@dbc.mtview.ca.us>tanstaaf
       which produces a digest value of
          c4c9334bac560ecc979e58001b3e22fb