• USER
  To authenticate using the USER and PASS command combination, the client must first issue the USER command. If the POP3 server responds with a positive status indicator ("+OK"), then the client may issue either the PASS command to complete the authentication, or the QUIT command to terminate the POP3 session. If the POP3 server responds with a negative status indicator ("-ERR") to the USER command, then the client may either issue a new authentication command or may issue the QUIT command. Format of the command: USER name

  USER ddement

• PASS
  When the client issues the PASS command, the POP3 server uses the argument pair from the USER and PASS commands to determine if the client should be given access to the appropriate mailbox. Since the PASS command has exactly one argument, a POP3 server may treat spaces in the argument as part of the password, instead of as argument separators.

  PASS my password

• LIST
  Format of this command is: LIST msg_number
  If an argument was given and the POP3 server issues a positive response with a line containing information for that message. If no argument was given and the POP3 server issues a positive response, then the response given is multi-line. After the initial +OK, for each message in the maildrop, the POP3 server responds with a line containing information for that message. This line contains the number of the message and amount of bytes in the message. This line is also called a "scan listing" for that message. If there are no messages in the maildrop, then the POP3 server responds with no scan listings--it issues a positive response followed by a line containing a termination octet and a CRLF pair.

  LIST
  LIST 10

• RETR
  Format: RETR msg_number
  Argument is required for this command. If the POP3 server issues a positive response, then the response given is multi-line. After the initial +OK, the POP3 server sends the message corresponding to the given message-number, being careful to byte-stuff the termination character (as with all multi-line responses).

  RETR 1

• DELE
  Format: DELE msg_number
  The POP3 server marks the message as deleted. Any future reference to the message-number associated with the message in a POP3 command generates an error. The POP3 server does not actually delete the message until the POP3 session enters the UPDATE state.
• NOOP
  The POP3 server does nothing, it merely replies with a positive response.
• RSET
  If any messages have been marked as deleted by the POP3 server, they are unmarked. The POP3 server then replies with a positive response.
• STAT
  The POP3 server issues a positive response with a line containing information for the maildrop. This line is called a "drop listing" for that maildrop. In order to simplify parsing, all POP3 servers are required to use a certain format for drop listings. The positive response consists of "+OK" followed by a single space, the number of messages in the maildrop, a single space, and the size of the maildrop in octets. This memo makes no requirement on what follows the maildrop size. Minimal implementations should just end that line of the response with a CRLF pair. More advanced implementations may include other information.
• QUIT
  The POP3 server removes all messages marked as deleted from the maildrop and replies as to the status of this operation. If there is an error, such as a resource shortage, encountered while removing messages, the maildrop may result in having some or none of the messages marked as deleted be removed. In no case may the server remove any messages not marked as deleted. Whether the removal was successful or not, the server then releases any exclusive-access lock on the maildrop and closes the TCP connection.
• TOP, UIDL, APOP

Typical scenario:

USER loginname
PASS password
STAT
LIST
RETR 4
DELE 4
QUIT

C:\> telnet mail.marshall.edu 110
+OK POP3D(*) Server PMDFV6.1-1.2 at Tue, 10 Sep 2002 13:42:48 PST <01KMC63OH77Q9ilbo.Marshall.edu>
USER dementiev
+OK password please
PASS ***********
+OK Mailbox open, 7 messages
+OK 23 messages
LIST
1 2000
2 2400
3 1920
4 1280
5 1440
6 3440
7 1920
.
RETR 1
+OK 990 octets
Return-path: 
Received: from muxc02.marshall.edu
 ("port 3258"@muxc02.marshall.edu [10.101.4.83])
 by mail.Marshall.edu (PMDF V6.1-1 #41031)
 with ESMTP id <01KM6LHFECFE91WIYG@mail.Marshall.edu> for
 _dementiev@mail.Marshall.edu (ORCPT _dementiev@marshall.edu); Fri,
 06 Sep 2002 13:59:01 -0400 (EDT)
Received: by muxc02.marshall.edu with Internet Mail Service (5.5.2653.19)
 id ; Fri, 06 Sep 2002 14:07:28 -0400
Content-return: allowed
Date: Fri, 06 Sep 2002 14:07:19 -0400
From: "Dementiev, Daniel" 
Subject: test
To: "'_dementiev@marshall.edu'" <_dementiev@Marshall.edu>
Message-id: <973A8A3969EF064B8C462506CBA8D4590BBDF5@muxc02.marshall.edu>
MIME-version: 1.0
X-Mailer: Internet Mail Service (5.5.2653.19)
Content-type: text/plain; charset=iso-8859-1
Content-transfer-encoding: 7BIT
Original-recipient: rfc822;_dementiev@marshall.edu

This is a test message for IST362 Network Protocol class.

Daniel

.
DELE 1
+OK Message deleted
QUIT
+OK bye
Connection to host lost.
c:\>

Official information: RFC 1939 - Post Office Protocol (POP) - Version 3.