rfc 1939 - Post Office Protocol - Version 3

PiS

posted Apr 3 '07 at 12:46 am

Network Working Group                                           J. Myers
Request for Comments: 1939                               Carnegie Mellon
STD: 53                                                          M. Rose
Obsoletes: 1725                             Dover Beach Consulting, Inc.
Category: Standards Track                                       May 1996

Post Office Protocol - Version 3

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements. Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol. Distribution of this memo is unlimited.

Table of Contents

   1. Introduction ................................................    2
   2. A Short Digression ..........................................    2
   3. Basic Operation .............................................    3
   4. The AUTHORIZATION State .....................................    4
      QUIT Command ................................................    5
   5. The TRANSACTION State .......................................    5
      STAT Command ................................................    6
      LIST Command ................................................    6
      RETR Command ................................................    8
      DELE Command ................................................    8
      NOOP Command ................................................    9
      RSET Command ................................................    9
   6. The UPDATE State ............................................   10
      QUIT Command ................................................   10
   7. Optional POP3 Commands ......................................   11
      TOP Command .................................................   11
      UIDL Command ................................................   12
      USER Command ................................................   13
      PASS Command ................................................   14
      APOP Command ................................................   15
   8. Scaling and Operational Considerations ......................   16
   9. POP3 Command Summary ........................................   18
   10. Example POP3 Session .......................................   19
   11. Message Format .............................................   19
   12. References .................................................   20
   13. Security Considerations ....................................   20
   14. Acknowledgements ...........................................   20
   15. Authors' Addresses .........................................   21
   Appendix A. Differences from RFC 1725 ..........................   22

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RFC 1939 POP3 May 1996

Appendix B. Command Index ...................................... 23

1. Introduction

   On certain types of smaller nodes in the Internet it is often
   impractical to maintain a message transport system (MTS). For
   example, a workstation may not have sufficient resources (cycles,
   disk space) in order to permit a SMTP server [RFC821] and associated
   local mail delivery system to be kept resident and continuously
   running. Similarly, it may be expensive (or impossible) to keep a
   personal computer interconnected to an IP-style network for long
   amounts of time (the node is lacking the resource known as
   "connectivity").

   Despite this, it is often very useful to be able to manage mail on
   these smaller nodes, and they often support a user agent (UA) to aid
   the tasks of mail handling. To solve this problem, a node which can
   support an MTS entity offers a maildrop service to these less endowed
   nodes. The Post Office Protocol - Version 3 (POP3) is intended to
   permit a workstation to dynamically access a maildrop on a server
   host in a useful fashion. Usually, this means that the POP3 protocol
   is used to allow a workstation to retrieve mail that the server is
   holding for it.

   POP3 is not intended to provide extensive manipulation operations of
   mail on the server; normally, mail is downloaded and then deleted. A
   more advanced (and complex) protocol, IMAP4, is discussed in
   [RFC1730].

   For the remainder of this memo, the term "client host" refers to a
   host making use of the POP3 service, while the term "server host"
   refers to a host which offers the POP3 service.

2. A Short Digression

   This memo does not specify how a client host enters mail into the
   transport system, although a method consistent with the philosophy of
   this memo is presented here:

      When the user agent on a client host wishes to enter a message
      into the transport system, it establishes an SMTP connection to
      its relay host and sends all mail to it. This relay host could
      be, but need not be, the POP3 server host for the client host. Of
      course, the relay host must accept mail for delivery to arbitrary
      recipient addresses, that functionality is not required of all
      SMTP servers.

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RFC 1939 POP3 May 1996

3. Basic Operation

   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.

   Commands in the POP3 consist of a case-insensitive keyword, possibly
   followed by one or more arguments. All commands are terminated by a
   CRLF pair. Keywords and arguments consist of printable ASCII
   characters. Keywords and arguments are each separated by a single
   SPACE character. Keywords are three or four characters long. Each
   argument may be up to 40 characters long.

   Responses in the POP3 consist of a status indicator and a keyword
   possibly followed by additional information. All responses are
   terminated by a CRLF pair. Responses may be up to 512 characters
   long, including the terminating CRLF. There are currently two status
   indicators: positive ("+OK") and negative ("-ERR"). Servers MUST
   send the "+OK" and "-ERR" in upper case.

   Responses to certain commands are multi-line. In these cases, which
   are clearly indicated below, after sending the first line of the
   response and a CRLF, any additional lines are sent, each terminated
   by a CRLF pair. When all lines of the response have been sent, a
   final line is sent, consisting of a termination octet (decimal code
   046, ".") and a CRLF pair. If any line of the multi-line response
   begins with the termination octet, the line is "byte-stuffed" by
   pre-pending the termination octet to that line of the response.
   Hence a multi-line response is terminated with the five octets
   "CRLF.CRLF". When examining a multi-line response, the client checks
   to see if the line begins with the termination octet. If so and if
   octets other than CRLF follow, the first octet of the line (the
   termination octet) is stripped away. If so and if CRLF immediately
   follows the termination character, then the response from the POP
   server is ended and the line containing ".CRLF" is not considered
   part of the multi-line response.

   A POP3 session progresses through a number of states during its
   lifetime. Once the TCP connection has been opened and the POP3
   server has sent the greeting, the session enters the AUTHORIZATION
   state. In this state, the client must identify itself to the POP3
   server. Once the client has successfully done this, the server
   acquires resources associated with the client's maildrop, and the
   session enters the TRANSACTION state. In this state, the client
   requests actions on the part of the POP3 server. When the client has

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RFC 1939 POP3 May 1996

   issued the QUIT command, the session enters the UPDATE state. In
   this state, the POP3 server releases any resources acquired during
   the TRANSACTION state and says goodbye. The TCP connection is then
   closed.

   A server MUST respond to an unrecognized, unimplemented, or
   syntactically invalid command by responding with a negative status
   indicator. A server MUST respond to a command issued when the
   session is in an incorrect state by responding with a negative status
   indicator. There is no general method for a client to distinguish
   between a server which does not implement an optional command and a
   server which is unwilling or unable to process the command.

   A POP3 server MAY have an inactivity autologout timer. Such a timer
   MUST be of at least 10 minutes' duration. The receipt of any command
   from the client during that interval should suffice to reset the
   autologout timer. When the timer expires, the session does NOT enter
   the UPDATE state--the server should close the TCP connection without
   removing any messages or sending any response to the client.

4. The AUTHORIZATION State

   Once the TCP connection has been opened by a POP3 client, the POP3
   server issues a one line greeting. This can be any positive
   response. An example might be:

S: +OK POP3 server ready

   The POP3 session is now in the AUTHORIZATION state. The client must
   now identify and authenticate itself to the POP3 server. Two
   possible mechanisms for doing this are described in this document,
   the USER and PASS command combination and the APOP command. Both
   mechanisms are described later in this document. Additional
   authentication mechanisms are described in [RFC1734]. While there is
   no single authentication mechanism that is required of all POP3
   servers, a POP3 server must of course support at least one
   authentication mechanism.

   Once the POP3 server has determined through the use of any
   authentication command that the client should be given access to the
   appropriate maildrop, the POP3 server then acquires an exclusive-
   access lock on the maildrop, as necessary to prevent messages from
   being modified or removed before the session enters the UPDATE state.
   If the lock is successfully acquired, the POP3 server responds with a
   positive status indicator. The POP3 session now enters the
   TRANSACTION state, with no messages marked as deleted. If the
   maildrop cannot be opened for some reason (for example, a lock can
   not be acquired, the client is denied access to the appropriate

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RFC 1939 POP3 May 1996

   maildrop, or the maildrop cannot be parsed), the POP3 server responds
   with a negative status indicator. (If a lock was acquired but the
   POP3 server intends to respond with a negative status indicator, the
   POP3 server must release the lock prior to rejecting the command.)
   After returning a negative status indicator, the server may close the
   connection. If the server does not close the connection, the client
   may either issue a new authentication command and start again, or the
   client may issue the QUIT command.

   After the POP3 server has opened the maildrop, it assigns a message-
   number to each message, and notes the size of each message in octets.
   The first message in the maildrop is assigned a message-number of
   "1", the second is assigned "2", and so on, so that the nth message
   in a maildrop is assigned a message-number of "n". In POP3 commands
   and responses, all message-numbers and message sizes are expressed in
   base-10 (i.e., decimal).

Here is the summary for the QUIT command when used in the
AUTHORIZATION state:

QUIT

Arguments: none

Restrictions: none

Possible Responses:
+OK

         Examples:
             C: QUIT
             S: +OK dewey POP3 server signing off

5. The TRANSACTION State

   Once the client has successfully identified itself to the POP3 server
   and the POP3 server has locked and opened the appropriate maildrop,
   the POP3 session is now in the TRANSACTION state. The client may now
   issue any of the following POP3 commands repeatedly. After each
   command, the POP3 server issues a response. Eventually, the client
   issues the QUIT command and the POP3 session enters the UPDATE state.

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RFC 1939 POP3 May 1996

Here are the POP3 commands valid in the TRANSACTION state:

STAT

Arguments: none

Restrictions:
may only be given in the TRANSACTION state

         Discussion:
             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.

                NOTE: This memo STRONGLY discourages implementations
                from supplying additional information in the drop
                listing. Other, optional, facilities are discussed
                later on which permit the client to parse the messages
                in the maildrop.

Note that messages marked as deleted are not counted in
either total.

Possible Responses:
+OK nn mm

         Examples:
             C: STAT
             S: +OK 2 320

LIST [msg]

         Arguments:
             a message-number (optional), which, if present, may NOT
             refer to a message marked as deleted

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RFC 1939 POP3 May 1996

Restrictions:
may only be given in the TRANSACTION state

         Discussion:
             If an argument was given and the POP3 server issues a
             positive response with a line containing information for
             that message. This line is called a "scan listing" 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 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.

             In order to simplify parsing, all POP3 servers are
             required to use a certain format for scan listings. A
             scan listing consists of the message-number of the
             message, followed by a single space and the exact size of
             the message in octets. Methods for calculating the exact
             size of the message are described in the "Message Format"
             section below. This memo makes no requirement on what
             follows the message size in the scan listing. Minimal
             implementations should just end that line of the response
             with a CRLF pair. More advanced implementations may
             include other information, as parsed from the message.

                NOTE: This memo STRONGLY discourages implementations
                from supplying additional information in the scan
                listing. Other, optional, facilities are discussed
                later on which permit the client to parse the messages
                in the maildrop.

Note that messages marked as deleted are not listed.

         Possible Responses:
             +OK scan listing follows
             -ERR no such message

         Examples:
             C: LIST
             S: +OK 2 messages (320 octets)
             S: 1 120

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RFC 1939 POP3 May 1996

             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

         Arguments:
             a message-number (required) which may NOT refer to a
             message marked as deleted

Restrictions:
may only be given in the TRANSACTION state

         Discussion:
             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).

         Possible Responses:
             +OK message follows
             -ERR no such message

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

DELE msg

         Arguments:
             a message-number (required) which may NOT refer to a
             message marked as deleted

Restrictions:
may only be given in the TRANSACTION state

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RFC 1939 POP3 May 1996

         Discussion:
             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.

         Possible Responses:
             +OK message deleted
             -ERR no such message

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

NOOP

Arguments: none

Restrictions:
may only be given in the TRANSACTION state

         Discussion:
             The POP3 server does nothing, it merely replies with a
             positive response.

Possible Responses:
+OK

         Examples:
             C: NOOP
             S: +OK

RSET

Arguments: none

Restrictions:
may only be given in the TRANSACTION state

         Discussion:
             If any messages have been marked as deleted by the POP3
             server, they are unmarked. The POP3 server then replies

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RFC 1939 POP3 May 1996

with a positive response.

Possible Responses:
+OK

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

6. The UPDATE State

   When the client issues the QUIT command from the TRANSACTION state,
   the POP3 session enters the UPDATE state. (Note that if the client
   issues the QUIT command from the AUTHORIZATION state, the POP3
   session terminates but does NOT enter the UPDATE state.)

   If a session terminates for some reason other than a client-issued
   QUIT command, the POP3 session does NOT enter the UPDATE state and
   MUST not remove any messages from the maildrop.

QUIT

Arguments: none

Restrictions: none

         Discussion:
             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.

         Possible Responses:
             +OK
             -ERR some deleted messages not removed

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

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RFC 1939 POP3 May 1996

S: +OK dewey POP3 server signing off (2 messages left)
...

7. Optional POP3 Commands

The POP3 commands discussed above must be supported by all minimal
implementations of POP3 servers.

   The optional POP3 commands described below permit a POP3 client
   greater freedom in message handling, while preserving a simple POP3
   server implementation.

      NOTE: This memo STRONGLY encourages implementations to support
      these commands in lieu of developing augmented drop and scan
      listings. In short, the philosophy of this memo is to put
      intelligence in the part of the POP3 client and not the POP3
      server.

TOP msg n

         Arguments:
             a message-number (required) which may NOT refer to to a
             message marked as deleted, and a non-negative number
             of lines (required)

Restrictions:
may only be given in the TRANSACTION state

         Discussion:
             If the POP3 server issues a positive response, then the
             response given is multi-line. After the initial +OK, the
             POP3 server sends the headers of the message, the blank
             line separating the headers from the body, and then the
             number of lines of the indicated message's body, being
             careful to byte-stuff the termination character (as with
             all multi-line responses).

             Note that if the number of lines requested by the POP3
             client is greater than than the number of lines in the
             body, then the POP3 server sends the entire message.

         Possible Responses:
             +OK top of message follows
             -ERR no such message

         Examples:
             C: TOP 1 10
             S: +OK

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RFC 1939 POP3 May 1996

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]

      Arguments:
          a message-number (optional), which, if present, may NOT
          refer to a message marked as deleted

Restrictions:
may only be given in the TRANSACTION state.

      Discussion:
          If an argument was given and the POP3 server issues a positive
          response with a line containing information for that message.
          This line is called a "unique-id listing" for that message.

          In order to simplify parsing, all POP3 servers are required to
          use a certain format for unique-id listings. A unique-id
          listing consists of the message-number of the message,
          followed by a single space and the unique-id of the message.
          No information follows the unique-id in the unique-id listing.

          The unique-id of a message is an arbitrary server-determined
          string, consisting of one to 70 characters in the range 0x21
          to 0x7E, which uniquely identifies a message within a
          maildrop and which persists across sessions. This
          persistence is required even if a session ends without
          entering the UPDATE state. The server should never reuse an
          unique-id in a given maildrop, for as long as the entity
          using the unique-id exists.

Note that messages marked as deleted are not listed.

While it is generally preferable for server implementations
to store arbitrarily assigned unique-ids in the maildrop,

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RFC 1939 POP3 May 1996

          this specification is intended to permit unique-ids to be
          calculated as a hash of the message. Clients should be able
          to handle a situation where two identical copies of a
          message in a maildrop have the same unique-id.

      Possible Responses:
          +OK unique-id listing follows
          -ERR no such message

      Examples:
          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

         Arguments:
             a string identifying a mailbox (required), which is of
             significance ONLY to the server

         Restrictions:
             may only be given in the AUTHORIZATION state after the POP3
             greeting or after an unsuccessful USER or PASS command

         Discussion:
             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.

             The server may return a positive response even though no
             such mailbox exists. The server may return a negative
             response if mailbox exists, but does not permit plaintext

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RFC 1939 POP3 May 1996

password authentication.

         Possible Responses:
             +OK name is a valid mailbox
             -ERR never heard of mailbox name

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

PASS string

Arguments:
a server/mailbox-specific password (required)

         Restrictions:
             may only be given in the AUTHORIZATION state immediately
             after a successful USER command

         Discussion:
             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 maildrop.

             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.

         Possible Responses:
             +OK maildrop locked and ready
             -ERR invalid password
             -ERR unable to lock maildrop

         Examples:
             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)

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RFC 1939 POP3 May 1996

APOP name digest

         Arguments:
             a string identifying a mailbox and a MD5 digest string
             (both required)

         Restrictions:
             may only be given in the AUTHORIZATION state after the POP3
             greeting or after an unsuccessful USER or PASS command

         Discussion:
             Normally, each POP3 session starts with a USER/PASS
             exchange. This results in a server/user-id specific
             password being sent in the clear on the network. For
             intermittent use of POP3, this may not introduce a sizable
             risk. However, many POP3 client implementations connect to
             the POP3 server on a regular basis -- to check for new
             mail. Further the interval of session initiation may be on
             the order of five minutes. Hence, the risk of password
             capture is greatly enhanced.

An alternate method of authentication is required which
provides for both or

Network Working Group&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; J. Myers Request for Comments: 1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Carnegie Mellon STD: 53&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; M. Rose Obsoletes: 1725&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Dover Beach Consulting, Inc. Category: Standards Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; May 1996 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Post Office Protocol - Version 3 Status of this Memo &nbsp;&nbsp; This document specifies an Internet standards track protocol for the &nbsp;&nbsp; Internet community, and requests discussion and suggestions for &nbsp;&nbsp; improvements.&nbsp; Please refer to the current edition of the "Internet &nbsp;&nbsp; Official Protocol Standards" (STD 1) for the standardization state &nbsp;&nbsp; and status of this protocol.&nbsp; Distribution of this memo is unlimited. Table of Contents &nbsp;&nbsp; 1. Introduction ................................................&nbsp;&nbsp;&nbsp; 2 &nbsp;&nbsp; 2. A Short Digression ..........................................&nbsp;&nbsp;&nbsp; 2 &nbsp;&nbsp; 3. Basic Operation .............................................&nbsp;&nbsp;&nbsp; 3 &nbsp;&nbsp; 4. The AUTHORIZATION State .....................................&nbsp;&nbsp;&nbsp; 4 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; QUIT Command ................................................&nbsp;&nbsp;&nbsp; 5 &nbsp;&nbsp; 5. The TRANSACTION State .......................................&nbsp;&nbsp;&nbsp; 5 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; STAT Command ................................................&nbsp;&nbsp;&nbsp; 6 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; LIST Command ................................................&nbsp;&nbsp;&nbsp; 6 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; RETR Command ................................................&nbsp;&nbsp;&nbsp; 8 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; DELE Command ................................................&nbsp;&nbsp;&nbsp; 8 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; NOOP Command ................................................&nbsp;&nbsp;&nbsp; 9 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; RSET Command ................................................&nbsp;&nbsp;&nbsp; 9 &nbsp;&nbsp; 6. The UPDATE State ............................................&nbsp;&nbsp; 10 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; QUIT Command ................................................&nbsp;&nbsp; 10 &nbsp;&nbsp; 7. Optional POP3 Commands ......................................&nbsp;&nbsp; 11 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; TOP Command .................................................&nbsp;&nbsp; 11 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; UIDL Command ................................................&nbsp;&nbsp; 12 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; USER Command ................................................&nbsp;&nbsp; 13 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; PASS Command ................................................&nbsp;&nbsp; 14 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; APOP Command ................................................&nbsp;&nbsp; 15 &nbsp;&nbsp; 8. Scaling and Operational Considerations ......................&nbsp;&nbsp; 16 &nbsp;&nbsp; 9. POP3 Command Summary ........................................&nbsp;&nbsp; 18 &nbsp;&nbsp; 10. Example POP3 Session .......................................&nbsp;&nbsp; 19 &nbsp;&nbsp; 11. Message Format .............................................&nbsp;&nbsp; 19 &nbsp;&nbsp; 12. References .................................................&nbsp;&nbsp; 20 &nbsp;&nbsp; 13. Security Considerations ....................................&nbsp;&nbsp; 20 &nbsp;&nbsp; 14. Acknowledgements ...........................................&nbsp;&nbsp; 20 &nbsp;&nbsp; 15. Authors' Addresses .........................................&nbsp;&nbsp; 21 &nbsp;&nbsp; Appendix A. Differences from RFC 1725 ..........................&nbsp;&nbsp; 22 &nbsp; Myers &amp; Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Standards Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; [Page 1] RFC 1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; POP3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; May 1996 &nbsp;&nbsp; Appendix B. Command Index ......................................&nbsp;&nbsp; 23 1. Introduction &nbsp;&nbsp; On certain types of smaller nodes in the Internet it is often &nbsp;&nbsp; impractical to maintain a message transport system (MTS).&nbsp; For &nbsp;&nbsp; example, a workstation may not have sufficient resources (cycles, &nbsp;&nbsp; disk space) in order to permit a SMTP server [RFC821] and associated &nbsp;&nbsp; local mail delivery system to be kept resident and continuously &nbsp;&nbsp; running.&nbsp; Similarly, it may be expensive (or impossible) to keep a &nbsp;&nbsp; personal computer interconnected to an IP-style network for long &nbsp;&nbsp; amounts of time (the node is lacking the resource known as &nbsp;&nbsp; "connectivity"). &nbsp;&nbsp; Despite this, it is often very useful to be able to manage mail on &nbsp;&nbsp; these smaller nodes, and they often support a user agent (UA) to aid &nbsp;&nbsp; the tasks of mail handling.&nbsp; To solve this problem, a node which can &nbsp;&nbsp; support an MTS entity offers a maildrop service to these less endowed &nbsp;&nbsp; nodes.&nbsp; The Post Office Protocol - Version 3 (POP3) is intended to &nbsp;&nbsp; permit a workstation to dynamically access a maildrop on a server &nbsp;&nbsp; host in a useful fashion.&nbsp; Usually, this means that the POP3 protocol &nbsp;&nbsp; is used to allow a workstation to retrieve mail that the server is &nbsp;&nbsp; holding for it. &nbsp;&nbsp; POP3 is not intended to provide extensive manipulation operations of &nbsp;&nbsp; mail on the server; normally, mail is downloaded and then deleted.&nbsp; A &nbsp;&nbsp; more advanced (and complex) protocol, IMAP4, is discussed in &nbsp;&nbsp; [RFC1730]. &nbsp;&nbsp; For the remainder of this memo, the term "client host" refers to a &nbsp;&nbsp; host making use of the POP3 service, while the term "server host" &nbsp;&nbsp; refers to a host which offers the POP3 service. 2. A Short Digression &nbsp;&nbsp; This memo does not specify how a client host enters mail into the &nbsp;&nbsp; transport system, although a method consistent with the philosophy of &nbsp;&nbsp; this memo is presented here: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; When the user agent on a client host wishes to enter a message &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; into the transport system, it establishes an SMTP connection to &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; its relay host and sends all mail to it.&nbsp; This relay host could &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; be, but need not be, the POP3 server host for the client host.&nbsp; Of &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; course, the relay host must accept mail for delivery to arbitrary &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; recipient addresses, that functionality is not required of all &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; SMTP servers. &nbsp; &nbsp; Myers &amp; Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Standards Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; [Page 2] RFC 1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; POP3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; May 1996 3. Basic Operation &nbsp;&nbsp; Initially, the server host starts the POP3 service by listening on &nbsp;&nbsp; TCP port 110.&nbsp; When a client host wishes to make use of the service, &nbsp;&nbsp; it establishes a TCP connection with the server host.&nbsp; When the &nbsp;&nbsp; connection is established, the POP3 server sends a greeting.&nbsp; The &nbsp;&nbsp; client and POP3 server then exchange commands and responses &nbsp;&nbsp; (respectively) until the connection is closed or aborted. &nbsp;&nbsp; Commands in the POP3 consist of a case-insensitive keyword, possibly &nbsp;&nbsp; followed by one or more arguments.&nbsp; All commands are terminated by a &nbsp;&nbsp; CRLF pair.&nbsp; Keywords and arguments consist of printable ASCII &nbsp;&nbsp; characters.&nbsp; Keywords and arguments are each separated by a single &nbsp;&nbsp; SPACE character.&nbsp; Keywords are three or four characters long. Each &nbsp;&nbsp; argument may be up to 40 characters long. &nbsp;&nbsp; Responses in the POP3 consist of a status indicator and a keyword &nbsp;&nbsp; possibly followed by additional information.&nbsp; All responses are &nbsp;&nbsp; terminated by a CRLF pair.&nbsp; Responses may be up to 512 characters &nbsp;&nbsp; long, including the terminating CRLF.&nbsp; There are currently two status &nbsp;&nbsp; indicators: positive ("+OK") and negative ("-ERR").&nbsp; Servers MUST &nbsp;&nbsp; send the "+OK" and "-ERR" in upper case. &nbsp;&nbsp; Responses to certain commands are multi-line.&nbsp; In these cases, which &nbsp;&nbsp; are clearly indicated below, after sending the first line of the &nbsp;&nbsp; response and a CRLF, any additional lines are sent, each terminated &nbsp;&nbsp; by a CRLF pair.&nbsp; When all lines of the response have been sent, a &nbsp;&nbsp; final line is sent, consisting of a termination octet (decimal code &nbsp;&nbsp; 046, ".") and a CRLF pair.&nbsp; If any line of the multi-line response &nbsp;&nbsp; begins with the termination octet, the line is "byte-stuffed" by &nbsp;&nbsp; pre-pending the termination octet to that line of the response. &nbsp;&nbsp; Hence a multi-line response is terminated with the five octets &nbsp;&nbsp; "CRLF.CRLF".&nbsp; When examining a multi-line response, the client checks &nbsp;&nbsp; to see if the line begins with the termination octet.&nbsp; If so and if &nbsp;&nbsp; octets other than CRLF follow, the first octet of the line (the &nbsp;&nbsp; termination octet) is stripped away.&nbsp; If so and if CRLF immediately &nbsp;&nbsp; follows the termination character, then the response from the POP &nbsp;&nbsp; server is ended and the line containing ".CRLF" is not considered &nbsp;&nbsp; part of the multi-line response. &nbsp;&nbsp; A POP3 session progresses through a number of states during its &nbsp;&nbsp; lifetime.&nbsp; Once the TCP connection has been opened and the POP3 &nbsp;&nbsp; server has sent the greeting, the session enters the AUTHORIZATION &nbsp;&nbsp; state.&nbsp; In this state, the client must identify itself to the POP3 &nbsp;&nbsp; server.&nbsp; Once the client has successfully done this, the server &nbsp;&nbsp; acquires resources associated with the client's maildrop, and the &nbsp;&nbsp; session enters the TRANSACTION state.&nbsp; In this state, the client &nbsp;&nbsp; requests actions on the part of the POP3 server.&nbsp; When the client has &nbsp; Myers &amp; Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Standards Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; [Page 3] RFC 1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; POP3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; May 1996 &nbsp;&nbsp; issued the QUIT command, the session enters the UPDATE state.&nbsp; In &nbsp;&nbsp; this state, the POP3 server releases any resources acquired during &nbsp;&nbsp; the TRANSACTION state and says goodbye.&nbsp; The TCP connection is then &nbsp;&nbsp; closed. &nbsp;&nbsp; A server MUST respond to an unrecognized, unimplemented, or &nbsp;&nbsp; syntactically invalid command by responding with a negative status &nbsp;&nbsp; indicator.&nbsp; A server MUST respond to a command issued when the &nbsp;&nbsp; session is in an incorrect state by responding with a negative status &nbsp;&nbsp; indicator.&nbsp; There is no general method for a client to distinguish &nbsp;&nbsp; between a server which does not implement an optional command and a &nbsp;&nbsp; server which is unwilling or unable to process the command. &nbsp;&nbsp; A POP3 server MAY have an inactivity autologout timer.&nbsp; Such a timer &nbsp;&nbsp; MUST be of at least 10 minutes' duration.&nbsp; The receipt of any command &nbsp;&nbsp; from the client during that interval should suffice to reset the &nbsp;&nbsp; autologout timer.&nbsp; When the timer expires, the session does NOT enter &nbsp;&nbsp; the UPDATE state--the server should close the TCP connection without &nbsp;&nbsp; removing any messages or sending any response to the client. 4. The AUTHORIZATION State &nbsp;&nbsp; Once the TCP connection has been opened by a POP3 client, the POP3 &nbsp;&nbsp; server issues a one line greeting.&nbsp; This can be any positive &nbsp;&nbsp; response.&nbsp; An example might be: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S:&nbsp; +OK POP3 server ready &nbsp;&nbsp; The POP3 session is now in the AUTHORIZATION state.&nbsp; The client must &nbsp;&nbsp; now identify and authenticate itself to the POP3 server.&nbsp; Two &nbsp;&nbsp; possible mechanisms for doing this are described in this document, &nbsp;&nbsp; the USER and PASS command combination and the APOP command.&nbsp; Both &nbsp;&nbsp; mechanisms are described later in this document.&nbsp; Additional &nbsp;&nbsp; authentication mechanisms are described in [RFC1734].&nbsp; While there is &nbsp;&nbsp; no single authentication mechanism that is required of all POP3 &nbsp;&nbsp; servers, a POP3 server must of course support at least one &nbsp;&nbsp; authentication mechanism. &nbsp;&nbsp; Once the POP3 server has determined through the use of any &nbsp;&nbsp; authentication command that the client should be given access to the &nbsp;&nbsp; appropriate maildrop, the POP3 server then acquires an exclusive- &nbsp;&nbsp; access lock on the maildrop, as necessary to prevent messages from &nbsp;&nbsp; being modified or removed before the session enters the UPDATE state. &nbsp;&nbsp; If the lock is successfully acquired, the POP3 server responds with a &nbsp;&nbsp; positive status indicator.&nbsp; The POP3 session now enters the &nbsp;&nbsp; TRANSACTION state, with no messages marked as deleted.&nbsp; If the &nbsp;&nbsp; maildrop cannot be opened for some reason (for example, a lock can &nbsp;&nbsp; not be acquired, the client is denied access to the appropriate &nbsp; Myers &amp; Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Standards Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; [Page 4] RFC 1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; POP3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; May 1996 &nbsp;&nbsp; maildrop, or the maildrop cannot be parsed), the POP3 server responds &nbsp;&nbsp; with a negative status indicator.&nbsp; (If a lock was acquired but the &nbsp;&nbsp; POP3 server intends to respond with a negative status indicator, the &nbsp;&nbsp; POP3 server must release the lock prior to rejecting the command.) &nbsp;&nbsp; After returning a negative status indicator, the server may close the &nbsp;&nbsp; connection.&nbsp; If the server does not close the connection, the client &nbsp;&nbsp; may either issue a new authentication command and start again, or the &nbsp;&nbsp; client may issue the QUIT command. &nbsp;&nbsp; After the POP3 server has opened the maildrop, it assigns a message- &nbsp;&nbsp; number to each message, and notes the size of each message in octets. &nbsp;&nbsp; The first message in the maildrop is assigned a message-number of &nbsp;&nbsp; "1", the second is assigned "2", and so on, so that the nth message &nbsp;&nbsp; in a maildrop is assigned a message-number of "n".&nbsp; In POP3 commands &nbsp;&nbsp; and responses, all message-numbers and message sizes are expressed in &nbsp;&nbsp; base-10 (i.e., decimal). &nbsp;&nbsp; Here is the summary for the QUIT command when used in the &nbsp;&nbsp; AUTHORIZATION state: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; QUIT &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Arguments: none &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Restrictions: none &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Possible Responses: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; +OK &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Examples: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: QUIT &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK dewey POP3 server signing off 5. The TRANSACTION State &nbsp;&nbsp; Once the client has successfully identified itself to the POP3 server &nbsp;&nbsp; and the POP3 server has locked and opened the appropriate maildrop, &nbsp;&nbsp; the POP3 session is now in the TRANSACTION state.&nbsp; The client may now &nbsp;&nbsp; issue any of the following POP3 commands repeatedly.&nbsp; After each &nbsp;&nbsp; command, the POP3 server issues a response.&nbsp; Eventually, the client &nbsp;&nbsp; issues the QUIT command and the POP3 session enters the UPDATE state. &nbsp; &nbsp; &nbsp; &nbsp; Myers &amp; Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Standards Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; [Page 5] RFC 1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; POP3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; May 1996 &nbsp;&nbsp; Here are the POP3 commands valid in the TRANSACTION state: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; STAT &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Arguments: none &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Restrictions: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; may only be given in the TRANSACTION state &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Discussion: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; The POP3 server issues a positive response with a line &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; containing information for the maildrop.&nbsp; This line is &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; called a "drop listing" for that maildrop. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; In order to simplify parsing, all POP3 servers are &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; required to use a certain format for drop listings.&nbsp; The &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; positive response consists of "+OK" followed by a single &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; space, the number of messages in the maildrop, a single &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; space, and the size of the maildrop in octets.&nbsp; This memo &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; makes no requirement on what follows the maildrop size. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Minimal implementations should just end that line of the &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; response with a CRLF pair.&nbsp; More advanced implementations &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; may include other information. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; NOTE: This memo STRONGLY discourages implementations &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; from supplying additional information in the drop &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; listing.&nbsp; Other, optional, facilities are discussed &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; later on which permit the client to parse the messages &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; in the maildrop. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Note that messages marked as deleted are not counted in &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; either total. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Possible Responses: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; +OK nn mm &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Examples: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: STAT &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK 2 320 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; LIST [msg] &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Arguments: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; a message-number (optional), which, if present, may NOT &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; refer to a message marked as deleted &nbsp; &nbsp; Myers &amp; Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Standards Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; [Page 6] RFC 1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; POP3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; May 1996 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Restrictions: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; may only be given in the TRANSACTION state &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Discussion: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; If an argument was given and the POP3 server issues a &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; positive response with a line containing information for &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; that message.&nbsp; This line is called a "scan listing" for &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; that message. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; If no argument was given and the POP3 server issues a &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; positive response, then the response given is multi-line. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; After the initial +OK, for each message in the maildrop, &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; the POP3 server responds with a line containing &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; information for that message.&nbsp; This line is also called a &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; "scan listing" for that message.&nbsp; If there are no &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; messages in the maildrop, then the POP3 server responds &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; with no scan listings--it issues a positive response &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; followed by a line containing a termination octet and a &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; CRLF pair. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; In order to simplify parsing, all POP3 servers are &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; required to use a certain format for scan listings.&nbsp; A &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; scan listing consists of the message-number of the &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; message, followed by a single space and the exact size of &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; the message in octets.&nbsp; Methods for calculating the exact &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; size of the message are described in the "Message Format" &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; section below.&nbsp; This memo makes no requirement on what &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; follows the message size in the scan listing.&nbsp; Minimal &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; implementations should just end that line of the response &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; with a CRLF pair.&nbsp; More advanced implementations may &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; include other information, as parsed from the message. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; NOTE: This memo STRONGLY discourages implementations &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; from supplying additional information in the scan &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; listing.&nbsp; Other, optional, facilities are discussed &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; later on which permit the client to parse the messages &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; in the maildrop. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Note that messages marked as deleted are not listed. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Possible Responses: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; +OK scan listing follows &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; -ERR no such message &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Examples: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: LIST &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK 2 messages (320 octets) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: 1 120 &nbsp; Myers &amp; Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Standards Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; [Page 7] RFC 1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; POP3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; May 1996 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: 2 200 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: . &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ... &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: LIST 2 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK 2 200 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ... &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: LIST 3 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: -ERR no such message, only 2 messages in maildrop &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; RETR msg &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Arguments: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; a message-number (required) which may NOT refer to a &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; message marked as deleted &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Restrictions: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; may only be given in the TRANSACTION state &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Discussion: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; If the POP3 server issues a positive response, then the &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; response given is multi-line.&nbsp; After the initial +OK, the &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; POP3 server sends the message corresponding to the given &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; message-number, being careful to byte-stuff the termination &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; character (as with all multi-line responses). &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Possible Responses: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; +OK message follows &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; -ERR no such message &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Examples: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: RETR 1 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK 120 octets &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: &lt;the POP3 server sends the entire message here&gt; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: . &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; DELE msg &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Arguments: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; a message-number (required) which may NOT refer to a &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; message marked as deleted &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Restrictions: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; may only be given in the TRANSACTION state &nbsp; &nbsp; Myers &amp; Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Standards Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; [Page 8] RFC 1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; POP3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; May 1996 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Discussion: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; The POP3 server marks the message as deleted.&nbsp; Any future &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; reference to the message-number associated with the message &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; in a POP3 command generates an error.&nbsp; The POP3 server does &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; not actually delete the message until the POP3 session &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; enters the UPDATE state. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Possible Responses: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; +OK message deleted &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; -ERR no such message &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Examples: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: DELE 1 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK message 1 deleted &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ... &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: DELE 2 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: -ERR message 2 already deleted &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; NOOP &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Arguments: none &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Restrictions: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; may only be given in the TRANSACTION state &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Discussion: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; The POP3 server does nothing, it merely replies with a &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; positive response. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Possible Responses: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; +OK &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Examples: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: NOOP &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; RSET &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Arguments: none &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Restrictions: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; may only be given in the TRANSACTION state &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Discussion: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; If any messages have been marked as deleted by the POP3 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; server, they are unmarked.&nbsp; The POP3 server then replies &nbsp; Myers &amp; Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Standards Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; [Page 9] RFC 1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; POP3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; May 1996 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; with a positive response. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Possible Responses: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; +OK &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Examples: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: RSET &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK maildrop has 2 messages (320 octets) 6. The UPDATE State &nbsp;&nbsp; When the client issues the QUIT command from the TRANSACTION state, &nbsp;&nbsp; the POP3 session enters the UPDATE state.&nbsp; (Note that if the client &nbsp;&nbsp; issues the QUIT command from the AUTHORIZATION state, the POP3 &nbsp;&nbsp; session terminates but does NOT enter the UPDATE state.) &nbsp;&nbsp; If a session terminates for some reason other than a client-issued &nbsp;&nbsp; QUIT command, the POP3 session does NOT enter the UPDATE state and &nbsp;&nbsp; MUST not remove any messages from the maildrop. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; QUIT &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Arguments: none &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Restrictions: none &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Discussion: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; The POP3 server removes all messages marked as deleted &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; from the maildrop and replies as to the status of this &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; operation.&nbsp; If there is an error, such as a resource &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; shortage, encountered while removing messages, the &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; maildrop may result in having some or none of the messages &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; marked as deleted be removed.&nbsp; In no case may the server &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; remove any messages not marked as deleted. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Whether the removal was successful or not, the server &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; then releases any exclusive-access lock on the maildrop &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; and closes the TCP connection. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Possible Responses: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; +OK &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; -ERR some deleted messages not removed &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Examples: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: QUIT &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK dewey POP3 server signing off (maildrop empty) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ... &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: QUIT &nbsp; Myers &amp; Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Standards Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; [Page 10] RFC 1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; POP3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; May 1996 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK dewey POP3 server signing off (2 messages left) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ... 7. Optional POP3 Commands &nbsp;&nbsp; The POP3 commands discussed above must be supported by all minimal &nbsp;&nbsp; implementations of POP3 servers. &nbsp;&nbsp; The optional POP3 commands described below permit a POP3 client &nbsp;&nbsp; greater freedom in message handling, while preserving a simple POP3 &nbsp;&nbsp; server implementation. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; NOTE: This memo STRONGLY encourages implementations to support &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; these commands in lieu of developing augmented drop and scan &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; listings.&nbsp; In short, the philosophy of this memo is to put &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; intelligence in the part of the POP3 client and not the POP3 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; server. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; TOP msg n &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Arguments: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; a message-number (required) which may NOT refer to to a &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; message marked as deleted, and a non-negative number &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; of lines (required) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Restrictions: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; may only be given in the TRANSACTION state &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Discussion: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; If the POP3 server issues a positive response, then the &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; response given is multi-line.&nbsp; After the initial +OK, the &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; POP3 server sends the headers of the message, the blank &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; line separating the headers from the body, and then the &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; number of lines of the indicated message's body, being &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; careful to byte-stuff the termination character (as with &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; all multi-line responses). &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Note that if the number of lines requested by the POP3 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; client is greater than than the number of lines in the &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; body, then the POP3 server sends the entire message. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Possible Responses: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; +OK top of message follows &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; -ERR no such message &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Examples: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: TOP 1 10 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK &nbsp; Myers &amp; Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Standards Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; [Page 11] RFC 1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; POP3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; May 1996 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: &lt;the POP3 server sends the headers of the &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; message, a blank line, and the first 10 lines &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; of the body of the message&gt; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: . &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ... &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: TOP 100 3 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: -ERR no such message &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; UIDL [msg] &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Arguments: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; a message-number (optional), which, if present, may NOT &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; refer to a message marked as deleted &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Restrictions: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; may only be given in the TRANSACTION state. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Discussion: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; If an argument was given and the POP3 server issues a positive &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; response with a line containing information for that message. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; This line is called a "unique-id listing" for that message. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; If no argument was given and the POP3 server issues a positive &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; response, then the response given is multi-line.&nbsp; After the &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; initial +OK, for each message in the maildrop, the POP3 server &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; responds with a line containing information for that message. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; This line is called a "unique-id listing" for that message. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; In order to simplify parsing, all POP3 servers are required to &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; use a certain format for unique-id listings.&nbsp; A unique-id &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; listing consists of the message-number of the message, &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; followed by a single space and the unique-id of the message. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; No information follows the unique-id in the unique-id listing. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; The unique-id of a message is an arbitrary server-determined &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; string, consisting of one to 70 characters in the range 0x21 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; to 0x7E, which uniquely identifies a message within a &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; maildrop and which persists across sessions.&nbsp; This &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; persistence is required even if a session ends without &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; entering the UPDATE state.&nbsp; The server should never reuse an &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; unique-id in a given maildrop, for as long as the entity &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; using the unique-id exists. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Note that messages marked as deleted are not listed. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; While it is generally preferable for server implementations &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; to store arbitrarily assigned unique-ids in the maildrop, &nbsp; Myers &amp; Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Standards Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; [Page 12] RFC 1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; POP3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; May 1996 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; this specification is intended to permit unique-ids to be &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; calculated as a hash of the message.&nbsp; Clients should be able &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; to handle a situation where two identical copies of a &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; message in a maildrop have the same unique-id. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Possible Responses: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; +OK unique-id listing follows &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; -ERR no such message &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Examples: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: UIDL &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: 1 whqtswO00WBw418f9t5JxYwZ &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: 2 QhdPYR:00WBw1Ph7x7 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: . &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ... &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: UIDL 2 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK 2 QhdPYR:00WBw1Ph7x7 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ... &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: UIDL 3 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: -ERR no such message, only 2 messages in maildrop &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; USER name &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Arguments: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; a string identifying a mailbox (required), which is of &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; significance ONLY to the server &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Restrictions: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; may only be given in the AUTHORIZATION state after the POP3 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; greeting or after an unsuccessful USER or PASS command &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Discussion: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; To authenticate using the USER and PASS command &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; combination, the client must first issue the USER &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; command.&nbsp; If the POP3 server responds with a positive &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; status indicator ("+OK"), then the client may issue &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; either the PASS command to complete the authentication, &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; or the QUIT command to terminate the POP3 session.&nbsp; If &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; the POP3 server responds with a negative status indicator &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ("-ERR") to the USER command, then the client may either &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; issue a new authentication command or may issue the QUIT &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; command. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; The server may return a positive response even though no &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; such mailbox exists.&nbsp; The server may return a negative &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; response if mailbox exists, but does not permit plaintext &nbsp; Myers &amp; Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Standards Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; [Page 13] RFC 1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; POP3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; May 1996 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; password authentication. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Possible Responses: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; +OK name is a valid mailbox &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; -ERR never heard of mailbox name &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Examples: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: USER frated &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: -ERR sorry, no mailbox for frated here &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ... &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: USER mrose &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK mrose is a real hoopy frood &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; PASS string &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Arguments: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; a server/mailbox-specific password (required) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Restrictions: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; may only be given in the AUTHORIZATION state immediately &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; after a successful USER command &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Discussion: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; When the client issues the PASS command, the POP3 server &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; uses the argument pair from the USER and PASS commands to &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; determine if the client should be given access to the &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; appropriate maildrop. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Since the PASS command has exactly one argument, a POP3 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; server may treat spaces in the argument as part of the &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; password, instead of as argument separators. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Possible Responses: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; +OK maildrop locked and ready &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; -ERR invalid password &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; -ERR unable to lock maildrop &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Examples: &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: USER mrose &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK mrose is a real hoopy frood &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: PASS secret &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: -ERR maildrop already locked &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ... &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: USER mrose &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK mrose is a real hoopy frood &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C: PASS secret &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; S: +OK mrose's maildrop has 2 messages (320 octets) &nbsp; Myers &amp; Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Standards Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;

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