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Code Editor : rfc1939.txt

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

Myers & Rose                Standards Track                     [Page 3]

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

Myers & Rose                Standards Track                    [Page 11]

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 origin authentication and replay
             protection, but which does not involve sending a password
             in the clear over the network.  The APOP command provides
             this functionality.

A POP3 server which implements the APOP command will
             include a timestamp in its banner greeting.  The syntax of
             the timestamp corresponds to the `msg-id' in [RFC822], and
             MUST be different each time the POP3 server issues a banner
             greeting.  For example, on a UNIX implementation in which a
             separate UNIX process is used for each instance of a POP3
             server, the syntax of the timestamp might be:

<process-ID.clock@hostname>

where `process-ID' is the decimal value of the process's
             PID, clock is the decimal value of the system clock, and
             hostname is the fully-qualified domain-name corresponding
             to the host where the POP3 server is running.

The POP3 client makes note of this timestamp, and then
             issues the APOP command.  The `name' parameter has
             identical semantics to the `name' parameter of the USER
             command. The `digest' parameter is calculated by applying
             the MD5 algorithm [RFC1321] to a string consisting of the
             timestamp (including angle-brackets) followed by a shared

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RFC 1939                          POP3                          May 1996

secret.  This shared secret is a string known only to the
             POP3 client and server.  Great care should be taken to
             prevent unauthorized disclosure of the secret, as knowledge
             of the secret will allow any entity to successfully
             masquerade as the named user.  The `digest' parameter
             itself is a 16-octet value which is sent in hexadecimal
             format, using lower-case ASCII characters.

When the POP3 server receives the APOP command, it verifies
             the digest provided.  If the digest is correct, the POP3
             server issues a positive response, and the POP3 session
             enters the TRANSACTION state.  Otherwise, a negative
             response is issued and the POP3 session remains in the
             AUTHORIZATION state.

Note that as the length of the shared secret increases, so
             does the difficulty of deriving it.  As such, shared
             secrets should be long strings (considerably longer than
             the 8-character example shown below).

Possible Responses:
             +OK maildrop locked and ready
             -ERR permission denied

Examples:
             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  `tan-
             staaf'.  Hence, the MD5 algorithm is applied to the string

<1896.697170952@dbc.mtview.ca.us>tanstaaf

which produces a digest value of

c4c9334bac560ecc979e58001b3e22fb

8. Scaling and Operational Considerations

Since some of the optional features described above were added to the
   POP3 protocol, experience has accumulated in using them in large-
   scale commercial post office operations where most of the users are
   unrelated to each other.  In these situations and others, users and
   vendors of POP3 clients have discovered that the combination of using
   the UIDL command and not issuing the DELE command can provide a weak
   version of the "maildrop as semi-permanent repository" functionality
   normally associated with IMAP.  Of course the other capabilities of

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IMAP, such as polling an existing connection for newly arrived
   messages and supporting multiple folders on the server, are not
   present in POP3.

When these facilities are used in this way by casual users, there has
   been a tendency for already-read messages to accumulate on the server
   without bound.  This is clearly an undesirable behavior pattern from
   the standpoint of the server operator.  This situation is aggravated
   by the fact that the limited capabilities of the POP3 do not permit
   efficient handling of maildrops which have hundreds or thousands of
   messages.

Consequently, it is recommended that operators of large-scale multi-
   user servers, especially ones in which the user's only access to the
   maildrop is via POP3, consider such options as:

*  Imposing a per-user maildrop storage quota or the like.

A disadvantage to this option is that accumulation of messages may
      result in the user's inability to receive new ones into the
      maildrop.  Sites which choose this option should be sure to inform
      users of impending or current exhaustion of quota, perhaps by
      inserting an appropriate message into the user's maildrop.

*  Enforce a site policy regarding mail retention on the server.

Sites are free to establish local policy regarding the storage and
      retention of messages on the server, both read and unread.  For
      example, a site might delete unread messages from the server after
      60 days and delete read messages after 7 days.  Such message
      deletions are outside the scope of the POP3 protocol and are not
      considered a protocol violation.

Server operators enforcing message deletion policies should take
      care to make all users aware of the policies in force.

Clients must not assume that a site policy will automate message
      deletions, and should continue to explicitly delete messages using
      the DELE command when appropriate.

It should be noted that enforcing site message deletion policies
      may be confusing to the user community, since their POP3 client
      may contain configuration options to leave mail on the server
      which will not in fact be supported by the server.

One special case of a site policy is that messages may only be
      downloaded once from the server, and are deleted after this has
      been accomplished.  This could be implemented in POP3 server

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software by the following mechanism: "following a POP3 login by a
      client which was ended by a QUIT, delete all messages downloaded
      during the session with the RETR command".  It is important not to
      delete messages in the event of abnormal connection termination
      (ie, if no QUIT was received from the client) because the client
      may not have successfully received or stored the messages.
      Servers implementing a download-and-delete policy may also wish to
      disable or limit the optional TOP command, since it could be used
      as an alternate mechanism to download entire messages.

9. POP3 Command Summary

Minimal POP3 Commands:

USER name               valid in the AUTHORIZATION state
         PASS string
         QUIT

STAT                    valid in the TRANSACTION state
         LIST [msg]
         RETR msg
         DELE msg
         NOOP
         RSET
         QUIT

Optional POP3 Commands:

APOP name digest        valid in the AUTHORIZATION state

TOP msg n               valid in the TRANSACTION state
         UIDL [msg]

POP3 Replies:

+OK
         -ERR

Note that with the exception of the STAT, LIST, and UIDL commands,
      the reply given by the POP3 server to any command is significant
      only to "+OK" and "-ERR".  Any text occurring after this reply
      may be ignored by the client.

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10. Example POP3 Session

S: <wait for connection on TCP port 110>
      C: <open connection>
      S:    +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
      C:    APOP mrose c4c9334bac560ecc979e58001b3e22fb
      S:    +OK mrose's maildrop has 2 messages (320 octets)
      C:    STAT
      S:    +OK 2 320
      C:    LIST
      S:    +OK 2 messages (320 octets)
      S:    1 120
      S:    2 200
      S:    .
      C:    RETR 1
      S:    +OK 120 octets
      S:    <the POP3 server sends message 1>
      S:    .
      C:    DELE 1
      S:    +OK message 1 deleted
      C:    RETR 2
      S:    +OK 200 octets
      S:    <the POP3 server sends message 2>
      S:    .
      C:    DELE 2
      S:    +OK message 2 deleted
      C:    QUIT
      S:    +OK dewey POP3 server signing off (maildrop empty)
      C:  <close connection>
      S:  <wait for next connection>

11. Message Format

All messages transmitted during a POP3 session are assumed to conform
   to the standard for the format of Internet text messages [RFC822].

It is important to note that the octet count for a message on the
   server host may differ from the octet count assigned to that message
   due to local conventions for designating end-of-line.  Usually,
   during the AUTHORIZATION state of the POP3 session, the POP3 server
   can calculate the size of each message in octets when it opens the
   maildrop.  For example, if the POP3 server host internally represents
   end-of-line as a single character, then the POP3 server simply counts
   each occurrence of this character in a message as two octets.  Note
   that lines in the message which start with the termination octet need
   not (and must not) be counted twice, since the POP3 client will
   remove all byte-stuffed termination characters when it receives a
   multi-line response.

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12. References

[RFC821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
       821, USC/Information Sciences Institute, August 1982.

[RFC822] Crocker, D., "Standard for the Format of ARPA-Internet Text
       Messages", STD 11, RFC 822, University of Delaware, August 1982.

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
       MIT Laboratory for Computer Science, April 1992.

[RFC1730] Crispin, M., "Internet Message Access Protocol - Version
       4", RFC 1730, University of Washington, December 1994.

[RFC1734] Myers, J., "POP3 AUTHentication command", RFC 1734,
       Carnegie Mellon, December 1994.

13. Security Considerations

It is conjectured that use of the APOP command provides origin
   identification and replay protection for a POP3 session.
   Accordingly, a POP3 server which implements both the PASS and APOP
   commands should not allow both methods of access for a given user;
   that is, for a given mailbox name, either the USER/PASS command
   sequence or the APOP command is allowed, but not both.

Further, note that as the length of the shared secret increases, so
   does the difficulty of deriving it.

Servers that answer -ERR to the USER command are giving potential
   attackers clues about which names are valid.

Use of the PASS command sends passwords in the clear over the
   network.

Use of the RETR and TOP commands sends mail in the clear over the
   network.

Otherwise, security issues are not discussed in this memo.

14. Acknowledgements

The POP family has a long and checkered history.  Although primarily
   a minor revision to RFC 1460, POP3 is based on the ideas presented in
   RFCs 918, 937, and 1081.

In addition, Alfred Grimstad, Keith McCloghrie, and Neil Ostroff
   provided significant comments on the APOP command.

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15. Authors' Addresses

John G. Myers
   Carnegie-Mellon University
   5000 Forbes Ave
   Pittsburgh, PA 15213

EMail: jgm+@cmu.edu

Marshall T. Rose
   Dover Beach Consulting, Inc.
   420 Whisman Court
   Mountain View, CA  94043-2186

EMail: mrose@dbc.mtview.ca.us

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Appendix A. Differences from RFC 1725

This memo is a revision to RFC 1725, a Draft Standard.  It makes the
   following changes from that document:

- clarifies that command keywords are case insensitive.

- specifies that servers must send "+OK" and "-ERR" in
        upper case.

- specifies that the initial greeting is a positive response,
        instead of any string which should be a positive response.

- clarifies behavior for unimplemented commands.

- makes the USER and PASS commands optional.

- clarified the set of possible responses to the USER command.

- reverses the order of the examples in the USER and PASS
        commands, to reduce confusion.

- clarifies that the PASS command may only be given immediately
        after a successful USER command.

- clarified the persistence requirements of UIDs and added some
        implementation notes.

- specifies a UID length limitation of one to 70 octets.

- specifies a status indicator length limitation
        of 512 octets, including the CRLF.

- clarifies that LIST with no arguments on an empty mailbox
        returns success.

- adds a reference from the LIST command to the Message Format
        section

- clarifies the behavior of QUIT upon failure

- clarifies the security section to not imply the use of the
        USER command with the APOP command.

- adds references to RFCs 1730 and 1734

- clarifies the method by which a UA may enter mail into the
        transport system.

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RFC 1939                          POP3                          May 1996

- clarifies that the second argument to the TOP command is a
        number of lines.

- changes the suggestion in the Security Considerations section
        for a server to not accept both PASS and APOP for a given user
        from a "must" to a "should".

- adds a section on scaling and operational considerations

Appendix B. Command Index

APOP .......................................................   15
       DELE .......................................................    8
       LIST .......................................................    6
       NOOP .......................................................    9
       PASS .......................................................   14
       QUIT .......................................................    5
       QUIT .......................................................   10
       RETR .......................................................    8
       RSET .......................................................    9
       STAT .......................................................    6
       TOP ........................................................   11
       UIDL .......................................................   12
       USER .......................................................   13

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