# RFC1939

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    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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       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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    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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       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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       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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       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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             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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                 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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             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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                 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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                 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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                 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.

              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 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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              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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                 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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          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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                 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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    RFC 1939                          POP3                          May 1996

       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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    RFC 1939                          POP3                          May 1996

          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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    RFC 1939                          POP3                          May 1996

    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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    RFC 1939                          POP3                          May 1996

    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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