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rfc:rfc772

Network Working Group S. Sluizer Request for Comments: 772 J. Postel

                                                                   ISI
                                                        September 1980
                       MAIL TRANSFER PROTOCOL

PREFACE

 This is a first draft of this protocol and comments are very
 definitely requested.

INTRODUCTION

 The objective of Mail Transfer Protocol (MTP) is to transfer mail
 reliably and efficiently.
 This paper assumes knowledge of the following protocols described in
 the ARPA Internet Protocol Handbook.  The reader will note strong
 similarities to portions of the File Transfer Protocol; in part, this
 is due to the original ARPA Network implementation of computer mail
 as a feature of FTP.
    The ARPANET Host-to-Host Protocol [Network Control Protocol] (NCP)
    The Transmission Control Protocol (TCP)
    The TELNET Protocol (TELNET)
    The File Transfer Protocol (FTP)

DISCUSSION

 In this section, the terminology and the MTP model are discussed.
 The terms defined in this section are only those that have special
 significance in MTP.  Some of the terminology is very specific to the
 MTP model; some readers may wish to turn to the section on the MTP
 model while reviewing the terminology.
 TERMINOLOGY
    ASCII
       The ASCII character set as defined in the ARPA Internet
       Protocol Handbook.  In MTP, ASCII characters are defined to be
       the lower half of an eight-bit code set (i.e., the most
       significant bit is zero) and is called NVT-ASCII.
                                 1

September 1980 RFC 772 Mail Transfer Protocol

    control connection
       The TCP full-duplex communication path or two NCP simplex
       communication paths between a sender-MTP and a receiver-MTP for
       the exchange of commands, replies, and mail text.  The control
       connection operates according to the TELNET Protocol.
    data mode
       The mail is transmitted over the control connection as a stream
       of octets.  (In FTP terminology this is called stream mode.)
    data structure
       The internal structure of mail is considered to be a continuous
       sequence of data octets.  (In FTP terminology this is called
       file-structure.)
    data representation
       The internal representation of all data (i.e., mail) is in
       NVT-ASCII.
    host
       A computer in the internetwork environment on which mailboxes
       reside.
    MTP commands
       A set of commands which comprise the control information
       flowing from the sender-MTP to the receiver-MTP.
    mail
       An ordered set of computer data of arbitrary length, which
       conforms to the standard set in RFC 733 (Standard for the
       Format of ARPA Network Text Messages).
    mailbox
       A character string (address) which identifies a user to whom
       mail is to be sent.  Mailbox normally consists of the host and
       user specifications.  The standard mailbox naming convention is
       defined to be "user@host".  Additionally, the "container" in
       which mail is stored.
                                 2

RFC 772 September 1980

                                                Mail Transfer Protocol
    NVT
       The Network Virtual Terminal as defined in the TELNET Protocol.
    octet
       Bytes in MTP are octets (8 bits).  This is not necessarily the
       same byte size in which data is stored in a host.
    reply
       A reply is an acknowledgment (positive or negative) sent from
       receiver to sender via the control connection in response to a
       MTP command.  The general form of a reply is a completion code
       (including error codes) followed by a text string.  The codes
       are for use by programs and the text is usually intended for
       human users.
    receiver-MTP process
       A process which transfers mail in cooperation with a sender-MTP
       process.  It "listens" on its port/socket L for a connection
       from a sender-MTP and establishes a control connection using
       the TELNET Protocol.  It receives MTP commands from the
       sender-MTP, sends replies, and governs the transfer of mail.
    sender-MTP process
       A process which transfers mail in cooperation with a
       receiver-MTP process.  A local language may be used in the user
       interface command/reply dialogue.  The sender-MTP initiates the
       control connection from its port/socket U to the receiver-MTP
       process.  It initiates MTP commands, receives replies, and
       governs the transfer of mail.
    user
       A human being (or a process on behalf of a human being) wishing
       to obtain mail transfer service.  In addition, a recipient of
       computer mail.
                                 3

September 1980 RFC 772 Mail Transfer Protocol

 THE MTP MODEL
    With the above definitions in mind, the following model (shown in
    Figure 1) may be diagrammed for an MTP service.
  1. ———– ————

| | | | ——–

                |          |      MTP       |          |<-->| User |
                | Receiver-|Commands/Replies|  Sender- |    --------
    --------    |   MTP    |<-------------->|    MTP   |    --------
    | Mail |<-->|          |      Mail      |          |<-->| Mail |
    |System|    |          |                |          |    |System|
    --------    ------------                ------------    --------
    
                Receiver-MTP                 Sender-MTP
                         Model for MTP Use
                              Figure 1
    In the model described in Figure 1, the sender-MTP initiates the
    TCP/NCP control connection which follows the TELNET Protocol.  At
    the initiation of the user, standard MTP commands are generated by
    the sender-MTP and transmitted to the receiver-MTP via the control
    connection.  Standard replies are sent from the receiver-MTP to
    the sender-MTP over the control connection in response to the
    commands.  In addition, mail is sent over the control connection.

MAIL TRANSFER FUNCTIONS

 The control connection is used for the transfer of commands which
 describe the functions to be performed, the replies to commands, as
 well as the actual transfer of mail.  Mail is transferred only via
 the control connection.
 The communication channel from the sender-MTP to the receiver-MTP is
 established by a TCP/NCP control connection from the sender to a
 standard receiver port/socket.  The sender-MTP is responsible for
 sending MTP commands, interpreting the replies received, and sending
 the mail; the receiver-MTP interprets commands, sends replies, and
 receives the mail.
                                 4

RFC 772 September 1980

                                                Mail Transfer Protocol
 MAIL REPRESENTATION AND STORAGE
    Mail is transferred from a storage device in the sending host to a
    storage device in the receiving host.  It may be necessary to
    perform certain transformations on the mail because data storage
    representations in the two systems are different.  For example,
    NVT-ASCII has different data storage representations in different
    systems.  PDP-10's generally store NVT-ASCII as five 7-bit ASCII
    characters, left-justified in a 36-bit word.  360's store
    NVT-ASCII as four 8-bit EBCDIC codes in a 32-bit word.  Multics
    stores NVT-ASCII as four 9-bit characters in a 36-bit word.
    For the sake of simplicity, all data must be represented in MTP as
    NVT-ASCII.  This means that characters must be converted into the
    standard NVT-ASCII representation when transmitting text,
    regardless of whether the sending and receiving hosts are
    dissimilar.  The sender converts the data from its internal
    character representation to the standard 8-bit NVT-ASCII
    representation (see the TELNET specification).  The receiver
    converts the data from the standard form to its own internal form.
    In accordance with this standard, the <CRLF> sequence should be
    used to denote the end of a line of text.
    The mail in MTP has no internal structure and is considered to be
    a continuous sequence of data octets.
 ERROR RECOVERY AND RESTART
    There is no provision for detecting bits lost or scrambled in data
    transfer; this level of error control is handled by the TCP/NCP.
    In addition, there is no restart procedure provided to protect
    senders from gross system failures (including failures of a host,
    an MTP-process, or the underlying network).

MTP COMMANDS

 COMMAND SEMANTICS
    The MTP commands define the mail transfer or the mail system
    function requested by the user.  The syntax of mailboxes must
    conform to receiver site conventions (with standard defaults
    applicable).  In response to an MTP transfer command, the mail
    shall always be transferred over the control connection.
    The Mail Transfer Protocol follows the specifications of the
    TELNET Protocol for all communications over the control
                                 5

September 1980 RFC 772 Mail Transfer Protocol

    connection.  Although the language used for TELNET communication
    can be a negotiated option, the "TELNET language" and the
    corresponding "TELNET end of line code" are required to be
    NVT-ASCII and <CRLF> respectively.  No other specifications of the
    TELNET Protocol will be cited.
    MTP commands are NVT-ASCII strings terminated by <CRLF>.  The
    command codes themselves are alphabetic characters terminated by
    the character <SP> (space) if parameters follow and <CRLF>
    otherwise.
    The MTP commands are discussed below.  In the description of a few
    of the commands in this section the possible replies are given
    explicitly.  MTP replies are discussed in the next section.
       MAIL (MAIL)
          This command allows a sender-MTP to send mail over the
          control connection.  The argument field contains a sender
          and optional path sequence.  If the path sequence is
          present, it consists of an optional list of hosts and a
          destination mailbox.  When the list of hosts is present, it
          is source routing information and indicates that the mail
          must be forwarded to the first host on the list.  Following
          this command line the receiver treats all subsequent
          characters as mail text from the sender.  The mail text is
          terminated by the character sequence "CRLF.CRLF".
          As mail is forwarded along the path sequence, each
          forwarding host must remove itself from the list.  When mail
          reaches its ultimate destination (the path sequence has only
          a (possibly empty) destination mailbox), the receiver
          inserts it into the destination mailbox in accordance with
          its host mail conventions.  If the second argument field is
          blank (one or more spaces) or empty (<CRLF>), the mail is
          destined for a printer or other designated place for site
          general delivery mail.  The mail may be marked as sent from
          the sender as specified by the first argument field.
       MAIL RECIPIENT SCHEME QUESTION (MRSQ)
          This MTP command is used to select a scheme for the
          transmission of mail to several users at the same host.  The
          schemes are to list the recipients first, or to send the
          mail first.
                                 6

RFC 772 September 1980

                                                Mail Transfer Protocol
       MAIL RECIPIENT (MRCP)
          This command is used to identify the individual recipients
          of the mail in the transmission of mail for multiple users
          at one host.
       HELP (HELP)
          This command causes the receiver to send helpful information
          regarding its implementation status over the control
          connection to the receiver.  The command may take an
          argument (e.g., any command name) and return more specific
          information as a response.  The reply is type 211 or 214.
       QUIT (QUIT)
          This command specifies that the receiver must close the
          control connection.
       NOOP (NOOP)
          This command does not affect any parameters or previously
          entered commands.  It specifies no action other than that
          the receiver send an OK reply.
 COMMAND SYNTAX
    The commands (and their functions and semantics) are TELNET
    NVT-ASCII strings transmitted over the control connection.  The
    functions and semantics of commands are described in the section
    on MTP Commands.  The reply sequences are discussed in the section
    on Sequencing of Commands and Replies.  Scenarios illustrating the
    use of commands are provided in the section on Typical MTP
    Scenarios.  The command syntax is specified in this section.
    The commands begin with a command code followed by an argument
    field.  The command codes are four alphabetic characters.  Upper
    and lower case alphabetic characters are to be treated
    identically.  Thus any of the following may represent the mail
    command:
       MAIL    Mail    mail    MaIl    mAIl
    This also applies to any symbols representing parameter values,
    such as R or r for RECIPIENT first.  The command codes and the
    argument fields are separated by one or more spaces.
                                 7

September 1980 RFC 772 Mail Transfer Protocol

    The argument field consists of a variable length character string
    ending with the character sequence <CRLF>.  It should be noted
    that the receiver is to take no action until the end of line code
    is received.
    The syntax is specified below in NVT-ASCII.  All characters in the
    argument field are ASCII characters.  Square brackets denote an
    optional argument field.  If the option is not taken, the
    appropriate default is implied.
    The following are the MTP commands:
       MAIL <SP> FROM:<sender> [<SP> TO:<path>] <CRLF>
       MRSQ [<SP> <scheme>] <CRLF>
       MRCP <SP> TO:<path> <CRLF>
       HELP [<SP> <string>] <CRLF>
       QUIT <CRLF>
       NOOP <CRLF>
    The syntax of the above argument fields (using BNF notation where
    applicable) is given below.  The "..." notation indicates that a
    field may be repeated one or more times.
       <sender> ::= "<" <mailbox> ">"
       <path> ::= "<" ["@" <host> "," ...] <mailbox> ">"
       <scheme> ::= "R" | "T" | "?"
       <string> ::= <char> | <char><string>
       <mailbox> ::= <user> "@" <host>
       <host> ::= <string>
       <user> ::= <string>
       <char> ::= any of the 128 ASCII characters except <CR> and <LF>
                                 8

RFC 772 September 1980

                                                Mail Transfer Protocol
 CONTROL FUNCTIONS
    Most time-sharing systems provide mechanisms to allow a terminal
    user to regain control of a "runaway" process.  When used locally,
    such systems have access to all user-supplied signals, whether
    these are normal characters or special "out of band" signals.
    When terminals are connected to the system through the network,
    the system does not necessarily have access to all user signals;
    the network's flow control mechanisms may cause such signals to be
    buffered elsewhere, for example in the user's host.
    To counter this problem, the TELNET "Synch" mechanism is used.  A
    Synch signal consists of a TCP Urgent or an NCP Interrupt
    notification, coupled with the TELNET command DATA MARK (DM).
    This notification, which is not subject to the flow control
    pertaining to the TELNET connection, is used to invoke special
    handling of the data stream by the process which receives it.  In
    this mode the data stream is immediately scanned for a TELNET
    Interrupt Process (IP) command.  (The rationale for the use of the
    TELNET IP command is to allow an existing server TELNET module to
    sit "under" the MTP.  If this code were directly implemented in
    the MTP the IP command would be unnecessary.)  The TELNET command
    DM is the synchronizing mark in the data stream which indicates
    that any special signal has already occurred and the recipient can
    return to normal processing of the data stream.  For a more
    complete understanding of this mechanism, see the TELNET Protocol
    Specification in the Internet Protocol Handbook.
    The effect of this mechanism is to to discard all characters (up
    to the DM) between the sender of the Synch and its recipeint.
    Thus, all characters in the control connection are ignored until
    the TELNET command DM is received.  The full sequence is
    illustrated below.  Each vertical bar (|) represents the boundary
    between data octets; IAC refers to the TELNET command code
    Interpret As Command.
                     Old                       New
                  -+-+-+-+-+-----+---+--+---+--+-  
                ...|M|A|I|L| ... |IAC|IP|IAC|DM|...
                  -+-+-+-+-+-----+---+--+---+--+-  
                                 9

September 1980 RFC 772 Mail Transfer Protocol

MTP REPLIES

 Replies to Mail Transfer Protocol commands are devised to ensure the
 synchronization of requests and actions in the process of mail
 transfer, and to guarantee that the sender-MTP always knows the state
 of the receiver.  Every command must generate at least one reply,
 although there may be more than one.  In the latter case, the
 multiple replies must be easily distinguished.  Additionally, some
 commands must occur sequentially, such as MRSQ T->MAIL->MRCP or
 MRSQ R->MRCP->MAIL.  Replies to these sequences show the existence of
 an intermediate state if all preceding commands have been successful.
 A failure at any point in the sequence necessitates the repetition of
 the entire sequence from the beginning.
    The details of the command-reply sequence are made explicit in the
    section on State Diagrams.
 An MTP reply consists of a three digit number (transmitted as three
 alphanumeric characters) followed by some text.  The number is
 intended for use by automata to determine what state to enter next;
 the text is meant for the human user.  It is intended that the three
 digits contain enough encoded information that the sender-MTP will
 not need to examine the text and may either discard it or pass it on
 to the user, as appropriate.  In particular, the text may be
 receiver-dependent, so there are likely to be varying texts for each
 reply code.
 Formally, a reply is defined to be the sequence:  a three-digit code,
 space <SP>, one line of text (where the maximum line length is 65),
 and a terminal <CRLF>.  Occasionally the text is longer than a single
 line; in these cases the complete text must be bracketed so the
 sender-MTP knows when it can stop reading the reply.  This requires a
 special first line format to indicate a multiple line reply, and
 another on the last line to so designate it.  Both lines will contain
 the appropriate reply code which indicates the transaction state.
    Thus the format for multi-line replies is that the first line will
    begin with the exact required reply code, followed immediately by
    a Hyphen, "-" (also known as minus), followed by text.  The last
    line will begin with the same code, followed immediately by space
    <SP>, optionally some text, and <CRLF>.
                                 10

RFC 772 September 1980

                                                Mail Transfer Protocol
       For example:
                              123-First line
                              Second line
                                234 A line beginning with numbers
                              123 The last line
    The sender-MTP then simply needs to search for the second
    occurrence of the same reply code followed by <SP> (space> at the
    beginning of a line, and ignore all intermediary lines.  If an
    intermediary line begins with a three-digit number, the receiver
    must pad the front to avoid confusion.
       This scheme allows standard system routines to be used for
       reply information, with "artificial" first and last lines
       tacked on.  In the rare cases where these routines are able to
       generate three digits and a space at the beginning of any line,
       the beginning of each text line should be offset by some
       neutral text, like space.
    This scheme assumes that multi-line replies may not be nested.  In
    general, reply nesting will not occur except for random system
    messages (also called spontaneous replies) which may interrupt
    another reply.  System messages (i.e., those not processed by the
    receiver-MTP) will NOT carry reply codes and may occur anywhere in
    the command-reply sequence.  They may be ignored by the sender-MTP
    as they are only information for the human user.
 The three digits of the reply each have a special significance.  This
 is intended to allow a range of very simple to very sophisticated
 response by the sender-MTP.  The first digit denotes whether the
 response is good, bad or incomplete.  (Referring to the state
 diagram) an unsophisticated sender-MTP will be able to determine its
 next action (proceed as planned, redo, retrench, etc.) by simply
 examining this first digit.  A sender-MTP that wants to know
 approximately what kind of error occurred (e.g., mail system error,
 command syntax error) may examine the second digit, reserving the
 third digit for the finest gradation of information.
    There are five values for the first digit of the reply code:
       1yz   Positive Preliminary reply
          The requested action is being initiated; expect another
          reply before proceeding with a new command.  (The sender-MTP
          sending another command before the completion reply would be
                                 11

September 1980 RFC 772 Mail Transfer Protocol

          in violation of protocol.  However, receiver-MTP processes
          should queue any commands that arrive while a preceding
          command is in progress.)
       2yz   Positive Completion reply
          The requested action has been successfully completed.  A new
          request may be initiated.
       3yz   Positive Intermediate reply
          The command has been accepted, but the requested action is
          being held in abeyance, pending receipt of further
          information.  The sender-MTP should send another command
          specifying this information.  This reply is used in command
          sequence groups.
       4yz   Transient Negative Completion reply
          The command was not accepted and the requested action did
          not occur.  However, the error condition is temporary and
          the action may be requested again.  The sender should return
          to the beginning of the command sequence (if any).  It is
          difficult to assign a meaning to "transient" when two
          different sites (receiver- and sender- MTPs) must agree on
          the interpretation.  Each reply in this category might have
          a different time value, but the sender-MTP is encouraged to
          try again.  A rule of thumb to determine if a reply fits
          into the 4yz or the 5yz category (see below) is that replies
          are 4yz if they can be repeated without any change in
          command form or in properties of the sender or receiver.
          (E.g., the command is repeated identically; the receiver
          does not put up a new implementation).
       5yz   Permanent Negative Completion reply
          The command was not accepted and the requested action did
          not occur.  The sender-MTP is discouraged from repeating the
          exact request (in the same sequence).  Even some "permanent"
          error conditions can be corrected, so the human user may
          want to direct the sender-MTP to reinitiate the command
          sequence by direct action at some point in the future (e.g.,
          after the spelling has been changed, or the user has altered
          his/her directory status.)
                                 12

RFC 772 September 1980

                                                Mail Transfer Protocol
    The second digit encodes responses in specific categories:
       x0z   Syntax -- These replies refer to syntax errors,
             syntactically correct commands that don't fit any
             functional category, and unimplemented or superfluous
             commands.
       x1z   Information --  These are replies to requests for
             information, such as status or help.
       x2z   Connections -- These are replies referring to the control
             connection.
       x3z   Unspecified as yet.
       x4z   Unspecified as yet.
       x5z   Mail system -- These replies indicate the status of the
             receiver mail system vis-a-vis the requested transfer or
             other mail system action.
    The third digit gives a finer gradation of meaning in each
    category specified by the second digit.  The list of replies below
    will illustrate this.  Each reply text is recommended rather than
    mandatory, and may even change according to the command with which
    it is associated.  On the other hand, the reply codes must
    strictly follow the specifications in this section.  Receiver
    implementations should not invent new codes for slightly different
    situations from the ones described here, but rather adapt codes
    already defined.
       A command such as NOOP whose successful execution does not
       offer the sender-MTP any new information will return a 200
       reply.  The response is 502 when the command requests an
       unimplemented non-site-specific action.  A refinement of that
       is the 504 reply for a command that IS implemented, but that
       requests an unimplemented parameter.
 REPLY CODES BY FUNCTION GROUPS
    200 Command okay
    500 Syntax error, command unrecognized
       [This may include errors such as command line too long]
    501 Syntax error in parameters or arguments
    502 Command not implemented
    503 Bad sequence of commands
                                 13

September 1980 RFC 772 Mail Transfer Protocol

     
    211 System status, or system help reply
    214 Help message
       [Information on how to use the receiver or the meaning of a
       particular non-standard command; this reply is useful only to
       the human user]
    215 <scheme> is the preferred scheme
     
    120 <host> Service ready in nnn minutes
    220 <host> Service ready for new user
    221 <host> Service closing control connection
    421 <host> Service not available, closing control connection
       [This may be a reply to any command if the service knows it
       must shut down]
     
    151 User not local; will forward to <user>@<host>
    152 User unknown; mail will be forwarded by the operator
    250 Requested mail action okay, completed
    450 Requested mail action not taken: mailbox unavailable
       [E.g., mailbox busy]
    550 Requested action not taken: mailbox unavailable
       [E.g., mailbox not found, no access]
    451 Requested action aborted: local error in processing
    452 Requested action not taken: insufficient system storage space
    552 Requested mail action aborted: exceeded storage allocation
       [For current mailbox location]
    553 Requested action not taken: mailbox name not allowed
    354 Start mail input; end with <CR><LF>.<CR><LF>
    
 NUMERIC ORDER LIST OF REPLY CODES
    120 <host> Service ready in nnn minutes
    151 User not local; will forward to <user>@<host>
    152 User unknown; mail will be forwarded by the operator
    200 Command okay
    211 System status, or system help reply
    214 Help message
       [Information on how to use the receiver or the meaning of a
       particular non-standard command; this reply is useful only to
       the human user]
    215 <scheme> is the preferred scheme
    220 <host> Service ready for new user
    221 <host> Service closing control connection
    250 Requested mail action okay, completed
    354 Start mail input; end with <CR><LF>.<CR><LF>
                                 14

RFC 772 September 1980

                                                Mail Transfer Protocol
    421 <host> Service not available, closing control connection
       [This may be a reply to any command if the service knows it
       must shut down]
    450 Requested mail action not taken: mailbox unavailable
       [E.g., mailbox busy]
    451 Requested action aborted: local error in processing
    452 Requested action not taken: insufficient system storage space
    500 Syntax error, command unrecognized
       [This may include errors such as command line too long]
    501 Syntax error in parameters or arguments
    502 Command not implemented
    503 Bad sequence of commands
    550 Requested action not taken: mailbox unavailable
       [E.g., mailbox not found, no access]
    552 Requested mail action aborted: exceeded storage allocation
       [For current mailbox location]
    553 Requested action not taken: mailbox name not allowed
    

DISCUSSION OF MAIL TRANSFER

 The basic command for transmitting mail is MAIL.  This command causes
 the transmitted data to be entered into the recipient's mailbox.
    MAIL <SP> "FROM:" <sender> [<SP> "TO:" <path>] <CRLF>
       <sender> is a mailbox and <path> is a source routing list of
       hosts and destination mailbox.  If accepted, it returns a 354
       reply and considers all succeeding lines to be the message
       text.  It is terminated by a line containing only a period,
       upon which a 250 completion reply is returned.  Various errors
       are possible.
 There are two possible preliminary replies that a receiver may use to
 indicate that it is accepting mail for a user whose mailbox is not at
 that receiver.
    151 User not local; will forward to <user>@<host>
       This reply indicates that the receiver knows the user's mailbox
       is on another host and will take responsibility for forwarding
       the mail to that host.  For example, at BBN (or ISI) there are
       several hosts.  Each has a list of many of the users on the
       hosts.  Each host can accept mail for any user on their list
       and forward it to the correct host.
                                 15

September 1980 RFC 772 Mail Transfer Protocol

    152 User Unknown; mail will be forwarded by the operator
       This reply indicates that the host does not recognize the user
       name, but that it will accept the mail and have the operator
       attempt to deliver it.  This is useful if the user name is
       misspelled, but may be a disservice if the mail is really
       undeliverable.
 If forwarding by the operator is unacceptable or if the user would
 prefer to send the mail directly to the recipient's actual host, the
 dialogue may be terminated upon receipt of one of these preliminary
 responses.
 There are two MTP commands which allow the text of a message to be
 mailed to several recipients simultaneously; such message
 transmission is far more efficient than the practice of sending the
 text again and again for each additional recipient at a site.  In
 one, all recipients are specified first, and then the text is sent.
 In the other, the order is reversed and the text is sent first,
 followed by the recipients.  Both schemes are necessary because
 neither by itself is optimal for all systems, as will be explained
 later.  To select a particular scheme, the MRSQ command is used; to
 specify recipients after a scheme is chosen, MRCP commands are given;
 and to furnish text, the MAIL command is used.
 SCHEME SELECTION:  MRSQ
    MRSQ is the means by which a sender-MTP can test for MRSQ/MRCP
    implementation, select a particular scheme, reset its state, and
    even do some rudimentary negotiation.  Its format is as follows:
       MRSQ [<SP> <scheme>] <CRLF>
       <scheme> is a single character.  The following are defined:
          R  Recipients first.  If this is not implemented, T must be.
          T  Text first.  If this is not implemented, R must be.
          ?  Request for preference.  This must always be implemented.
          No argument means a "selection" of none of the schemes (the
          default).
       Possible replies are:
          200 OK, we'll use specified scheme
          215 <scheme> This is the scheme I prefer
          501 I understand MRSQ but can't use that scheme
          5xx Command unrecognized or unimplemented
                                 16

RFC 772 September 1980

                                                Mail Transfer Protocol
    There are three aspects of MRSQ.  The first is that an MRSQ with
    no argument must always return a 200 reply and restore the default
    state of having no scheme selected.  Any other reply implies that
    MRSQ and hence MRCP are not understood or cannot be performed
    correctly.
    The second is that the use of "?" as a <scheme> asks the MTP
    receiver to return a 215 reply in which the receiver specifies a
    "preferred" scheme.  The format of this reply is simple:
       215 <SP> <scheme> [<SP> <arbitrary text>] <CRLF>
       Any other reply (e.g., 4xx or 5xx) implies that MRSQ and MRCP
       are not implemented, because "?" must always be implemented if
       MRSQ is.
    The third important point about MRSQ is that it always has the
    side effect of resetting all schemes to their initial state.  This
    reset must be done no matter what the reply will be -- 200, 215,
    or 501.  The actions necessary for a reset will be explained when
    discussing how each scheme actually works.
 MESSAGE TEXT SPECIFICATION:  MAIL
    Regardless of which scheme (if any) has been selected, a MAIL
    command with a non-null "TO" argument will behave exactly as
    before; the MRSQ/MRCP commands have no effect on it.  However, a
    normal MAIL command does have the same side effect as MRSQ; it
    "resets" the current scheme to its initial state.
    It is only when the "TO" argument is null (e.g., MAIL FROM:<X@Y>
    <CRLF>) that the particular scheme chosen is important.  Rather
    than producing an error (as most receivers currently do), the
    receiver will accept message text for this "null" specification.
    What it does with it depends on which scheme is in effect, and
    will be described in the section on Scheme Mechanics.
                                 17

September 1980 RFC 772 Mail Transfer Protocol

 RECIPIENT SPECIFICATION:  MRCP
    In order to specify recipient names (i.e., mailboxes) and receive
    some acknowledgment (or refusal) for each name, the following
    command is used:
       MRCP <SP> TO:<path> <CRLF>
       Reply for no scheme:
          503 No scheme specified yet; use MRSQ
       Replies for scheme T are identical to those for MAIL.
       Replies for scheme R (recipients first):
          200 OK, name stored
          452 Recipient table full, this name not stored
          553 Recipient name rejected
          4xx Temporary error, try this name again later
          5xx Permanent error, report to sender
    Note that use of this command is an error if no scheme has been
    selected yet; an MRSQ <scheme> must have been given if MRCP is to
    be used.
 SCHEME MECHANICS:  MRSQ R (RECIPIENTS-FIRST)
    In the recipients-first scheme, MRCP is used to specify names
    which the MTP receiver stores in a list or table.  Normally the
    reply for each MRCP will be either a 200 for acceptance or a
    4xx/5xx rejection code.  All 5xx codes are permanent rejections
    (e.g., user not known) which should be reported to the human user,
    whereas 4xx codes in general connote some temporary error that may
    be rectified later.  None of the 4xx/5xx replies impinge on
    previous or succeeding MRCP commands, except for 452 which
    indicates that no further MRCPs will succeed unless a message is
    sent to the already stored recipients or a reset is done.
                                 18

RFC 772 September 1980

                                                Mail Transfer Protocol
    Sending message text to stored recipients is done by giving a MAIL
    command with no "TO" argument; that is, just MAIL <SP> <sender>
    <CRLF>.  Transmission of the message text is exactly the same as
    for normal MAIL.  However, a positive acknowledgment at the end of
    transmission means the message has been sent to ALL recipients
    that were remembered with MRCP, and a failure code means that it
    should be considered to have failed for ALL of these specified
    recipients.  This applies regardless of the actual error code.
    Regardless of what the reply signifies, all stored recipient names
    are flushed and forgotten -- in other words, things are reset to
    their initial state.  This purging of the recipient name list must
    also be done as the reset side effect of any use of MRSQ.
    A 452 reply to an MRCP can be handled by using MAIL to specify the
    message for currently stored recipients, and then sending more
    MRCPs and another MAIL, as many times as necessary.  For example,
    if a receiver only had room for 10 names this would result in a
    50-recipient message being sent 5 times, to 10 different
    recipients each time.
    If a sender attempts to specify message text (MAIL with no "TO"
    argument) before any successful MRCPs have been given, this should
    be treated exactly as a "normal" MAIL with a null recipient would
    be; some receivers return an error, such as "550 Null recipient".
    See the example in Appendix A for a mail transfer using MRSQ R.
 SCHEME MECHANICS:  MRSQ T (TEXT-FIRST)
    In the text-first scheme, MAIL with no "TO" argument is used to
    specify message text, which the receiver stores away.  Succeeding
    MRCPs are then treated as if they were MAIL commands, except that
    none of the text transfer manipulations are done; the stored
    message text is sent to the specified recipient, and a reply code
    is returned identical to that which an actual MAIL would invoke.
    (Note that ANY 2xx code indicates success.)
    The stored message text is not forgotten until the next MAIL or
    MRSQ, which will either replace it with new text or flush it
    entirely.  Any use of MRSQ will reset this scheme by flushing
    stored text, as will any use of MAIL with a non-null argument.
    If an MRCP is seen before any message text has been stored, the
    sender in effect is trying to send a null message; some receivers
    might allow this, others would return an error code.
                                 19

September 1980 RFC 772 Mail Transfer Protocol

    See the example in Appendix B for a mail transfer using MRSQ T.
 WHY TWO SCHEMES ANYWAY?
    Because neither by itself is optimal for all systems.  MRSQ R
    allows more of a "bulk" mailing because everything is saved up and
    then mailed simultaneously.  This is very useful for systems such
    as ITS where the MTP-receiver does not itself write mail directly,
    but hands it on to a central mailer demon of great power.  The
    more information (e.g., recipients) associated with a single
    "hand-off", the more efficiently mail can be delivered.
    By contrast, MRSQ T is geared to receiver-MTPs which want to
    deliver mail directly, in one-by-one incremental fashion.  For
    each given recipient this scheme returns an individual
    success/failure reply code which may depend on variable mail
    system factors such as exceeding disk allocation, mailbox access
    conflicts, and so forth.  If these receiver-MTPs tried to emulate
    MRSQ Rs bulk mailing, they would have to ensure that a success
    reply to the MAIL indeed meant that it had been delivered to ALL
    recipients specified -- not just some.
 NOTES:
  • Because these commands are not required in the minimum

implementation of MTP, one must be prepared to deal with sites

      which don't recognize either MRSQ or MRCP.  "MRSQ" and "MRSQ ?"
      are explicitly designed as tests to see whether either scheme is
      implemented.  MRCP is not designed as a test, and a failure
      return of the "unimplemented" variety could be confused with "No
      scheme selected yet", or even with "Recipient unknown".
  • There is no way to indicate in a positive response to "MRSQ ?"

that the preferred "scheme" for a receiver is that of the

      default state; i.e., none of the multi-recipient schemes.  The
      rationale is that in this case, it would be pointless to
      implement MRSQ/MRCP at all, and the response would therefore be
      negative.
                                 20

RFC 772 September 1980

                                                Mail Transfer Protocol
  • One reason that the use of MAIL is restricted to null "TO"

arguments with this multi-recipient extension is the ambiguity

      that would result if a non-null "TO" argument were allowed.  For
      example, if MRSQ R was in effect and some MRCPs had been given,
      and a MAIL FROM:<X@Y> TO:<FOO><CRLF> was done, there would be no
      way to distinguish a failure reply for mailbox "FOO" from a
      global failure for all recipients specified.  A similar
      situation exists for MRSQ T; it would not be clear whether the
      text was stored and the mailbox failed, or vice versa, or both.
  • "Resets" are done by all MRSQs and "normal" MAILs to avoid

confusion and overly complicated implementation. The MRSQ

      command implies a change or uncertainty of status, and the MAIL
      command would otherwise have to use some independent mechanisms
      to avoid clobbering the data bases (e.g., message text storage
      area) used by the T/R schemes.  However, once a scheme is
      selected, it remains "in effect" just as an FTP "TYPE A" remains
      selected.  The recommended way for doing a reset, without
      changing the current selection, is with "MRSQ ?".  Remember that
      "MRSQ" alone reverts to the no-scheme state.
  • It is permissible to intersperse other MTP commands among the

MRSQ/MRCP/MAIL sequences.

                                 21

September 1980 RFC 772 Mail Transfer Protocol

DECLARATIVE SPECIFICATIONS

 MINIMUM IMPLEMENTATION
    In order to make MTP workable without needless error messages, the
    following minimum implementation is required for all receivers:
       COMMANDS -- QUIT
                   MAIL
                   NOOP
    In terms of FTP, the values of the transfer parameters must be:
       TYPE -- ASCII
       MODE -- STREAM
       STRU -- FILE-STRUCTURE
    All hosts must use the above values for mail transfer.
 CONNECTIONS
    The receiver-MTP shall "listen" on Port L.  The sender-MTP shall
    initiate the TCP/NCP control connection.  The control connection
    consists of a full-duplex connection under TCP; it is two simplex
    connections under NCP.  Receiver- and sender- MTPs should follow
    the conventions of the TELNET Protocol as specified in the ARPA
    Internet Protocol Handbook.  Receivers are under no obligation to
    provide for editing of command lines and may specify that it be
    done in the sender host.  The control connection shall be closed
    by the receiver at the sender's request after all transfers and
    replies are completed.
 SEQUENCING OF COMMANDS AND REPLIES
    The communication between the sender and receiver is intended to
    be an alternating dialogue.  As such, the sender issues an MTP
    command and the receiver responds with a prompt primary reply.
    The sender should wait for this initial primary success or failure
    response before sending further commands.
    Certain commands require a second reply for which the sender
    should also wait.  These replies may, for example, report on the
    progress or completion of mail transfer.  They are secondary
    replies to mail transfer commands.
    One important group of informational replies is the connection
                                 22

RFC 772 September 1980

                                                Mail Transfer Protocol
    greetings.  Under normal circumstances, a receiver will send a 220
    reply, "awaiting input", when the connection is completed.  The
    sender should wait for this greeting message before sending any
    commands.  If the receiver is unable to accept input right away,
    it should send a 120 "expected delay" reply immediately and a 220
    reply when ready.  The sender will then know not to hang up if
    there is a delay.
       Note: all the greeting type replies have the official name of
       the server host as the first word following the reply code.
    The table below lists alternative success and failure replies for
    each command.  These must be strictly adhered to; a receiver may
    substitute text in the replies, but the meaning and action implied
    by the code numbers and by the specific command reply sequence
    cannot be altered.
    COMMAND-REPLY SEQUENCES
       In this section, the command-reply sequence is presented.  Each
       command is listed with its possible replies; command groups are
       listed together.  Preliminary replies are listed first (with
       their succeeding replies indented under them), then positive
       and negative completion, and finally intermediary replies with
       the remaining commands from the sequence following.  The 421
       reply (service not available, closing control connection) may
       be given at any point if the MTP-receiver knows it must shut
       down.  This listing forms the basis for the state diagrams,
       which will be presented separately.
          CONNECTION ESTABLISHMENT
             120
                220
             220
             421
          MAIL ACTION COMMANDS
             MAIL
                151, 152
                   354
                      250
                      451, 552
                354
                   250
                   451, 552
                450, 550, 452, 553
                500, 501, 502, 421
                                 23

September 1980 RFC 772 Mail Transfer Protocol

             MRSQ
                200, 215
                500, 501, 502, 421
             MRCP
                151, 152
                   200
                200
                450, 550, 452, 553
                500, 501, 502, 503, 421
             QUIT
                221
          INFORMATIONAL COMMANDS
             HELP
                211, 214
                500, 501, 502, 421
          MISCELLANEOUS COMMANDS
             NOOP
                200
                500 421

STATE DIAGRAMS

 Here we present state diagrams for a very simple minded MTP
 implementation.  Only the first digit of the reply codes is used.
 There is one state diagram for each group of MTP commands.
 The command groupings were determined by constructing a model for
 each command and then collecting together the commands with
 structurally identical models.
 For each command there are three possible outcomes:  "success" (S),
 "failure" (F), and "error" (E). In the state diagrams below we use
 the symbol B for "begin", and the symbol W for "wait for reply".
                                 24

RFC 772 September 1980

                                                Mail Transfer Protocol
 We first present the diagram that represents the most MTP commands:
    
                             1,3    +---+
                        ----------->| E |
                       |            +---+
                       |
    +---+    cmd    +---+    2      +---+
    | B |---------->| W |---------->| S |
    +---+           +---+           +---+
                       |
                       |     4,5    +---+
                        ----------->| F |
                                    +---+
    
    This diagram models the commands:
       HELP, MRCP, MRSQ, NOOP, QUIT.
                                 25

September 1980 RFC 772 Mail Transfer Protocol

 A more complex diagram models the MAIL command:
    
                 ----  1
                |    |
    +---+  cmd   -->+---+     2     +---+
    | B |---------->| W |---------->| E |
    +---+           +---+        -->+---+
                     | |        |
              3      | | 4,5    |
       --------------  ------   |
      |                      |  |   +---+
      |               ------------->| S |
      |              |   1,3 |  |   +---+
      |             2|  --------
      |              | |     |
      V              | |     |
    +---+   text    +---+ 4,5 ----->+---+
    |   |---------->| W |---------->| F |
    +---+           +---+           +---+
    
    Note that the "text" here is a series of lines sent from the
    sender to the receiver with no response expected until the last
    line is sent.  (The last line must consist of only a single
    period.)
                                 26

RFC 772 September 1980

                                                Mail Transfer Protocol
 Finally we present a generalized diagram that could be used to model
 the command and reply interchange:
    
             ------------------------------------
            |                                    |
    Begin   |                                    |
      |     V                                    |
      |   +---+  cmd   +---+ 2         +---+     |
       -->|   |------->|   |---------->|   |     |
          |   |        | W |           | S |-----|
       -->|   |     -->|   |-----      |   |     |
      |   +---+    |   +---+ 4,5 |     +---+     |
      |     |      |    | |      |               |
      |     |      |   1| |3     |     +---+     |
      |     |      |    | |      |     |   |     |
      |     |       ----  |       ---->| F |-----
      |     |             |            |   |
      |     |             |            +---+
       -------------------
            |
            |
            V
           End
                                 27

September 1980 RFC 772 Mail Transfer Protocol

CONNECTION ESTABLISHMENT

 The MTP control connection is established via TCP/NCP between the
 receiver process port/socket L and the sender process port/socket U.
 This protocol is assigned the service port/socket 57 (71 octal), that
 is L=57.
                                 28

RFC 772 September 1980

                                                Mail Transfer Protocol

APPENDIX A

                Example of MRSQ R (Recipients-first)
 This is an example of how MRSQ R is used.  First the sender must
 establish that the receiver in fact implements MRSQ.
    S: MRSQ <CRLF>
    R: 200 OK, no scheme selected
 An MRSQ with a null argument always returns a 200 if implemented,
 selecting the default "scheme", i.e., none of them.  If MRSQ were not
 implemented, a code of 4xx or 5xx would be returned.
    S: MRSQ R <CRLF>
    R: 200 OK, using that scheme
 All is well; now the recipients can be specified.
    S: MRCP TO:<Foo@Y> <CRLF>
    R: 200 OK
    S: MRCP TO:<Raboof@Y> <CRLF>
    R: 553  No such user here
    S: MRCP TO:<bar@Y> <CRLF>
    R: 200 OK
    S: MRCP TO:<@Y,@X,fubar@Z> <CRLF>
    R: 200 OK
 Note that the failure of "Raboof" has no effect on the storage of
 mail for "Foo", "bar" or the mail to be forwarded to "fubar@Z"
 through host "X".  Now the message text is furnished, by giving a
 MAIL command with no "TO" argument.
    S: MAIL FROM:<waldo@A><CRLF>
    R: 354 Type mail, ended by <CRLF>.<CRLF>
    S: Blah blah blah blah....etc. etc. etc.
    S: <CRLF>.<CRLF>
    R: 250 Mail sent
 The mail text has now been sent to "Foo" and "bar" as well as
 forwarded to "fubar@Z".
                                 29

September 1980 RFC 772 Mail Transfer Protocol

APPENDIX B

                   Example of MRSQ T (Text-first)
 Using the same message as the previous example to establish that the
 receiver implements MRSQ.
    S: MRSQ ? <CRLF>
    R: 215 T Text first, please
 MRSQ is indeed implemented, and the receiver says that it prefers
 "T", but that needn't stop the sender from trying something else.
    S: MRSQ R <CRLF>
    R: 501 Sorry, I really can't do that
 It's possible that it could have understood "R" also, but in general
 it's best to use the "preferred" scheme, since the receiver knows
 which is most efficient for its particular site.
    S: MRSQ T <CRLF>
    R: 200 OK, using that scheme
 Scheme "T" is now selected, and the message text is sent by giving a
 mail command with no "TO" argument.
    S: MAIL FROM:<WALDO@A><CRLF>
    R: 354 Type mail, ended by <CRLF>.<CRLF>
    S: Blah blah blah blah....etc. etc. etc.
    S: <CRLF>.<CRLF>
    R: 250 Mail stored
 Now recipients can be specified.
    S: MRCP TO:<Foo@Y> <CRLF>
    R: 250 Stored mail sent
    S: MRCP TO:<Raboof@Y> <CRLF>
    R: 553  No such user here
    S: MRCP TO:<bar@Y> <CRLF>
    R: 250 Stored mail sent
    S: MRCP TO:<@Y,@X,fubar@Z> <CRLF>
    R: 200 OK
                                 30

RFC 772 September 1980

                                                Mail Transfer Protocol
 The text has now been sent to "Foo" and "bar" at host "Y" and will be
 forwarded to "fubar@Z" through host "X", and still remains stored.  A
 new message can be sent with another MAIL/MRCP ... sequence, but a
 careful sender would reset the state using the exchange below.
    S: MRSQ ? <CRLF>
    R: 215 T Text first, please
 Which resets the state without altering the scheme in effect.
                                 31
/data/webs/external/dokuwiki/data/pages/rfc/rfc772.txt · Last modified: 1992/10/15 21:56 by 127.0.0.1

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