    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.

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    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.

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    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.

                      ------------                ------------
                      |          |                |          |    --------
                      |          |      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.

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    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

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    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.

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    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.

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    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