                             MAIL TRANSFER PROTOCOL

                                Suzanne Sluizer

                                      and

                               Jonathan B. Postel

                                    RFC 780

                                    May 1981

                         Information Sciences Institute
                       University of Southern California
                               4676 Admiralty Way
                       Marina del Rey, California  90291

                                 (213) 822-1511

    May 1981                                                         RFC 780
    Mail Transfer Protocol

                               TABLE OF CONTENTS

       1.  INTRODUCTION .................................................. 1

       2.  THE MTP MODEL ................................................. 2

       3.  BASIC MAIL .................................................... 4

          3.1.  Forwarding ............................................... 5
          3.2.  Source Routing ........................................... 6

       4.  MULTI-RECIPIENT MAIL .......................................... 8

          4.1.  Scheme Selection: MRSQ ................................... 8
          4.2.  Message Text Specification: MAIL ......................... 9
          4.3.  Recipient Specification: MRCP ........................... 10
          4.4.  Scheme Mechanics: Recipients First ...................... 10
          4.5.  Scheme Mechanics: Text First ............................ 12
          4.6.  Discussion .............................................. 12

       5.  SPECIFICATIONS ............................................... 16

          5.1.  MTP Commands ............................................ 16
          5.1.1.  Command Semantics ..................................... 16
          5.1.2.  Command Syntax ........................................ 18
          5.2.  MTP Replies ............................................. 22
          5.2.1.  Reply Codes by Function Group ......................... 23
          5.2.2.  Reply Codes in Numeric Order .......................... 24
          5.3.  Sequencing of Commands and Replies ...................... 25
          5.4.  State Diagrams .......................................... 28
          5.5.  Details ................................................. 30
          5.5.1.  Minimum Implementation ................................ 30
          5.5.2.  Transparency .......................................... 30
          5.5.3.  Sizes ................................................. 30

       APPENDIX A:  TCP ................................................. 32
       APPENDIX B:  NCP ................................................. 33
       APPENDIX C:  NITS ................................................ 34
       APPENDIX D:  X.25 ................................................ 35
       APPENDIX E:  Theory of Reply Codes ............................... 36

       GLOSSARY ......................................................... 39

       REFERENCES ....................................................... 42

    Network Working Group                                         S. Sluizer
    Request for Comments: 780                                      J. Postel
                                                                         ISI
    Replaces: RFC 772                                               May 1981

                             MAIL TRANSFER PROTOCOL

    1.  INTRODUCTION

       The objective of Mail Transfer Protocol (MTP) is to transfer mail
       reliably and efficiently.

       MTP is designed to be independent of the particular transmission
       subsystem and requires only a reliable ordered data stream channel.
       Appendices describe the use of MTP with various transport services.
       A Glossary provides the definitions of terms as used in this
       document.

       An important feature of MTP is its capability to relay mail from one
       transport environment to another.  A transport service provides an
       interprocess communication environment (IPCE).  An IPCE may cover one
       network, several networks, or a subset of a network.  A process can
       communicate directly with another process anywhere in its own IPCE.
       Mail is a special case of interprocess communication.  Mail can be
       communicated between proceses in different IPCEs by relaying through
       a process connected to two (or more) IPCEs.  More specifically, mail
       can be relayed between hosts on different transport systems by a host
       on both transport systems.  It is important to realize that transport
       systems (or IPCEs) are not one-to-one with networks.

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    May 1981                                                         RFC 780
    Mail Transfer Protocol

    2.  THE MTP MODEL

       The MTP design is based on the following model of communication:  at
       the initiation of the user, the sender-MTP establishes the
       full-duplex transmission channel.  MTP commands are generated by the
       sender-MTP and sent to the receiver-MTP.  MTP replies are sent from
       the receiver-MTP to the sender-MTP in response to the commands.

       In the simplest case, once the transmission channel is established
       the MTP-sender sends a MAIL command indicating the sender and
       receiver of the mail.  If the MTP-receiver can accept the mail it
       responds with a go ahead reply.  Then the MTP-sender sends the mail
       data, terminating with a special sequence.  If the MTP-receiver
       successfully processes the mail it responds with an OK reply.

         -------------------------------------------------------------

                   +----------+                +----------+
       +------+    |          |                |          |
       | User |<-->|          |      MTP       |          |
       +------+    |  Sender- |Commands/Replies| Receiver-|
       +------+    |   MTP    |<-------------->|    MTP   |    +------+
       | File |<-->|          |    and Mail    |          |<-->| File |
       |System|    |          |                |          |    |System|
       +------+    +----------+                +----------+    +------+

                    Sender-MTP                 Receiver-MTP

                               Model for MTP Use

                                    Figure 1

         -------------------------------------------------------------

       The MTP provides mechanisms for the transmission of mail; directly
       from the sending user's host to the receiving user's host when the
       two host are connected to the same transport service, or via one or
       more relay MTP-servers when the source and destination hosts are not
       connected to the same transport service.

       To be able to provide the relay capability the MTP-server must be
       supplied with the name of the ultimate destination host as well as
       the destination mailbox name.

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    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

       The arguments to the MAIL command are a FROM path and a TO path.  The
       TO path is a source route while the FROM path is a return route
       (which may be used to return a message to the sender when an error
       occurs with a relayed message).

       The preceding discussion has outlined the transmission of one copy of
       one message from a source to a destination host and the possibility
       of relaying messages between different transport services.  The MTP
       additionally supports the transmission of one copy of a message
       addressed to multiple recipients.

       In order for mail to be successfully transmitted the destination
       users must be known at the destination receiver-MTP and the mail data
       must be correctly received and stored.  In the single recipient case
       discussed above the positive response to the MAIL command indicated
       the recipient was known, and the final OK response indicated the mail
       was received and stored.

       To support multi-recipient mail, MTP provides two procedures:
       Text-First, and Recipients-First.  In the text-first scheme the mail
       data is sent and acknowledged, then each recipient identification is
       sent and acknowledged (or refused) separately.  In the
       recipients-first scheme the recipients are negotiated first, then the
       text is sent and acknowledged (for all recipients at once).  The
       choice of scheme is up to the MTP-receiver, and depends on the way
       mail is handled in the destination host.

       The multi-recipient mail procedures are optional and the
       determination of which scheme to use is negotiated.  The use of the
       multi-recipient schemes is strongly encouraged by the economy they
       provide in transmission and processing.

       The mail commands and replies have a rigid syntax.  Replies also have
       a numeric code.  In the following, examples appear which use actual
       commands and replies.  The complete lists of commands and replies
       appears in Section 5 on specifications.

       Commands and replies are not case sensitive.  That is, a command or
       reply word may be upper case, lower case, or any mixture of upper and
       lower case.  Note that this is not true of mailbox user names.  For
       some hosts the user name is case sensitive, and MTP implementations
       must take case to preserve the case of user names as they appear in
       mailbox arguments.

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    May 1981                                                         RFC 780
    Mail Transfer Protocol

    3.  BASIC MAIL

       The basic command for transmitting mail is MAIL.  This command causes
       the transmitted data to be entered into the recipient's mailbox, or
       accepted for relaying to the destination host.

       The mail text is also sent on the transmission channel.  This
       requires  that the end of the text be signalled so that the command
       and reply dialog can be resumed.  MTP signals the end of the mail
       text by sending a line containing only a period.  A transparency
       procedure is used to prevent this interfering with the users text
       (see Section 5.5.2).

          MAIL <SP> FROM:<sender-path> <SP> TO:<receiver-path> <CRLF>

             The <sender-path> contains the source mailbox; the
             <receiver-path> contains the destination mailbox.  If accepted,
             the receiver-MTP returns a 354 reply and considers all
             succeeding lines to be the message text.  The message text is
             terminated by a line containing only a period, upon which a 250
             completion reply is returned.  Various errors are possible.

             Actually the <sender-path> and <receiver-path> are more than
             just the mailboxes, they may be source routes.  The
             <receiver-path> is a source routing list of hosts and
             destination mailbox; the <sender-path> is a reverse source
             routing list of hosts and source mailbox.

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    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

         -------------------------------------------------------------

                          Example of MAIL (Basic Mail)

          This MAIL command specifies the mail is sent by Waldo at host A,
          and is to be delivered to Foo at host Y.  Here we assume that host
          A contacts host Y directly.

             S: MAIL FROM:<waldo@A> TO:<Foo@Y> <CRLF>
             R: 354 Start mail input; end with <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".

                                   Example 1

         -------------------------------------------------------------

       3.1.  FORWARDING

          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 host.

             151 User not local; will forward to <user>@<host>

                This reply indicates that the receiver-MTP knows the user's
                mailbox is on another host and will take responsibility for
                forwarding the mail to that host.  This reply is only sent
                when the sender would not expect the mail to be forwarded.
                That is, when <receiver-path> as given in the command
                indicates mail relaying, this reply will not be used.  This
                reply could be used for an organization with several hosts
                when each has a list of many of the users on the hosts.  A
                host can accept mail for any user on its list and forward it
                to the correct host.

             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.

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    Mail Transfer Protocol

          If forwarding by the operator is unacceptable or if the
          sending-user would prefer to send the mail directly to the
          recipient's actual host, the action may be aborted.

          The MTP-sender must accept or reject the proposal in the
          preliminary reply by sending a continue (CONT) or abort (ABRT)
          command.  In the case of the continue, the next reply from the
          MTP-receiver will be any of the replies expected for the MAIL
          command, most likely "354 Start mail input, ...".  In the case of
          the abort, the next reply from the MTP-receiver will be "201
          Command okay, action aborted".

       3.2.  SOURCE ROUTING

          The receiver-path may be a source route of the form
          "@ONE,@TWO,JOE@THREE", where ONE, TWO, and THREE are hosts.  This
          form is used to emphasize the distinction between an address and a
          route.

          At some distant future time it might be necessary to expand the
          mailbox format to include a region identifier, such as
          "user@host@region".  If this occured the MTP  path convention
          could be expanded to
          "host@region,host@region,...user@host@region". For example,
          "ONE@R1,TWO@R2,JOE@THREE@R3".

          The mailbox is an absolute address, and the route is information
          about how to get there.  The two concepts should not be confused.

          The elements of the receiver-path are to be moved to the
          sender-path as the message is relayed from one MTP to another. The
          sender-path is a reverse source route, that is, a source route to
          the originator of the message.  When an MTP deletes its identifier
          from the receiver-path and inserts it into the sender-path, it
          must use the name it is known by in the environment it is sending
          into, not the environment the mail came from, in case the MTP is
          known be different names in different environments.

          When source routing is used the receiver-MTP will receive mail to
          be relayed to another MTP.  The receiver-MTP may accept the task
          of relaying the mail or reject it in the same way it accepts or
          reject mail for a local user.  It does not use the 151 "User not
          local" or 152 "User unknown" preliminary replies.  Once the
          receiver-MTP accepts the relaying task it receives the mail text
          and transforms the command arguments by removing its own
          identifier from the receiver-path and inserting it in the

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    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

          beginning of the sender-path.  The receiver-MTP then becomes a
          sender-MTP and establishes a transmission channel to the next MTP
          in the receiver-path and sends it the mail.

          If an MTP has accepted the task of relaying the mail and later
          finds that the receiver-path is incorrect or that the mail cannot
          be delivered for whatever reason, then it must construct a
          notification message and send it to the originator of the
          undeliverable mail as indicated by the sender-path.  This
          notification message must be from the MTP at this host.  That is,
          the sender-path of the notification message itself will be
          "MTP@<host>", and in the notification message header the From
          field will be "MTP at <host>".  Of course, MTPs should not send
          notification messages about problems with notification messages.

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    May 1981                                                         RFC 780
    Mail Transfer Protocol

    4.  MULTI-RECIPIENT MAIL

       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 host.  In one
       scheme, all recipients are specified first, and then the text is
       sent.  In the other scheme, the order is reversed and the text is
       sent first, followed by the recipients.  The sender-MTP suggests the
       scheme it would prefer, but receiver-MTP controls which scheme is
       actually used.  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.

       Both schemes are necessary 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.  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.

       4.1.  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.

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    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

                No argument means a "selection" of none of the schemes (the
                default).

             Possible replies are:
                200 OK, use the specified scheme
                215 <scheme> This is the scheme I prefer
                504 I understand MRSQ but can't use that scheme
                5xx Command unrecognized or unimplemented

          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> <string>] <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 reseting all schemes to their initial state.  This
          reset must be done no matter what the reply will be -- 200, 215,
          or 504.  The actions necessary for a reset will be explained when
          discussing how each scheme actually works.

          Note that the receiver gets to choose which scheme is used.  The
          sender must be prepared to do either.

       4.2.  MESSAGE TEXT SPECIFICATION:  MAIL

          Regardless of which scheme (if any) has been selected, a MAIL
          command with a non-null receiver-path 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" all schemes to their initial state.

          It is only when the receiver-path argument is null that the
          particular scheme chosen is important.

             MAIL FROM:<sender-path> <CRLF>

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    Mail Transfer Protocol

          Rather than producing an error, the receiver will accept message
          text for this "null" recipient specification.  What it does with
          it depends on which scheme is in effect, and will be described in
          the section on Scheme Mechanics.

       4.3.  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:<receiver-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
                550 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.

       4.4.  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.

          Sending message text to stored recipients is done by giving a MAIL
          command with no receiver-path argument; that is, just MAIL <SP>
          FROM: <sender-path> <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

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    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

          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 (or MAIL with a non-null receiver-path
          argument).

          A 452 reply (out of storage space) 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
          receiver-path 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".

          -------------------------------------------------------------

                      Example of MRSQ R (Recipients First)

             First the sender must establish that the receiver 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

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    Mail Transfer Protocol

                S: MRCP TO:<Raboof@Y> <CRLF>
                R: 550 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 relayed to "fubar@Z"
             through host "X".  Now the message text is furnished, by giving
             a MAIL command with no receiver-path argument.

                S: MAIL FROM:<waldo@A><CRLF>
                R: 354 Start mail input; end with <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
             relayed to "fubar@Z".

                                   Example 2

          -------------------------------------------------------------

       4.5.  SCHEME MECHANICS:  MRSQ T (TEXT-FIRST)

          In the text-first scheme, MAIL with no receiver-path 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 receiver-path
          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.

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    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

          -------------------------------------------------------------

                         Example of MRSQ T (Text First)

             First the sender must 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: 504 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 receiver-path argument.

                S: MAIL FROM:<WALDO@A><CRLF>
                R: 354 Start mail input; end with <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: 550 No such user here

                S: MRCP TO:<bar@Y> <CRLF>
                R: 250 Stored mail sent

                S: MRCP TO:<@Y,@X,fubar@Z> <CRLF>
                R: 250 Mail accepted for relaying

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    May 1981                                                         RFC 780
    Mail Transfer Protocol

             The text has now been sent to "Foo" and "bar" at host "Y" and
             will be relayed 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.

                                   Example 3

          -------------------------------------------------------------

       4.6.  DISCUSSION

          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.

          One reason that the use of MAIL is restricted to null
          receiver-path arguments with this multi-recipient extension is the
          ambiguity that would result if a non-null receiver-path 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@Z><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" of all schemes 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

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    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

          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.  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.

    Sluizer & Postel                                               [Page 15]

    May 1981                                                         RFC 780
    Mail Transfer Protocol

    5.  SPECIFICATIONS

       5.1.  MTP COMMANDS

          5.1.1.  COMMAND SEMANTICS

             The MTP commands define the mail transfer or the mail system
             function requested by the user.  MTP commands are character
             strings terminated by <CRLF>.  The command codes themselves are
             alphabetic characters terminated by <SP> if parameters follow
             and <CRLF> otherwise.  The syntax of mailboxes must conform to
             receiver site conventions. The MTP commands are discussed
             below.  MTP replies are discussed in the Section 5.2.

             MAIL (MAIL)

                This command is used to send mail over the transmission
                channel.  The argument field contains a sender-path sequence
                and optional receiver-path sequence.

                The sender-path sequence consists of an optional list of
                hosts and the sender mailbox.  When the list of hosts is
                present, it is "reverse" source routing information and
                indicates that the mail was relayed through each host on the
                list (the first host in the list was the most recent relay).
                This list is used as source routing to return non-delivery
                notices to the sender.  As each relay host adds itself to
                the beginning of the list, it must use its name as known in
                the network to which it is relaying the mail rather than the
                network from which the mail came (if they are different).

                If the receiver-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 relayed to the first
                host on the list.

                The receiver treats the lines following the command as mail
                text from the sender.  The mail text is terminated by the
                character sequence "<CRLF>.<CRLF>", (see Section 5.5.2 on
                Transparency).

                As mail is relayed along the receiver-path sequence, each
                relay host must remove itself from the path sequence and put
                itself at the beginning of the sender-path sequence.  When
                mail reaches its ultimate destination (the receiver-path

    [Page 16]                                               Sluizer & Postel

    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

                sequence has only a destination mailbox), the receiver-MTP
                inserts it into the destination mailbox in accordance with
                its host mail conventions.  (For example, "MAIL FROM:<X@Y>
                TO:<@A,@B,C@D> <CRLF>" will eventually be relayed as "MAIL
                FROM:<@A,X@Y> TO:<@B,C@D> <CRLF>.)

                If the receiver-path sequence is empty, the mail is destined
                for a printer or other designated place for host general
                delivery mail (if allowed at this host).  The mail may be
                marked as sent from the sender as specified in the
                sender-path sequence 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 recipients-first, or text-first.

             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 transmission
                channel to the receiver.  The command may take an argument
                (e.g., any command name) and return more specific
                information as a response.

             QUIT (QUIT)

                This command specifies that the receiver must close the
                transmission channel.

             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.

    Sluizer & Postel                                               [Page 17]

    May 1981                                                         RFC 780
    Mail Transfer Protocol

             CONTINUE (CONT)

                This command specifies that the previously specified action
                is to be continued.  This is sent only following a
                preliminary reply.

             ABORT (ABRT)

                This command specifies that the previously specified action
                is to be aborted.  This is sent only following a preliminary
                reply.  It specifies no further action other than that the
                receiver send an OK reply.

          5.1.2.  COMMAND SYNTAX

             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.

             But, note that in the sender-path and receiver-path arguments
             case is important.  In particular, in some hosts the user "foo"
             is different from the user "Foo".

             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
             the line is received.

             Square brackets denote an optional argument field.  If the
             option is not taken, the appropriate default is implied.  All
             characters are in the ASCII characters set.

    [Page 18]                                               Sluizer & Postel

    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

             The following are the MTP commands:

             MAIL <SP> FROM:<sender-path> [<SP> TO:<receiver-path>] <CRLF>

             MRSQ [<SP> <scheme>] <CRLF>

             MRCP <SP> TO:<receiver-path> <CRLF>

             HELP [<SP> <string>] <CRLF>

             QUIT <CRLF>

             NOOP <CRLF>

             CONT <CRLF>

             ABRT <CRLF>

    Sluizer & Postel                                               [Page 19]

    May 1981                                                         RFC 780
    Mail Transfer Protocol

             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-path> ::= <path>

                <receiver-path> ::= <path>

                <scheme> ::= "R" | "T" | "?"

                <string> ::= <char> | <char> <string>

                <path> ::= "<" ["@" <host> "," ...] <mailbox> ">"

                <host> ::= <a> <string> | "#" <number> | "[" <dotnum> "]"

                <mailbox> ::= <user> "@" <host>

                <user> ::= <string>

                <char> ::= <c> | '\' <c> | '\' <s>

                <dotnum> ::= <snum> "." <snum> "." <snum> "." <snum>

                <number> ::= <d> | <d> <number>

                <snum> ::= three digits representing an integer value in the
                range 0 through 255

                <specials> ::= '<', '>', '(', ')', '\', ',', ';', ':', '@',
                '"', and the control characters (ASCII codes 0 through 37
                octal inclusive and 177 octal)

                <a> ::= any one of the 26 letters A through Z in either case

                <c> ::= any one of the 128 ASCII characters except
                <specials>

                <d> ::= any one of the ten digits 0 through 9

                <s> ::= any one of <specials>

                Note that the backslash, '\', is a quote character, which is
                used to indicate that the next character is to be used
                literally instead of with its normal interpretation.  For

    [Page 20]                                               Sluizer & Postel

    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

                example, "Joe\,Smith" could be used to indicate a single
                nine character user field with comma being the fourth
                character of the field.

             Hosts are generally known by names which are translated to
             addresses  in each host.  Sometimes a host is not known to the
             translation function and communication is blocked.  To bypass
             this barrier numeric forms are also allowed for host "names".
             One form is a decimal integer prefixed by a pound sign, "#",
             which indicates the number is the address of the host.  Another
             form is four small decimal integers separated by dots and
             enclosed by brackets, e.g., "[123.255.37.321]", which indicates
             a 32 bit ARPA Internet Address in four eight bit fields.

    Sluizer & Postel                                               [Page 21]

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    Mail Transfer Protocol

       5.2.  MTP REPLIES

          Replies to MTP 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-MTP.  Every command must generate exactly one reply.
          Additionally, some commands must occur sequentially, such as
          MRSQ T->MAIL->MRCP or MRSQ R->MRCP->MAIL.

             The details of the command-reply sequence are made explicit in
             the Sections 5.3 and 5.4 on Sequencing and 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. Further
          explanation of the assignment of reply codes is given in the
          Appendix E on the Theory of Reply Codes.  Formally, a reply is
          defined to be the sequence:  a three-digit code, <SP>, one line of
          text, and <CRLF>.

    [Page 22]                                               Sluizer & Postel

    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

          5.2.1.  REPLY CODES BY FUNCTION GROUPS

             200 Command okay
             201 Command okay, action aborted
             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
             504 Command parameter not implemented

             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 transmission channel
             421 <host> Service not available, closing transmission channel
                [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
             552 Requested mail action aborted: exceeded storage allocation
                [For current mailbox location]
             553 Requested action not taken: mailbox name not allowed
                [E.g., mailbox syntax incorrect]
             354 Start mail input; end with <CRLF>.<CRLF>

    Sluizer & Postel                                               [Page 23]

    May 1981                                                         RFC 780
    Mail Transfer Protocol

          5.2.2.  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
             201 Command okay, action aborted
             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 transmission channel
             250 Requested mail action okay, completed

             354 Start mail input; end with <CRLF>.<CRLF>

             421 <host> Service not available, closing transmission channel
                [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

             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
             504 Command parameter not implemented
             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
                [E.g., mailbox syntax incorrect]

    [Page 24]                                               Sluizer & Postel

    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

       5.3.  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 response before sending further
          commands.

          The preliminary (1xx) and intermediate (3xx) replies indicate that
          further commands and information are required to complete the
          required action.  The preliminary replies require either a
          continue or abort command to proceed; the intermediate replies
          require action dependent further commands.

          One important reply is the connection greetings.  Under normal
          circumstances, a receiver will send a 220 "Awaiting input" reply
          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.  The sender can then indicate it is
          willing to wait via a continue command, or not via the abort
          command.  The receiver will respond to the abort with a 201 reply,
          and to the continue with the 220 reply when ready.

             Note: all the greeting type replies have the official name of
             the server host as the first word following the reply code.

                For example,

                   220 <SP> USC-ISIF <SP> Service ready <CRLF>

          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

             Each command is listed with its possible replies.  Preliminary
             replies are listed first with their succeeding replies indented
             under them, then success and failure completion, and finally
             intermediary replies with the remaining commands from the
             sequence following.  The prefixes used before the possible
             replies are "P" for preliminary, "I" for intermediate, "S" for
             success, "F" for failure, and "E" for error.  The 421 reply

    Sluizer & Postel                                               [Page 25]

    May 1981                                                         RFC 780
    Mail Transfer Protocol

             (service not available, closing transmission channel) may be
             given to any command if the MTP-receiver knows it must shut
             down.  This listing forms the basis for the State Diagrams, in
             Section 5.4.

                CONNECTION ESTABLISHMENT
                   P: 120 -> CONT -> S: 220
                                     F: 421
                             ABRT    S: 201
                                     F: 421
                   S: 220
                   F: 421
                MAIL
                   P: 151 -> CONT -> I: 354 -> text -> S: 250
                      152                              F: 451,552,450,
                                                          550,452,553
                             ABRT -> S: 201
                                     F: 451,552,450,550,452,553
                   I: 354 -> text -> S: 250
                                     F: 451,552,450,550,452,553
                   F: 451, 552, 450, 550, 452, 553
                   E: 500, 501, 502, 421
                MRSQ
                   S: 200, 215
                   E: 500, 501, 502, 504, 421
                MRCP
                   P: 151 -> CONT -> S: 200, 215, 250
                      152            F: 451,552,450,550,452,553
                             ABRT -> S: 201
                                     F: 451,552,450,550,452,553
                   S: 200, 215, 250
                   F: 451, 552, 450, 550, 452, 553
                   E: 500, 501, 502, 503, 421

    [Page 26]                                               Sluizer & Postel

    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

                QUIT
                   S: 221
                   E: 500, 421
                HELP
                   S: 211, 214
                   E: 500, 501, 502, 504, 421
                NOOP
                   S: 200
                   E: 500, 421
                CONT
                   S: depends on previous command
                   F: depends on previous command
                   E: 500, 501, 502, 504, 421
                ABRT
                   S: 201,
                   E: 500, 501, 502, 504, 421

    Sluizer & Postel                                               [Page 27]

    May 1981                                                         RFC 780
    Mail Transfer Protocol

       5.4.  STATE DIAGRAMS

          Following are 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".

          First, the diagram that represents most of the MTP commands:

                                      1,3    +---+
                                 ----------->| E |
                                |            +---+
                                |
             +---+    cmd    +---+    2      +---+
             | B |---------->| W |---------->| S |
             +---+           +---+           +---+
                                |
                                |     4,5    +---+
                                 ----------->| F |
                                             +---+

             This diagram models the commands:

                HELP, MRCP, MRSQ, NOOP, QUIT, ABRT.

    [Page 28]                                               Sluizer & Postel

    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

          A more complex diagram models the MAIL command:

                                  ABRT       +---+ 1,3
                     CONT ---- ------------->| W |-------
                         |    |              +---+       |
                         |    |1           4,5|  |2      V
             +---+  cmd   -->+---+ 2          |  |     +---+
             | B |---------->| W |-------------------->| E |
             +---+           +---+        ------------>+---+
                             3| |4,5     |    |  |
                              | |        |    |  |
                --------------  ------   |    |  |
               |                      |  |    |   ---->+---+
               |               ----------------------->| S |
               |              |       |  |    |        +---+
               |              |  --------     |
               |              | |     |       |
               V             2| |1,3  |       |
             +---+   text    +---+    |        ------->+---+
             |   |---------->| W |     --------------->| F |
             +---+           +---+-------------------->+---+
                                  4,5

             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.

    Sluizer & Postel                                               [Page 29]

    May 1981                                                         RFC 780
    Mail Transfer Protocol

       5.5.  DETAILS

          5.5.1.  MINIMUM IMPLEMENTATION

             In order to make MTP workable, the following minimum
             implementation is required for all receivers:

                COMMANDS -- MAIL
                            QUIT
                            NOOP

          5.5.2.  TRANSPARENCY

             Without some provision for data transparency the character
             sequence "<CRLF>.<CRLF>" ends the the mail text and cannot be
             sent by the user.  In general, users are not aware of such
             "forbidden"  sequences.  To allow all user composed text to be
             transmitted transparently the following procedures are used.

             1. Before sending a line of mail text the sender-MTP checks the
             first character of the line.  If it is a period, one additional
             period is inserted at the beginning of the line.

             2. When a line of mail text is received by the receiver-MTP it
             checks the the line.  If the line is composed of a single
             period it is the end of mail.  If the first character is a
             period and there are other characters on the line, the first
             character is deleted.

          5.5.3.  SIZES

             There are several objects that ought to have defined maximum
             sizes.

                user

                   The maximum total length of a user name is 40 characters.

                host

                   The maximum total length of a host name or number is 20
                   characters.

    [Page 30]                                               Sluizer & Postel

    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

                path

                   The maximum total length of a sender-path or
                   receiver-path is 100 characters.

                command line

                   The maximum total length of a command line including the
                   command word and the <CRLF> is 200 characters.

                reply line

                   The maximum total length of a reply line including the
                   reply code and the <CRLF> is 65 characters.

                text line

                   The maximum total length of a text line including the the
                   <CRLF> is 1000 characters.

             To the maximum extent possible implementation techniques which
             impose no limits at all to the length of these objects should
             be used.

    Sluizer & Postel                                               [Page 31]

    May 1981                                                         RFC 780
    Mail Transfer Protocol

    APPENDIX A

       TCP Transport service

          The Transmission Control Protocol [1] is used in the ARPA
          Internet, and in any network following the US DoD standards for
          internetwork protocols.

          Connection Establishment

             The MTP transmission channel is a TCP connection established
             between the sender process port U and the receiver process port
             L.  This single full duplex connection is used as the
             transmission channel.  This protocol is assigned the service
             port 57 (71 octal), that is L=57.

          Data Transfer

             The TCP connection supports the transmission of 8-bit bytes.
             The MTP data is 7-bit ASCII characters.  Each character is
             transmitted as a 8-bit byte with the high-order bit cleared to
             zero.

    [Page 32]                                               Sluizer & Postel

    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

    APPENDIX B

       NCP Transport service

          The ARPANET Host-to-Host Protocol [2] (implemented by the Network
          Control Program) may be used in the ARPANET.

          Connection Establishment

             The MTP transmission channel is established via NCP between the
             the sender process socket U and receiver process socket L.  The
             Initial Connection Protocol [3] is followed resulting in a pair
             of simplex connections.  This pair of connections is used as
             the transmission channel.  This protocol is assigned the
             contact socket 57 (71 octal), that is L=57.

          Data Transfer

             The NCP data connections are established in 8-bit byte mode.
             The MTP data is 7-bit ASCII characters.  Each character is
             transmitted as a 8-bit byte with the high-order bit cleared to
             zero.

    Sluizer & Postel                                               [Page 33]

    May 1981                                                         RFC 780
    Mail Transfer Protocol

    APPENDIX C

       NITS

          The Network Independent Transport Service [4] may be used.

          Connection Establishment

             The MTP transmission channel is established via NITS between
             the the sender process and receiver process.  The sender
             process executes the CONNECT primitive, and the waiting
             receiver process executes the ACCEPT primitive.

          Data Transfer

             The NITS connection supports the transmission of 8-bit bytes.
             The MTP data is 7-bit ASCII characters.  Each character is
             transmitted as a 8-bit byte with the high-order bit cleared to
             zero.

    [Page 34]                                               Sluizer & Postel

    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

    APPENDIX D

       X.25 Transport service

          It may be possible to use the X.25 service [5] as provided by the
          Public Data Networks directly, but there are indications that it
          is too error prone to qualify as a reliable channel.  It is
          suggested that a reliable end-to-end protocol such as TCP be used
          on top of X.25 connections.

    Sluizer & Postel                                               [Page 35]

    May 1981                                                         RFC 780
    Mail Transfer Protocol

    APPENDIX E

       Theory of Reply Codes

          The three digits of the reply each have a special significance.
          The first digit denotes whether the response is good, bad or
          incomplete.  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 command has been accepted, but the requested action
                   is being held in abeyance, pending confirmation of the
                   information in this reply.  The sender-MTP should send
                   another command specifying whether to continue or abort
                   the action.

                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

    [Page 36]                                               Sluizer & Postel

    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

                   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 the account status.)

             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
                      transmission channel.

                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
             illustrates 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.

    Sluizer & Postel                                               [Page 37]

    May 1981                                                         RFC 780
    Mail Transfer Protocol

             Receiver implementations should not invent new codes for
             slightly different situations from the ones described here, but
             rather adapt codes already defined.

             For example, 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.

          The reply text may be longer than a single line; in these cases
          the complete text must be marked so the sender-MTP knows when it
          can stop reading the reply.  This requires a special format to
          indicate a multiple line reply.

             The format for multi-line replies requires that every line,
             except the last, begin with the reply code, followed
             immediately by a hyphen, "-" (also known as minus), followed by
             text.  The last line will begin with the reply code, followed
             immediately by <SP>, optionally some text, and <CRLF>.

                For example:
                                    123-First line
                                    123-Second line
                                    123-234 text beginning with numbers
                                    123 The last line

             The sender-MTP then simply needs to search for the reply code
             followed by <SP> at the beginning of a line, and ignore all
             preceding lines.

    [Page 38]                                               Sluizer & Postel

    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

    GLOSSARY

       ASCII

          American Standard Code for Information Interchange [6].

       command

          A request for a mail service action sent by the sender-MTP to the
          receiver-MTP.

       host

          A computer in the internetwork environment on which mailboxes or
          MTP processes reside.

       line

          A line of text ending with a <CRLF>.

       mail

          A sequence of ASCII characters of arbitrary length, which conforms
          to the standard set in RFC 733 (Standard for the Format of ARPA
          Network Text Messages [7]).

       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.

       receiver-MTP process

          A process which transfers mail in cooperation with a sender-MTP
          process.  It waits for a connection to be established via the
          transport service.  It receives MTP commands from the sender-MTP,
          sends replies, and governs the transfer of mail.

    Sluizer & Postel                                               [Page 39]

    May 1981                                                         RFC 780
    Mail Transfer Protocol

       reply

          A reply is an acknowledgment (positive or negative) sent from
          receiver to sender via the transmission channel 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.

       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 transport
          service connection.  It initiates MTP commands, receives replies,
          and governs the transfer of mail.

       transmission channel

          A full-duplex communication path between a sender-MTP and a
          receiver-MTP for the exchange of commands, replies, and mail text.

       transport service

          Any reliable stream-oriented data communication services.  For
          example, NCP, TCP, NITS.

       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.

       word

          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.

       <CRLF>

          The characters carriage return and line feed (in that order).

    [Page 40]                                               Sluizer & Postel

    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

       <SP>

          The space character.

    Sluizer & Postel                                               [Page 41]

    May 1981                                                         RFC 780
    Mail Transfer Protocol

    REFERENCES

       [1]  TCP

          Postel, J., ed., "DOD Standard Transmission Control Protocol",
          IEN 129, RFC 761, USC/Information Sciences Institute,
          NTIS ADA082609, January 1980.  Appears in: Computer Communication
          Review, Special Interest Group on Data Communications, ACM, V.10,
          N.4, October 1980.

       [2]  NCP

          McKenzie,A., "Host/Host Protocol for the ARPA Network", NIC 8246,
          January 1972.  Also in:  Feinler, E. and J. Postel, eds., "ARPANET
          Protocol Handbook", NIC 7104, for the Defense Communications
          Agency by SRI International, Menlo Park, California, Revised
          January 1978.

       [3]  Initial Connection Protocol

          Postel, J., "Official Initial Connection Protocol", NIC 7101,
          11 June 1971.  Also in:  Feinler, E. and J. Postel, eds., "ARPANET
          Protocol Handbook", NIC 7104, for the Defense Communications
          Agency by SRI International, Menlo Park, California, Revised
          January 1978.

       [4]  NITS

          PSS/SG3, "A Network Independent Transport Service", Study Group 3,
          The Post Office PSS Users Group, February 1980.  Available from
          the DCPU, National Physical Laboratory, Teddington, UK.

       [5]  X.25

          CCITT, "Recommendation X.25 - Interface Between Data Terminal
          Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for
          Terminals Operating in the Packet Mode on Public Data Networks,"
          CCITT Orange Book, Vol. VIII.2, International Telephone and
          Telegraph Consultative Committee, Geneva, 1976.

    [Page 42]                                               Sluizer & Postel

    RFC 780                                                         May 1981
                                                      Mail Transfer Protocol

       [6]  ASCII

          ASCII, "USA Code for Information Interchange", United States of
          America Standards Institute, X3.4, 1968.  Also in:  Feinler, E.
          and J. Postel, eds., "ARPANET Protocol Handbook", NIC 7104, for
          the Defense Communications Agency by SRI International, Menlo
          Park, California, Revised January 1978.

       [7]  RFC 733

          Crocker, D., J. Vittal, K. Pogran, and D. Henderson, "Standard for
          the Format of ARPA Network Text Messages," RFC 733, NIC 41952,
          November 1977.  Also in:  Feinler, E. and J. Postel, eds.,
          "ARPANET Protocol Handbook", NIC 7104, for the Defense
          Communications Agency by SRI International, Menlo Park,
          California, Revised January 1978.

    Sluizer & Postel                                               [Page 43]