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Obsoleted by: 788

                         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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May 1981                                                         RFC 780
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.



































Sluizer & Postel                                                [Page 7]

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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May 1981                                                         RFC 780
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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May 1981                                                         RFC 780
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



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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]

May 1981                                                         RFC 780
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]


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