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rfc:rfc780
                                  
                                  
                                  
                                  
                                  
                       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.

Sluizer & Postel [Page 1]

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.
  1. ————————————————————
 
             +----------+                +----------+
 +------+    |          |                |          |
 | User |<-->|          |      MTP       |          |
 +------+    |  Sender- |Commands/Replies| Receiver-|
 +------+    |   MTP    |<-------------->|    MTP   |    +------+
 | File |<-->|          |    and Mail    |          |<-->| File |
 |System|    |          |                |          |    |System|
 +------+    +----------+                +----------+    +------+
 
              Sender-MTP                 Receiver-MTP
                         Model for MTP Use
                              Figure 1
  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.

[Page 2] Sluizer & Postel

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.

Sluizer & Postel [Page 3]

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.

[Page 4] Sluizer & Postel

RFC 780 May 1981

                                                Mail Transfer Protocol
  1. ————————————————————
                    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
  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.

Sluizer & Postel [Page 5]

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

[Page 6] Sluizer & Postel

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.

[Page 8] Sluizer & Postel

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>

Sluizer & Postel [Page 9]

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

[Page 10] Sluizer & Postel

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".
  1. ————————————————————
                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

Sluizer & Postel [Page 11]

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
  1. ————————————————————
 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.

[Page 12] Sluizer & Postel

RFC 780 May 1981

                                                Mail Transfer Protocol
  1. ————————————————————
                   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
  1. ————————————————————
 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

[Page 14] Sluizer & Postel

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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                                                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.

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       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.

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                                                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>

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       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

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                                                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.

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 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>.

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                                                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>
       

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    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]
       

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                                                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

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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

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                                                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

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 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.

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                                                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.

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 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.

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                                                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.

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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.

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                                                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.

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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.

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                                                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.

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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

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                                                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.

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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.

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                                                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.

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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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