RFC 821: Simple Mail Transfer Protocol (original) (raw)

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Obsoleted by: 2821 INTERNET STANDARD

RFC 821

                 SIMPLE MAIL TRANSFER PROTOCOL


                       Jonathan B. Postel


                          August 1982


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

                         (213) 822-1511


RFC 821 August 1982 Simple Mail Transfer Protocol

                       TABLE OF CONTENTS

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

2. THE SMTP MODEL ................................................ 2

3. THE SMTP PROCEDURE ............................................ 4

  [3.1](#section-3.1).  Mail ..................................................... [4](#page-4)
  [3.2](#section-3.2).  Forwarding ............................................... [7](#page-7)
  [3.3](#section-3.3).  Verifying and Expanding .................................. [8](#page-8)
  [3.4](#section-3.4).  Sending and Mailing ..................................... [11](#page-11)
  [3.5](#section-3.5).  Opening and Closing ..................................... [13](#page-13)
  [3.6](#section-3.6).  Relaying ................................................ [14](#page-14)
  [3.7](#section-3.7).  Domains ................................................. [17](#page-17)
  [3.8](#section-3.8).  Changing Roles .......................................... [18](#page-18)

4. THE SMTP SPECIFICATIONS ...................................... 19

  [4.1](#section-4.1).  SMTP Commands ........................................... [19](#page-19)
  [4.1.1](#section-4.1.1).  Command Semantics ..................................... [19](#page-19)
  [4.1.2](#section-4.1.2).  Command Syntax ........................................ [27](#page-27)
  [4.2](#section-4.2).  SMTP Replies ............................................ [34](#page-34)
  [4.2.1](#section-4.2.1).  Reply Codes by Function Group ......................... [35](#page-35)
  [4.2.2](#section-4.2.2).  Reply Codes in Numeric Order .......................... [36](#page-36)
  [4.3](#section-4.3).  Sequencing of Commands and Replies ...................... [37](#page-37)
  [4.4](#section-4.4).  State Diagrams .......................................... [39](#page-39)
  [4.5](#section-4.5).  Details ................................................. [41](#page-41)
  [4.5.1](#section-4.5.1).  Minimum Implementation ................................ [41](#page-41)
  [4.5.2](#section-4.5.2).  Transparency .......................................... [41](#page-41)
  [4.5.3](#section-4.5.3).  Sizes ................................................. [42](#page-42)

APPENDIX A: TCP ................................................. 44 APPENDIX B: NCP ................................................. 45 APPENDIX C: NITS ................................................ 46 APPENDIX D: X.25 ................................................ 47 APPENDIX E: Theory of Reply Codes ............................... 48 APPENDIX F: Scenarios ........................................... 51

GLOSSARY ......................................................... 64

REFERENCES ....................................................... 67



Network Working Group J. Postel Request for Comments: DRAFT ISI Replaces: RFC 788, 780, 772 August 1982

                 SIMPLE MAIL TRANSFER PROTOCOL

1. INTRODUCTION

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

SMTP is independent of the particular transmission subsystem and requires only a reliable ordered data stream channel. Appendices A, B, C, and D describe the use of SMTP with various transport services. A Glossary provides the definitions of terms as used in this document.

An important feature of SMTP is its capability to relay mail across transport service environments. A transport service provides an interprocess communication environment (IPCE). An IPCE may cover one network, several networks, or a subset of a network. It is important to realize that transport systems (or IPCEs) are not one-to-one with networks. A process can communicate directly with another process through any mutually known IPCE. Mail is an application or use of interprocess communication. Mail can be communicated between processes 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.

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August 1982 RFC 821 Simple Mail Transfer Protocol

2. THE SMTP MODEL

The SMTP design is based on the following model of communication: as the result of a user mail request, the sender-SMTP establishes a two-way transmission channel to a receiver-SMTP. The receiver-SMTP may be either the ultimate destination or an intermediate. SMTP commands are generated by the sender-SMTP and sent to the receiver-SMTP. SMTP replies are sent from the receiver-SMTP to the sender-SMTP in response to the commands.

Once the transmission channel is established, the SMTP-sender sends a MAIL command indicating the sender of the mail. If the SMTP-receiver can accept mail it responds with an OK reply. The SMTP-sender then sends a RCPT command identifying a recipient of the mail. If the SMTP-receiver can accept mail for that recipient it responds with an OK reply; if not, it responds with a reply rejecting that recipient (but not the whole mail transaction). The SMTP-sender and SMTP-receiver may negotiate several recipients. When the recipients have been negotiated the SMTP-sender sends the mail data, terminating with a special sequence. If the SMTP-receiver successfully processes the mail data it responds with an OK reply. The dialog is purposely lock-step, one-at-a-time.

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


           +----------+                +----------+

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

            Sender-SMTP                Receiver-SMTP

                       Model for SMTP Use

                            Figure 1

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

The SMTP provides mechanisms for the transmission of mail; directly from the sending user's host to the receiving user's host when the

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RFC 821 August 1982 Simple Mail Transfer Protocol

two host are connected to the same transport service, or via one or more relay SMTP-servers when the source and destination hosts are not connected to the same transport service.

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

The argument to the MAIL command is a reverse-path, which specifies who the mail is from. The argument to the RCPT command is a forward-path, which specifies who the mail is to. The forward-path is a source route, while the reverse-path is a return route (which may be used to return a message to the sender when an error occurs with a relayed message).

When the same message is sent to multiple recipients the SMTP encourages the transmission of only one copy of the data for all the recipients at the same destination host.

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 4 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 SMTP implementations must take case to preserve the case of user names as they appear in mailbox arguments. Host names are not case sensitive.

Commands and replies are composed of characters from the ASCII character set [1]. When the transport service provides an 8-bit byte (octet) transmission channel, each 7-bit character is transmitted right justified in an octet with the high order bit cleared to zero.

When specifying the general form of a command or reply, an argument (or special symbol) will be denoted by a meta-linguistic variable (or constant), for example, "" or "". Here the angle brackets indicate these are meta-linguistic variables. However, some arguments use the angle brackets literally. For example, an actual reverse-path is enclosed in angle brackets, i.e., "John.Smith@USC-ISI.ARPA" is an instance of (the angle brackets are actually transmitted in the command or reply).

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August 1982 RFC 821 Simple Mail Transfer Protocol

3. THE SMTP PROCEDURES

This section presents the procedures used in SMTP in several parts. First comes the basic mail procedure defined as a mail transaction. Following this are descriptions of forwarding mail, verifying mailbox names and expanding mailing lists, sending to terminals instead of or in combination with mailboxes, and the opening and closing exchanges. At the end of this section are comments on relaying, a note on mail domains, and a discussion of changing roles. Throughout this section are examples of partial command and reply sequences, several complete scenarios are presented in Appendix F.

3.1. MAIL

  There are three steps to SMTP mail transactions.  The transaction
  is started with a MAIL command which gives the sender
  identification.  A series of one or more RCPT commands follows
  giving the receiver information.  Then a DATA command gives the
  mail data.  And finally, the end of mail data indicator confirms
  the transaction.

     The first step in the procedure is the MAIL command.  The
     <reverse-path> contains the source mailbox.

        MAIL <SP> FROM:<reverse-path> <CRLF>

     This command tells the SMTP-receiver that a new mail
     transaction is starting and to reset all its state tables and
     buffers, including any recipients or mail data.  It gives the
     reverse-path which can be used to report errors.  If accepted,
     the receiver-SMTP returns a 250 OK reply.

     The <reverse-path> can contain more than just a mailbox.  The
     <reverse-path> is a reverse source routing list of hosts and
     source mailbox.  The first host in the <reverse-path> should be
     the host sending this command.

     The second step in the procedure is the RCPT command.

        RCPT <SP> TO:<forward-path> <CRLF>

     This command gives a forward-path identifying one recipient.
     If accepted, the receiver-SMTP returns a 250 OK reply, and
     stores the forward-path.  If the recipient is unknown the
     receiver-SMTP returns a 550 Failure reply.  This second step of
     the procedure can be repeated any number of times.

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RFC 821 August 1982 Simple Mail Transfer Protocol

     The <forward-path> can contain more than just a mailbox.  The
     <forward-path> is a source routing list of hosts and the
     destination mailbox.  The first host in the <forward-path>
     should be the host receiving this command.

     The third step in the procedure is the DATA command.

        DATA <CRLF>

     If accepted, the receiver-SMTP returns a 354 Intermediate reply
     and considers all succeeding lines to be the message text.
     When the end of text is received and stored the SMTP-receiver
     sends a 250 OK reply.

     Since the mail data is sent on the transmission channel the end
     of the mail data must be indicated so that the command and
     reply dialog can be resumed.  SMTP indicates the end of the
     mail data by sending a line containing only a period.  A
     transparency procedure is used to prevent this from interfering
     with the user's text (see [Section 4.5.2](#section-4.5.2)).

        Please note that the mail data includes the memo header
        items such as Date, Subject, To, Cc, From [[2](#ref-2)].

     The end of mail data indicator also confirms the mail
     transaction and tells the receiver-SMTP to now process the
     stored recipients and mail data.  If accepted, the
     receiver-SMTP returns a 250 OK reply.  The DATA command should
     fail only if the mail transaction was incomplete (for example,
     no recipients), or if resources are not available.

  The above procedure is an example of a mail transaction.  These
  commands must be used only in the order discussed above.
  Example 1 (below) illustrates the use of these commands in a mail
  transaction.

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August 1982 RFC 821 Simple Mail Transfer Protocol

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

                 Example of the SMTP Procedure

     This SMTP example shows mail sent by Smith at host Alpha.ARPA,
     to Jones, Green, and Brown at host Beta.ARPA.  Here we assume
     that host Alpha contacts host Beta directly.

        S: MAIL FROM:<Smith@Alpha.ARPA>
        R: 250 OK

        S: RCPT TO:<Jones@Beta.ARPA>
        R: 250 OK

        S: RCPT TO:<Green@Beta.ARPA>
        R: 550 No such user here

        S: RCPT TO:<Brown@Beta.ARPA>
        R: 250 OK

        S: DATA
        R: 354 Start mail input; end with <CRLF>.<CRLF>
        S: Blah blah blah...
        S: ...etc. etc. etc.
        S: <CRLF>.<CRLF>
        R: 250 OK

     The mail has now been accepted for Jones and Brown.  Green did
     not have a mailbox at host Beta.

                           Example 1

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

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RFC 821 August 1982 Simple Mail Transfer Protocol

3.2. FORWARDING

  There are some cases where the destination information in the
  <forward-path> is incorrect, but the receiver-SMTP knows the
  correct destination.  In such cases, one of the following replies
  should be used to allow the sender to contact the correct
  destination.

     251 User not local; will forward to <forward-path>

        This reply indicates that the receiver-SMTP knows the user's
        mailbox is on another host and indicates the correct
        forward-path to use in the future.  Note that either the
        host or user or both may be different.  The receiver takes
        responsibility for delivering the message.

     551 User not local; please try <forward-path>

        This reply indicates that the receiver-SMTP knows the user's
        mailbox is on another host and indicates the correct
        forward-path to use.  Note that either the host or user or
        both may be different.  The receiver refuses to accept mail
        for this user, and the sender must either redirect the mail
        according to the information provided or return an error
        response to the originating user.

  Example 2 illustrates the use of these responses.

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

                     Example of Forwarding

  Either

  S: RCPT TO:<Postel@USC-ISI.ARPA>
  R: 251 User not local; will forward to <Postel@USC-ISIF.ARPA>

  Or

  S: RCPT TO:<Paul@USC-ISIB.ARPA>
  R: 551 User not local; please try <Mockapetris@USC-ISIF.ARPA>

                           Example 2

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

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August 1982 RFC 821 Simple Mail Transfer Protocol

3.3. VERIFYING AND EXPANDING

  SMTP provides as additional features, commands to verify a user
  name or expand a mailing list.  This is done with the VRFY and
  EXPN commands, which have character string arguments.  For the
  VRFY command, the string is a user name, and the response may
  include the full name of the user and must include the mailbox of
  the user.  For the EXPN command, the string identifies a mailing
  list, and the multiline response may include the full name of the
  users and must give the mailboxes on the mailing list.

  "User name" is a fuzzy term and used purposely.  If a host
  implements the VRFY or EXPN commands then at least local mailboxes
  must be recognized as "user names".  If a host chooses to
  recognize other strings as "user names" that is allowed.

  In some hosts the distinction between a mailing list and an alias
  for a single mailbox is a bit fuzzy, since a common data structure
  may hold both types of entries, and it is possible to have mailing
  lists of one mailbox.  If a request is made to verify a mailing
  list a positive response can be given if on receipt of a message
  so addressed it will be delivered to everyone on the list,
  otherwise an error should be reported (e.g., "550 That is a
  mailing list, not a user").  If a request is made to expand a user
  name a positive response can be formed by returning a list
  containing one name, or an error can be reported (e.g., "550 That
  is a user name, not a mailing list").

  In the case of a multiline reply (normal for EXPN) exactly one
  mailbox is to be specified on each line of the reply.  In the case
  of an ambiguous request, for example, "VRFY Smith", where there
  are two Smith's the response must be "553 User ambiguous".

  The case of verifying a user name is straightforward as shown in
  example 3.

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RFC 821 August 1982 Simple Mail Transfer Protocol

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

                Example of Verifying a User Name

     Either

        S: VRFY Smith
        R: 250 Fred Smith <Smith@USC-ISIF.ARPA>

     Or

        S: VRFY Smith
        R: 251 User not local; will forward to <Smith@USC-ISIQ.ARPA>

     Or

        S: VRFY Jones
        R: 550 String does not match anything.

     Or

        S: VRFY Jones
        R: 551 User not local; please try <Jones@USC-ISIQ.ARPA>

     Or

        S: VRFY Gourzenkyinplatz
        R: 553 User ambiguous.

                           Example 3

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

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August 1982 RFC 821 Simple Mail Transfer Protocol

  The case of expanding a mailbox list requires a multiline reply as
  shown in example 4.

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

              Example of Expanding a Mailing List

     Either

        S: EXPN Example-People
        R: 250-Jon Postel <Postel@USC-ISIF.ARPA>
        R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA>
        R: 250-Sam Q. Smith <SQSmith@USC-ISIQ.ARPA>
        R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
        R: 250-<joe@foo-unix.ARPA>
        R: 250 <xyz@bar-unix.ARPA>

     Or

        S: EXPN Executive-Washroom-List
        R: 550 Access Denied to You.

                           Example 4

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

  The character string arguments of the VRFY and EXPN commands
  cannot be further restricted due to the variety of implementations
  of the user name and mailbox list concepts.  On some systems it
  may be appropriate for the argument of the EXPN command to be a
  file name for a file containing a mailing list, but again there is
  a variety of file naming conventions in the Internet.

  The VRFY and EXPN commands are not included in the minimum
  implementation ([Section 4.5.1](#section-4.5.1)), and are not required to work
  across relays when they are implemented.

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RFC 821 August 1982 Simple Mail Transfer Protocol

3.4. SENDING AND MAILING

  The main purpose of SMTP is to deliver messages to user's
  mailboxes.  A very similar service provided by some hosts is to
  deliver messages to user's terminals (provided the user is active
  on the host).  The delivery to the user's mailbox is called
  "mailing", the delivery to the user's terminal is called
  "sending".  Because in many hosts the implementation of sending is
  nearly identical to the implementation of mailing these two
  functions are combined in SMTP.  However the sending commands are
  not included in the required minimum implementation
  ([Section 4.5.1](#section-4.5.1)).  Users should have the ability to control the
  writing of messages on their terminals.  Most hosts permit the
  users to accept or refuse such messages.

  The following three command are defined to support the sending
  options.  These are used in the mail transaction instead of the
  MAIL command and inform the receiver-SMTP of the special semantics
  of this transaction:

     SEND <SP> FROM:<reverse-path> <CRLF>

        The SEND command requires that the mail data be delivered to
        the user's terminal.  If the user is not active (or not
        accepting terminal messages) on the host a 450 reply may
        returned to a RCPT command.  The mail transaction is
        successful if the message is delivered the terminal.

     SOML <SP> FROM:<reverse-path> <CRLF>

        The Send Or MaiL command requires that the mail data be
        delivered to the user's terminal if the user is active (and
        accepting terminal messages) on the host.  If the user is
        not active (or not accepting terminal messages) then the
        mail data is entered into the user's mailbox.  The mail
        transaction is successful if the message is delivered either
        to the terminal or the mailbox.

     SAML <SP> FROM:<reverse-path> <CRLF>

        The Send And MaiL command requires that the mail data be
        delivered to the user's terminal if the user is active (and
        accepting terminal messages) on the host.  In any case the
        mail data is entered into the user's mailbox.  The mail
        transaction is successful if the message is delivered the
        mailbox.

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August 1982 RFC 821 Simple Mail Transfer Protocol

  The same reply codes that are used for the MAIL commands are used
  for these commands.

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RFC 821 August 1982 Simple Mail Transfer Protocol

3.5. OPENING AND CLOSING

  At the time the transmission channel is opened there is an
  exchange to ensure that the hosts are communicating with the hosts
  they think they are.

  The following two commands are used in transmission channel
  opening and closing:

     HELO <SP> <domain> <CRLF>

     QUIT <CRLF>

  In the HELO command the host sending the command identifies
  itself; the command may be interpreted as saying "Hello, I am
  <domain>".

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

                 Example of Connection Opening

     R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
     S: HELO USC-ISIF.ARPA
     R: 250 BBN-UNIX.ARPA

                           Example 5

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

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

                 Example of Connection Closing

     S: QUIT
     R: 221 BBN-UNIX.ARPA Service closing transmission channel

                           Example 6

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

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August 1982 RFC 821 Simple Mail Transfer Protocol

3.6. RELAYING

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

  Conceptually the elements of the forward-path are moved to the
  reverse-path as the message is relayed from one server-SMTP to
  another.  The reverse-path is a reverse source route, (i.e., a
  source route from the current location of the message to the
  originator of the message).  When a server-SMTP deletes its
  identifier from the forward-path and inserts it into the
  reverse-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 server-SMTP is known by different names in
  different environments.

  If when the message arrives at an SMTP the first element of the
  forward-path is not the identifier of that SMTP the element is not
  deleted from the forward-path and is used to determine the next
  SMTP to send the message to.  In any case, the SMTP adds its own
  identifier to the reverse-path.

  Using source routing the receiver-SMTP receives mail to be relayed
  to another server-SMTP  The receiver-SMTP may accept or reject the
  task of relaying the mail in the same way it accepts or rejects
  mail for a local user.  The receiver-SMTP transforms the command
  arguments by moving its own identifier from the forward-path to
  the beginning of the reverse-path.  The receiver-SMTP then becomes
  a sender-SMTP, establishes a transmission channel to the next SMTP
  in the forward-path, and sends it the mail.

  The first host in the reverse-path should be the host sending the
  SMTP commands, and the first host in the forward-path should be
  the host receiving the SMTP commands.

  Notice that the forward-path and reverse-path appear in the SMTP
  commands and replies, but not necessarily in the message.  That
  is, there is no need for these paths and especially this syntax to
  appear in the "To:" , "From:", "CC:", etc. fields of the message
  header.

  If a server-SMTP has accepted the task of relaying the mail and

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RFC 821 August 1982 Simple Mail Transfer Protocol

  later finds that the forward-path is incorrect or that the mail
  cannot be delivered for whatever reason, then it must construct an
  "undeliverable mail" notification message and send it to the
  originator of the undeliverable mail (as indicated by the
  reverse-path).

  This notification message must be from the server-SMTP at this
  host.  Of course, server-SMTPs should not send notification
  messages about problems with notification messages.  One way to
  prevent loops in error reporting is to specify a null reverse-path
  in the MAIL command of a notification message.  When such a
  message is relayed it is permissible to leave the reverse-path
  null.  A MAIL command with a null reverse-path appears as follows:

     MAIL FROM:<>

  An undeliverable mail notification message is shown in example 7.
  This notification is in response to a message originated by JOE at
  HOSTW and sent via HOSTX to HOSTY with instructions to relay it on
  to HOSTZ.  What we see in the example is the transaction between
  HOSTY and HOSTX, which is the first step in the return of the
  notification message.

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August 1982 RFC 821 Simple Mail Transfer Protocol

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

        Example Undeliverable Mail Notification Message

     S: MAIL FROM:<>
     R: 250 ok
     S: RCPT TO:<@HOSTX.ARPA:JOE@HOSTW.ARPA>
     R: 250 ok
     S: DATA
     R: 354 send the mail data, end with .
     S: Date: 23 Oct 81 11:22:33
     S: From: SMTP@HOSTY.ARPA
     S: To: JOE@HOSTW.ARPA
     S: Subject: Mail System Problem
     S:
     S:   Sorry JOE, your message to SAM@HOSTZ.ARPA lost.
     S:   HOSTZ.ARPA said this:
     S:    "550 No Such User"
     S: .
     R: 250 ok

                           Example 7

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

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RFC 821 August 1982 Simple Mail Transfer Protocol

3.7. DOMAINS

  Domains are a recently introduced concept in the ARPA Internet
  mail system.  The use of domains changes the address space from a
  flat global space of simple character string host names to a
  hierarchically structured rooted tree of global addresses.  The
  host name is replaced by a domain and host designator which is a
  sequence of domain element strings separated by periods with the
  understanding that the domain elements are ordered from the most
  specific to the most general.

  For example, "USC-ISIF.ARPA", "Fred.Cambridge.UK", and
  "PC7.LCS.MIT.ARPA" might be host-and-domain identifiers.

  Whenever domain names are used in SMTP only the official names are
  used, the use of nicknames or aliases is not allowed.

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August 1982 RFC 821 Simple Mail Transfer Protocol

3.8. CHANGING ROLES

  The TURN command may be used to reverse the roles of the two
  programs communicating over the transmission channel.

  If program-A is currently the sender-SMTP and it sends the TURN
  command and receives an ok reply (250) then program-A becomes the
  receiver-SMTP.

  If program-B is currently the receiver-SMTP and it receives the
  TURN command and sends an ok reply (250) then program-B becomes
  the sender-SMTP.

  To refuse to change roles the receiver sends the 502 reply.

  Please note that this command is optional.  It would not normally
  be used in situations where the transmission channel is TCP.
  However, when the cost of establishing the transmission channel is
  high, this command may be quite useful.  For example, this command
  may be useful in supporting be mail exchange using the public
  switched telephone system as a transmission channel, especially if
  some hosts poll other hosts for mail exchanges.

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RFC 821 August 1982 Simple Mail Transfer Protocol

4. THE SMTP SPECIFICATIONS

4.1. SMTP COMMANDS

  4.1.1.  COMMAND SEMANTICS

     The SMTP commands define the mail transfer or the mail system
     function requested by the user.  SMTP 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 SMTP commands are discussed
     below.  The SMTP replies are discussed in the [Section 4.2](#section-4.2).

     A mail transaction involves several data objects which are
     communicated as arguments to different commands.  The
     reverse-path is the argument of the MAIL command, the
     forward-path is the argument of the RCPT command, and the mail
     data is the argument of the DATA command.  These arguments or
     data objects must be transmitted and held pending the
     confirmation communicated by the end of mail data indication
     which finalizes the transaction.  The model for this is that
     distinct buffers are provided to hold the types of data
     objects, that is, there is a reverse-path buffer, a
     forward-path buffer, and a mail data buffer.  Specific commands
     cause information to be appended to a specific buffer, or cause
     one or more buffers to be cleared.

     HELLO (HELO)

        This command is used to identify the sender-SMTP to the
        receiver-SMTP.  The argument field contains the host name of
        the sender-SMTP.

        The receiver-SMTP identifies itself to the sender-SMTP in
        the connection greeting reply, and in the response to this
        command.

        This command and an OK reply to it confirm that both the
        sender-SMTP and the receiver-SMTP are in the initial state,
        that is, there is no transaction in progress and all state
        tables and buffers are cleared.

Postel [Page 19]


August 1982 RFC 821 Simple Mail Transfer Protocol

     MAIL (MAIL)

        This command is used to initiate a mail transaction in which
        the mail data is delivered to one or more mailboxes.  The
        argument field contains a reverse-path.

        The reverse-path consists of an optional list of hosts and
        the sender mailbox.  When the list of hosts is present, it
        is a "reverse" source route 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 a
        source route 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 IPCE to which it is
        relaying the mail rather than the IPCE from which the mail
        came (if they are different).  In some types of error
        reporting messages (for example, undeliverable mail
        notifications) the reverse-path may be null (see Example 7).

        This command clears the reverse-path buffer, the
        forward-path buffer, and the mail data buffer; and inserts
        the reverse-path information from this command into the
        reverse-path buffer.

     RECIPIENT (RCPT)

        This command is used to identify an individual recipient of
        the mail data; multiple recipients are specified by multiple
        use of this command.

        The forward-path consists of an optional list of hosts and a
        required destination mailbox.  When the list of hosts is
        present, it is a source route and indicates that the mail
        must be relayed to the next host on the list.  If the
        receiver-SMTP does not implement the relay function it may
        user the same reply it would for an unknown local user
        (550).

        When mail is relayed, the relay host must remove itself from
        the beginning forward-path and put itself at the beginning
        of the reverse-path.  When mail reaches its ultimate
        destination (the forward-path contains only a destination
        mailbox), the receiver-SMTP inserts it into the destination
        mailbox in accordance with its host mail conventions.

[Page 20] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

           For example, mail received at relay host A with arguments

              FROM:<USERX@HOSTY.ARPA>
              TO:<@HOSTA.ARPA,@HOSTB.ARPA:USERC@HOSTD.ARPA>

           will be relayed on to host B with arguments

              FROM:<@HOSTA.ARPA:USERX@HOSTY.ARPA>
              TO:<@HOSTB.ARPA:USERC@HOSTD.ARPA>.

        This command causes its forward-path argument to be appended
        to the forward-path buffer.

     DATA (DATA)

        The receiver treats the lines following the command as mail
        data from the sender.  This command causes the mail data
        from this command to be appended to the mail data buffer.
        The mail data may contain any of the 128 ASCII character
        codes.

        The mail data is terminated by a line containing only a
        period, that is the character sequence "<CRLF>.<CRLF>" (see
        [Section 4.5.2](#section-4.5.2) on Transparency).  This is the end of mail
        data indication.

        The end of mail data indication requires that the receiver
        must now process the stored mail transaction information.
        This processing consumes the information in the reverse-path
        buffer, the forward-path buffer, and the mail data buffer,
        and on the completion of this command these buffers are
        cleared.  If the processing is successful the receiver must
        send an OK reply.  If the processing fails completely the
        receiver must send a failure reply.

        When the receiver-SMTP accepts a message either for relaying
        or for final delivery it inserts at the beginning of the
        mail data a time stamp line.  The time stamp line indicates
        the identity of the host that sent the message, and the
        identity of the host that received the message (and is
        inserting this time stamp), and the date and time the
        message was received.  Relayed messages will have multiple
        time stamp lines.

        When the receiver-SMTP makes the "final delivery" of a
        message it inserts at the beginning of the mail data a

Postel [Page 21]


August 1982 RFC 821 Simple Mail Transfer Protocol

        return path line.  The return path line preserves the
        information in the <reverse-path> from the MAIL command.
        Here, final delivery means the message leaves the SMTP
        world.  Normally, this would mean it has been delivered to
        the destination user, but in some cases it may be further
        processed and transmitted by another mail system.

           It is possible for the mailbox in the return path be
           different from the actual sender's mailbox, for example,
           if error responses are to be delivered a special error
           handling mailbox rather than the message senders.

        The preceding two paragraphs imply that the final mail data
        will begin with a  return path line, followed by one or more
        time stamp lines.  These lines will be followed by the mail
        data header and body [[2](#ref-2)].  See Example 8.

        Special mention is needed of the response and further action
        required when the processing following the end of mail data
        indication is partially successful.  This could arise if
        after accepting several recipients and the mail data, the
        receiver-SMTP finds that the mail data can be successfully
        delivered to some of the recipients, but it cannot be to
        others (for example, due to mailbox space allocation
        problems).  In such a situation, the response to the DATA
        command must be an OK reply.  But, the receiver-SMTP must
        compose and send an "undeliverable mail" notification
        message to the originator of the message.  Either a single
        notification which lists all of the recipients that failed
        to get the message, or separate notification messages must
        be sent for each failed recipient (see Example 7).  All
        undeliverable mail notification messages are sent using the
        MAIL command (even if they result from processing a SEND,
        SOML, or SAML command).

[Page 22] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

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

        Example of Return Path and Received Time Stamps

  Return-Path: <@GHI.ARPA,@DEF.ARPA,@ABC.ARPA:JOE@ABC.ARPA>
  Received: from GHI.ARPA by JKL.ARPA ; 27 Oct 81 15:27:39 PST
  Received: from DEF.ARPA by GHI.ARPA ; 27 Oct 81 15:15:13 PST
  Received: from ABC.ARPA by DEF.ARPA ; 27 Oct 81 15:01:59 PST
  Date: 27 Oct 81 15:01:01 PST
  From: JOE@ABC.ARPA
  Subject: Improved Mailing System Installed
  To: SAM@JKL.ARPA

  This is to inform you that ...

                           Example 8

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

     SEND (SEND)

        This command is used to initiate a mail transaction in which
        the mail data is delivered to one or more terminals.  The
        argument field contains a reverse-path.  This command is
        successful if the message is delivered to a terminal.

        The reverse-path consists of an optional list of hosts and
        the sender mailbox.  When the list of hosts is present, it
        is a "reverse" source route 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 a
        source route 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 IPCE to which it is
        relaying the mail rather than the IPCE from which the mail
        came (if they are different).

        This command clears the reverse-path buffer, the
        forward-path buffer, and the mail data buffer; and inserts
        the reverse-path information from this command into the
        reverse-path buffer.

     SEND OR MAIL (SOML)

        This command is used to initiate a mail transaction in which
        the mail data is delivered to one or more terminals or

Postel [Page 23]


August 1982 RFC 821 Simple Mail Transfer Protocol

        mailboxes. For each recipient the mail data is delivered to
        the recipient's terminal if the recipient is active on the
        host (and accepting terminal messages), otherwise to the
        recipient's mailbox.  The argument field contains a
        reverse-path.  This command is successful if the message is
        delivered to a terminal or the mailbox.

        The reverse-path consists of an optional list of hosts and
        the sender mailbox.  When the list of hosts is present, it
        is a "reverse" source route 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 a
        source route 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 IPCE to which it is
        relaying the mail rather than the IPCE from which the mail
        came (if they are different).

        This command clears the reverse-path buffer, the
        forward-path buffer, and the mail data buffer; and inserts
        the reverse-path information from this command into the
        reverse-path buffer.

     SEND AND MAIL (SAML)

        This command is used to initiate a mail transaction in which
        the mail data is delivered to one or more terminals and
        mailboxes. For each recipient the mail data is delivered to
        the recipient's terminal if the recipient is active on the
        host (and accepting terminal messages), and for all
        recipients to the recipient's mailbox.  The argument field
        contains a reverse-path.  This command is successful if the
        message is delivered to the mailbox.

        The reverse-path consists of an optional list of hosts and
        the sender mailbox.  When the list of hosts is present, it
        is a "reverse" source route 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 a
        source route 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 IPCE to which it is
        relaying the mail rather than the IPCE from which the mail
        came (if they are different).

        This command clears the reverse-path buffer, the

[Page 24] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

        forward-path buffer, and the mail data buffer; and inserts
        the reverse-path information from this command into the
        reverse-path buffer.

     RESET (RSET)

        This command specifies that the current mail transaction is
        to be aborted.  Any stored sender, recipients, and mail data
        must be discarded, and all buffers and state tables cleared.
        The receiver must send an OK reply.

     VERIFY (VRFY)

        This command asks the receiver to confirm that the argument
        identifies a user.  If it is a user name, the full name of
        the user (if known) and the fully specified mailbox are
        returned.

        This command has no effect on any of the reverse-path
        buffer, the forward-path buffer, or the mail data buffer.

     EXPAND (EXPN)

        This command asks the receiver to confirm that the argument
        identifies a mailing list, and if so, to return the
        membership of that list.  The full name of the users (if
        known) and the fully specified mailboxes are returned in a
        multiline reply.

        This command has no effect on any of the reverse-path
        buffer, the forward-path buffer, or the mail data buffer.

     HELP (HELP)

        This command causes the receiver to send helpful information
        to the sender of the HELP command.  The command may take an
        argument (e.g., any command name) and return more specific
        information as a response.

        This command has no effect on any of the reverse-path
        buffer, the forward-path buffer, or the mail data buffer.

Postel [Page 25]


August 1982 RFC 821 Simple Mail Transfer Protocol

     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.

        This command has no effect on any of the reverse-path
        buffer, the forward-path buffer, or the mail data buffer.

     QUIT (QUIT)

        This command specifies that the receiver must send an OK
        reply, and then close the transmission channel.

        The receiver should not close the transmission channel until
        it receives and replies to a QUIT command (even if there was
        an error).  The sender should not close the transmission
        channel until it send a QUIT command and receives the reply
        (even if there was an error response to a previous command).
        If the connection is closed prematurely the receiver should
        act as if a RSET command had been received (canceling any
        pending transaction, but not undoing any previously
        completed transaction), the sender should act as if the
        command or transaction in progress had received a temporary
        error (4xx).

     TURN (TURN)

        This command specifies that the receiver must either (1)
        send an OK reply and then take on the role of the
        sender-SMTP, or (2) send a refusal reply and retain the role
        of the receiver-SMTP.

        If program-A is currently the sender-SMTP and it sends the
        TURN command and receives an OK reply (250) then program-A
        becomes the receiver-SMTP.  Program-A is then in the initial
        state as if the transmission channel just opened, and it
        then sends the 220 service ready greeting.

        If program-B is currently the receiver-SMTP and it receives
        the TURN command and sends an OK reply (250) then program-B
        becomes the sender-SMTP.  Program-B is then in the initial
        state as if the transmission channel just opened, and it
        then expects to receive the 220 service ready greeting.

        To refuse to change roles the receiver sends the 502 reply.

[Page 26] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

     There are restrictions on the order in which these command may
     be used.

        The first command in a session must be the HELO command.
        The HELO command may be used later in a session as well.  If
        the HELO command argument is not acceptable a 501 failure
        reply must be returned and the receiver-SMTP must stay in
        the same state.

        The NOOP, HELP, EXPN, and VRFY commands can be used at any
        time during a session.

        The MAIL, SEND, SOML, or SAML commands begin a mail
        transaction.  Once started a mail transaction consists of
        one of the transaction beginning commands, one or more RCPT
        commands, and a DATA command, in that order.  A mail
        transaction may be aborted by the RSET command.  There may
        be zero or more transactions in a session.

        If the transaction beginning command argument is not
        acceptable a 501 failure reply must be returned and the
        receiver-SMTP must stay in the same state.  If the commands
        in a transaction are out of order a 503 failure reply must
        be returned and the receiver-SMTP must stay in the same
        state.

        The last command in a session must be the QUIT command.  The
        QUIT command can not be used at any other time in a session.

  4.1.2.  COMMAND SYNTAX

     The commands consist of a command code followed by an argument
     field.  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 "TO" or "to" for the forward-path.  Command codes and
     the argument fields are separated by one or more spaces.
     However, within the reverse-path and forward-path arguments
     case is important.  In particular, in some hosts the user
     "smith" is different from the user "Smith".

Postel [Page 27]


August 1982 RFC 821 Simple Mail Transfer Protocol

     The argument field consists of a variable length character
     string ending with the character sequence <CRLF>.  The receiver
     is to take no action until this sequence is received.

     Square brackets denote an optional argument field.  If the
     option is not taken, the appropriate default is implied.

[Page 28] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

     The following are the SMTP commands:

        HELO <SP> <domain> <CRLF>

        MAIL <SP> FROM:<reverse-path> <CRLF>

        RCPT <SP> TO:<forward-path> <CRLF>

        DATA <CRLF>

        RSET <CRLF>

        SEND <SP> FROM:<reverse-path> <CRLF>

        SOML <SP> FROM:<reverse-path> <CRLF>

        SAML <SP> FROM:<reverse-path> <CRLF>

        VRFY <SP> <string> <CRLF>

        EXPN <SP> <string> <CRLF>

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

        NOOP <CRLF>

        QUIT <CRLF>

        TURN <CRLF>

Postel [Page 29]


August 1982 RFC 821 Simple 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.

        <reverse-path> ::= <path>

        <forward-path> ::= <path>

        <path> ::= "<" [ <a-d-l> ":" ] <mailbox> ">"

        <a-d-l> ::= <at-domain> | <at-domain> "," <a-d-l>

        <at-domain> ::= "@" <domain>

        <domain> ::=  <element> | <element> "." <domain>

        <element> ::= <name> | "#" <number> | "[" <dotnum> "]"

        <mailbox> ::= <local-part> "@" <domain>

        <local-part> ::= <dot-string> | <quoted-string>

        <name> ::= <a> <ldh-str> <let-dig>

        <ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>

        <let-dig> ::= <a> | <d>

        <let-dig-hyp> ::= <a> | <d> | "-"

        <dot-string> ::= <string> | <string> "." <dot-string>

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

        <quoted-string> ::=  """ <qtext> """

        <qtext> ::=  "\" <x> | "\" <x> <qtext> | <q> | <q> <qtext>

        <char> ::= <c> | "\" <x>

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

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

        <CRLF> ::= <CR> <LF>

[Page 30] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

        <CR> ::= the carriage return character (ASCII code 13)

        <LF> ::= the line feed character (ASCII code 10)

        <SP> ::= the space character (ASCII code 32)

        <snum> ::= one, two, or three digits representing a decimal
                  integer value in the range 0 through 255

        <a> ::= any one of the 52 alphabetic characters A through Z
                  in upper case and a through z in lower case

        <c> ::= any one of the 128 ASCII characters, but not any
                  <special> or <SP>

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

        <q> ::= any one of the 128 ASCII characters except <CR>,
                  <LF>, quote ("), or backslash (\)

        <x> ::= any one of the 128 ASCII characters (no exceptions)

        <special> ::= "<" | ">" | "(" | ")" | "[" | "]" | "\" | "."
                  | "," | ";" | ":" | "@"  """ | the control
                  characters (ASCII codes 0 through 31 inclusive and
                  127)

     Note that the backslash, "\", is a quote character, which is
     used to indicate that the next character is to be used
     literally (instead of its normal interpretation).  For 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.  Note that the name elements of domains
     are the official names -- no use of nicknames or aliases is
     allowed.

     Sometimes a host is not known to the translation function and
     communication is blocked.  To bypass this barrier two 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., "[123.255.37.2]", which indicates a 32-bit ARPA Internet
     Address in four 8-bit fields.

Postel [Page 31]


August 1982 RFC 821 Simple Mail Transfer Protocol

     The time stamp line and the return path line are formally
     defined as follows:

     <return-path-line> ::= "Return-Path:" <SP><reverse-path><CRLF>

     <time-stamp-line> ::= "Received:" <SP> <stamp> <CRLF>

        <stamp> ::= <from-domain> <by-domain> <opt-info> ";"
                  <daytime>

        <from-domain> ::= "FROM" <SP> <domain> <SP>

        <by-domain> ::= "BY" <SP> <domain> <SP>

        <opt-info> ::= [<via>] [<with>] [<id>] [<for>]

        <via> ::= "VIA" <SP> <link> <SP>

        <with> ::= "WITH" <SP> <protocol> <SP>

        <id> ::= "ID" <SP> <string> <SP>

        <for> ::= "FOR" <SP> <path> <SP>

        <link> ::= The standard names for links are registered with
                  the Network Information Center.

        <protocol> ::= The standard names for protocols are
                  registered with the Network Information Center.

        <daytime> ::= <SP> <date> <SP> <time>

        <date> ::= <dd> <SP> <mon> <SP> <yy>

        <time> ::= <hh> ":" <mm> ":" <ss> <SP> <zone>

        <dd> ::= the one or two decimal integer day of the month in
                  the range 1 to 31.

        <mon> ::= "JAN" | "FEB" | "MAR" | "APR" | "MAY" | "JUN" |
                  "JUL" | "AUG" | "SEP" | "OCT" | "NOV" | "DEC"

        <yy> ::= the two decimal integer year of the century in the
                  range 00 to 99.

[Page 32] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

        <hh> ::= the two decimal integer hour of the day in the
                  range 00 to 24.

        <mm> ::= the two decimal integer minute of the hour in the
                  range 00 to 59.

        <ss> ::= the two decimal integer second of the minute in the
                  range 00 to 59.

        <zone> ::= "UT" for Universal Time (the default) or other
                  time zone designator (as in [[2](#ref-2)]).


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

                      Return Path Example

     Return-Path: <@CHARLIE.ARPA,@BAKER.ARPA:JOE@ABLE.ARPA>

                           Example 9

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

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

                    Time Stamp Line Example

  Received: FROM ABC.ARPA BY XYZ.ARPA ; 22 OCT 81 09:23:59 PDT

     Received: from ABC.ARPA by XYZ.ARPA via TELENET with X25
               id M12345 for Smith@PDQ.ARPA ; 22 OCT 81 09:23:59 PDT

                           Example 10

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

Postel [Page 33]


August 1982 RFC 821 Simple Mail Transfer Protocol

4.2. SMTP REPLIES

  Replies to SMTP commands are devised to ensure the synchronization
  of requests and actions in the process of mail transfer, and to
  guarantee that the sender-SMTP always knows the state of the
  receiver-SMTP.  Every command must generate exactly one reply.

     The details of the command-reply sequence are made explicit in
     [Section 5.3](#section-5.3) on Sequencing and [Section 5.4](#section-5.4) State Diagrams.

  An SMTP 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-SMTP need not 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 and context dependent, so there are
  likely to be varying texts for each reply code.  A discussion of
  the theory of reply codes is given in [Appendix E](#appendix-E).  Formally, a
  reply is defined to be the sequence:  a three-digit code, <SP>,
  one line of text, and <CRLF>, or a multiline reply (as defined in
  [Appendix E](#appendix-E)).  Only the EXPN and HELP commands are expected to
  result in multiline replies in normal circumstances, however
  multiline replies are allowed for any command.

[Page 34] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

  4.2.1.  REPLY CODES BY FUNCTION GROUPS

     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]

     220 <domain> Service ready
     221 <domain> Service closing transmission channel
     421 <domain> Service not available,
         closing transmission channel
        [This may be a reply to any command if the service knows it
        must shut down]

     250 Requested mail action okay, completed
     251 User not local; will forward to <forward-path>
     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: error in processing
     551 User not local; please try <forward-path>
     452 Requested action not taken: insufficient system storage
     552 Requested mail action aborted: exceeded storage allocation
     553 Requested action not taken: mailbox name not allowed
        [E.g., mailbox syntax incorrect]
     354 Start mail input; end with <CRLF>.<CRLF>
     554 Transaction failed

Postel [Page 35]


August 1982 RFC 821 Simple Mail Transfer Protocol

  4.2.2.  NUMERIC ORDER LIST OF REPLY CODES

     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]
     220 <domain> Service ready
     221 <domain> Service closing transmission channel
     250 Requested mail action okay, completed
     251 User not local; will forward to <forward-path>

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

     421 <domain> 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]
     551 User not local; please try <forward-path>
     552 Requested mail action aborted: exceeded storage allocation
     553 Requested action not taken: mailbox name not allowed
        [E.g., mailbox syntax incorrect]
     554 Transaction failed

[Page 36] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

4.3. SEQUENCING OF COMMANDS AND REPLIES

  The communication between the sender and receiver is intended to
  be an alternating dialogue, controlled by the sender.  As such,
  the sender issues a command and the receiver responds with a
  reply.  The sender must wait for this response before sending
  further commands.

  One important reply is the connection greeting.  Normally, a
  receiver will send a 220 "Service ready" reply when the connection
  is completed.  The sender should wait for this greeting message
  before sending any commands.

     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.ARPA <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.  The prefixes
     used before the possible replies are "P" for preliminary (not
     used in SMTP), "I" for intermediate, "S" for success, "F" for
     failure, and "E" for error.  The 421 reply (service not
     available, closing transmission channel) may be given to any
     command if the SMTP-receiver knows it must shut down.  This
     listing forms the basis for the State Diagrams in [Section 4.4](#section-4.4).

        CONNECTION ESTABLISHMENT
           S: 220
           F: 421
        HELO
           S: 250
           E: 500, 501, 504, 421
        MAIL
           S: 250
           F: 552, 451, 452
           E: 500, 501, 421

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August 1982 RFC 821 Simple Mail Transfer Protocol

        RCPT
           S: 250, 251
           F: 550, 551, 552, 553, 450, 451, 452
           E: 500, 501, 503, 421
        DATA
           I: 354 -> data -> S: 250
                             F: 552, 554, 451, 452
           F: 451, 554
           E: 500, 501, 503, 421
        RSET
           S: 250
           E: 500, 501, 504, 421
        SEND
           S: 250
           F: 552, 451, 452
           E: 500, 501, 502, 421
        SOML
           S: 250
           F: 552, 451, 452
           E: 500, 501, 502, 421
        SAML
           S: 250
           F: 552, 451, 452
           E: 500, 501, 502, 421
        VRFY
           S: 250, 251
           F: 550, 551, 553
           E: 500, 501, 502, 504, 421
        EXPN
           S: 250
           F: 550
           E: 500, 501, 502, 504, 421
        HELP
           S: 211, 214
           E: 500, 501, 502, 504, 421
        NOOP
           S: 250
           E: 500, 421
        QUIT
           S: 221
           E: 500
        TURN
           S: 250
           F: 502
           E: 500, 503

[Page 38] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

4.4. STATE DIAGRAMS

  Following are state diagrams for a simple-minded SMTP
  implementation.  Only the first digit of the reply codes is used.
  There is one state diagram for each group of SMTP 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 SMTP commands:


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


     This diagram models the commands:

        HELO, MAIL, RCPT, RSET, SEND, SOML, SAML, VRFY, EXPN, HELP,
        NOOP, QUIT, TURN.

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August 1982 RFC 821 Simple Mail Transfer Protocol

  A more complex diagram models the DATA command:


     +---+   DATA    +---+ 1,2                 +---+
     | B |---------->| W |-------------------->| E |
     +---+           +---+        ------------>+---+
                     3| |4,5     |
                      | |        |
        --------------   -----   |
       |                      |  |             +---+
       |               ----------     -------->| S |
       |              |       |      |         +---+
       |              |  ------------
       |              | |     |
       V           1,3| |2    |
     +---+   data    +---+     --------------->+---+
     |   |---------->| W |                     | F |
     +---+           +---+-------------------->+---+
                          4,5


     Note that the "data" here is a series of lines sent from the
     sender to the receiver with no response expected until the last
     line is sent.

[Page 40] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

4.5. DETAILS

  4.5.1.  MINIMUM IMPLEMENTATION

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

        COMMANDS -- HELO
                    MAIL
                    RCPT
                    DATA
                    RSET
                    NOOP
                    QUIT

  4.5.2.  TRANSPARENCY

     Without some provision for data transparency the character
     sequence "<CRLF>.<CRLF>" ends 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-SMTP 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-SMTP
        it checks 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.

     The mail data may contain any of the 128 ASCII characters.  All
     characters are to be delivered to the recipient's mailbox
     including format effectors and other control characters.  If
     the transmission channel provides an 8-bit byte (octets) data
     stream, the 7-bit ASCII codes are transmitted right justified
     in the octets with the high order bits cleared to zero.

        In some systems it may be necessary to transform the data as
        it is received and stored.  This may be necessary for hosts
        that use a different character set than ASCII as their local
        character set, or that store data in records rather than

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August 1982 RFC 821 Simple Mail Transfer Protocol

        strings.  If such transforms are necessary, they must be
        reversible -- especially if such transforms are applied to
        mail being relayed.

  4.5.3.  SIZES

     There are several objects that have required minimum maximum
     sizes.  That is, every implementation must be able to receive
     objects of at least these sizes, but must not send objects
     larger than these sizes.


      ****************************************************
      *                                                  *
      *  TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION  *
      *  TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
      *  OF THESE OBJECTS SHOULD BE USED.                *
      *                                                  *
      ****************************************************

        user

           The maximum total length of a user name is 64 characters.

        domain

           The maximum total length of a domain name or number is 64
           characters.

        path

           The maximum total length of a reverse-path or
           forward-path is 256 characters (including the punctuation
           and element separators).

        command line

           The maximum total length of a command line including the
           command word and the <CRLF> is 512 characters.

        reply line

           The maximum total length of a reply line including the
           reply code and the <CRLF> is 512 characters.

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RFC 821 August 1982 Simple Mail Transfer Protocol

        text line

           The maximum total length of a text line including the
           <CRLF> is 1000 characters (but not counting the leading
           dot duplicated for transparency).

        recipients buffer

           The maximum total number of recipients that must be
           buffered is 100 recipients.


      ****************************************************
      *                                                  *
      *  TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION  *
      *  TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
      *  OF THESE OBJECTS SHOULD BE USED.                *
      *                                                  *
      ****************************************************

     Errors due to exceeding these limits may be reported by using
     the reply codes, for example:

        500 Line too long.

        501 Path too long

        552 Too many recipients.

        552 Too much mail data.

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August 1982 RFC 821 Simple Mail Transfer Protocol

APPENDIX A

TCP Transport service

  The Transmission Control Protocol [[3](#ref-3)] is used in the ARPA
  Internet, and in any network following the US DoD standards for
  internetwork protocols.

  Connection Establishment

     The SMTP 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 25 (31 octal), that is L=25.

  Data Transfer

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

[Page 44] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

APPENDIX B

NCP Transport service

  The ARPANET Host-to-Host Protocol [[4](#ref-4)] (implemented by the Network
  Control Program) may be used in the ARPANET.

  Connection Establishment

     The SMTP transmission channel is established via NCP between
     the sender process socket U and receiver process socket L.  The
     Initial Connection Protocol [[5](#ref-5)] 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 25 (31 octal), that is L=25.

  Data Transfer

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

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August 1982 RFC 821 Simple Mail Transfer Protocol

APPENDIX C

NITS

  The Network Independent Transport Service [[6](#ref-6)] may be used.

  Connection Establishment

     The SMTP transmission channel is established via NITS between
     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 SMTP data is 7-bit ASCII characters.  Each character is
     transmitted as an 8-bit byte with the high-order bit cleared to
     zero.

[Page 46] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

APPENDIX D

X.25 Transport service

  It may be possible to use the X.25 service [[7](#ref-7)] as provided by the
  Public Data Networks directly, however, 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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August 1982 RFC 821 Simple 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-SMTP will be able to
  determine its next action (proceed as planned, redo, retrench,
  etc.) by simply examining this first digit.  A sender-SMTP 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-SMTP should send
           another command specifying whether to continue or abort
           the action.

              [Note: SMTP does not have any commands that allow this
              type of reply, and so does not have the continue or
              abort commands.]

        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-SMTP 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

[Page 48] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

           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- SMTPs) must
           agree on the interpretation.  Each reply in this category
           might have a different time value, but the sender-SMTP 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
           and 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-SMTP 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-SMTP 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

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August 1982 RFC 821 Simple Mail Transfer Protocol

     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.
     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-SMTP any new information will return
     a 250 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-SMTP knows when it
  can stop reading the reply.  This requires a special format to
  indicate a multiple line reply.

     The format for multiline 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

     In many cases the sender-SMTP then simply needs to search for
     the reply code followed by <SP> at the beginning of a line, and
     ignore all preceding lines.  In a few cases, there is important
     data for the sender in the reply "text".  The sender will know
     these cases from the current context.

[Page 50] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

APPENDIX F

Scenarios

  This section presents complete scenarios of several types of SMTP
  sessions.

A Typical SMTP Transaction Scenario

  This SMTP example shows mail sent by Smith at host USC-ISIF, to
  Jones, Green, and Brown at host BBN-UNIX.  Here we assume that
  host USC-ISIF contacts host BBN-UNIX directly.  The mail is
  accepted for Jones and Brown.  Green does not have a mailbox at
  host BBN-UNIX.

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

     R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
     S: HELO USC-ISIF.ARPA
     R: 250 BBN-UNIX.ARPA

     S: MAIL FROM:<Smith@USC-ISIF.ARPA>
     R: 250 OK

     S: RCPT TO:<Jones@BBN-UNIX.ARPA>
     R: 250 OK

     S: RCPT TO:<Green@BBN-UNIX.ARPA>
     R: 550 No such user here

     S: RCPT TO:<Brown@BBN-UNIX.ARPA>
     R: 250 OK

     S: DATA
     R: 354 Start mail input; end with <CRLF>.<CRLF>
     S: Blah blah blah...
     S: ...etc. etc. etc.
     S: .
     R: 250 OK

     S: QUIT
     R: 221 BBN-UNIX.ARPA Service closing transmission channel

                           Scenario 1

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

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August 1982 RFC 821 Simple Mail Transfer Protocol

Aborted SMTP Transaction Scenario

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

     R: 220 MIT-Multics.ARPA Simple Mail Transfer Service Ready
     S: HELO ISI-VAXA.ARPA
     R: 250 MIT-Multics.ARPA

     S: MAIL FROM:<Smith@ISI-VAXA.ARPA>
     R: 250 OK

     S: RCPT TO:<Jones@MIT-Multics.ARPA>
     R: 250 OK

     S: RCPT TO:<Green@MIT-Multics.ARPA>
     R: 550 No such user here

     S: RSET
     R: 250 OK

     S: QUIT
     R: 221 MIT-Multics.ARPA Service closing transmission channel

                           Scenario 2

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

[Page 52] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

Relayed Mail Scenario

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

     Step 1  --  Source Host to Relay Host

        R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
        S: HELO MIT-AI.ARPA
        R: 250 USC-ISIE.ARPA

        S: MAIL FROM:<JQP@MIT-AI.ARPA>
        R: 250 OK

        S: RCPT TO:<@USC-ISIE.ARPA:Jones@BBN-VAX.ARPA>
        R: 250 OK

        S: DATA
        R: 354 Start mail input; end with <CRLF>.<CRLF>
        S: Date: 2 Nov 81 22:33:44
        S: From: John Q. Public <JQP@MIT-AI.ARPA>
        S: Subject:  The Next Meeting of the Board
        S: To: Jones@BBN-Vax.ARPA
        S:
        S: Bill:
        S: The next meeting of the board of directors will be
        S: on Tuesday.
        S:                                              John.
        S: .
        R: 250 OK

        S: QUIT
        R: 221 USC-ISIE.ARPA Service closing transmission channel

Postel [Page 53]


August 1982 RFC 821 Simple Mail Transfer Protocol

     Step 2  --  Relay Host to Destination Host

        R: 220 BBN-VAX.ARPA Simple Mail Transfer Service Ready
        S: HELO USC-ISIE.ARPA
        R: 250 BBN-VAX.ARPA

        S: MAIL FROM:<@USC-ISIE.ARPA:JQP@MIT-AI.ARPA>
        R: 250 OK

        S: RCPT TO:<Jones@BBN-VAX.ARPA>
        R: 250 OK

        S: DATA
        R: 354 Start mail input; end with <CRLF>.<CRLF>
        S: Received: from MIT-AI.ARPA by USC-ISIE.ARPA ;
           2 Nov 81 22:40:10 UT
        S: Date: 2 Nov 81 22:33:44
        S: From: John Q. Public <JQP@MIT-AI.ARPA>
        S: Subject:  The Next Meeting of the Board
        S: To: Jones@BBN-Vax.ARPA
        S:
        S: Bill:
        S: The next meeting of the board of directors will be
        S: on Tuesday.
        S:                                              John.
        S: .
        R: 250 OK

        S: QUIT
        R: 221 USC-ISIE.ARPA Service closing transmission channel

                           Scenario 3

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

[Page 54] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

Verifying and Sending Scenario

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

     R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
     S: HELO MIT-MC.ARPA
     R: 250 SU-SCORE.ARPA

     S: VRFY Crispin
     R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>

     S: SEND FROM:<EAK@MIT-MC.ARPA>
     R: 250 OK

     S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
     R: 250 OK

     S: DATA
     R: 354 Start mail input; end with <CRLF>.<CRLF>
     S: Blah blah blah...
     S: ...etc. etc. etc.
     S: .
     R: 250 OK

     S: QUIT
     R: 221 SU-SCORE.ARPA Service closing transmission channel

                           Scenario 4

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

Postel [Page 55]


August 1982 RFC 821 Simple Mail Transfer Protocol

Sending and Mailing Scenarios

  First the user's name is verified, then  an attempt is made to
  send to the user's terminal.  When that fails, the messages is
  mailed to the user's mailbox.

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

     R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
     S: HELO MIT-MC.ARPA
     R: 250 SU-SCORE.ARPA

     S: VRFY Crispin
     R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>

     S: SEND FROM:<EAK@MIT-MC.ARPA>
     R: 250 OK

     S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
     R: 450 User not active now

     S: RSET
     R: 250 OK

     S: MAIL FROM:<EAK@MIT-MC.ARPA>
     R: 250 OK

     S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
     R: 250 OK

     S: DATA
     R: 354 Start mail input; end with <CRLF>.<CRLF>
     S: Blah blah blah...
     S: ...etc. etc. etc.
     S: .
     R: 250 OK

     S: QUIT
     R: 221 SU-SCORE.ARPA Service closing transmission channel

                           Scenario 5

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

[Page 56] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

  Doing the preceding scenario more efficiently.

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

     R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
     S: HELO MIT-MC.ARPA
     R: 250 SU-SCORE.ARPA

     S: VRFY Crispin
     R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>

     S: SOML FROM:<EAK@MIT-MC.ARPA>
     R: 250 OK

     S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
     R: 250 User not active now, so will do mail.

     S: DATA
     R: 354 Start mail input; end with <CRLF>.<CRLF>
     S: Blah blah blah...
     S: ...etc. etc. etc.
     S: .
     R: 250 OK

     S: QUIT
     R: 221 SU-SCORE.ARPA Service closing transmission channel

                           Scenario 6

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

Postel [Page 57]


August 1982 RFC 821 Simple Mail Transfer Protocol

Mailing List Scenario

  First each of two mailing lists are expanded in separate sessions
  with different hosts.  Then the message is sent to everyone that
  appeared on either list (but no duplicates) via a relay host.

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

     Step 1  --  Expanding the First List

        R: 220 MIT-AI.ARPA Simple Mail Transfer Service Ready
        S: HELO SU-SCORE.ARPA
        R: 250 MIT-AI.ARPA

        S: EXPN Example-People
        R: 250-<ABC@MIT-MC.ARPA>
        R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA>
        R: 250-Xenon Y. Zither <XYZ@MIT-AI.ARPA>
        R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
        R: 250-<joe@foo-unix.ARPA>
        R: 250 <xyz@bar-unix.ARPA>

        S: QUIT
        R: 221 MIT-AI.ARPA Service closing transmission channel

[Page 58] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

     Step 2  --  Expanding the Second List

        R: 220 MIT-MC.ARPA Simple Mail Transfer Service Ready
        S: HELO SU-SCORE.ARPA
        R: 250 MIT-MC.ARPA

        S: EXPN Interested-Parties
        R: 250-Al Calico <ABC@MIT-MC.ARPA>
        R: 250-<XYZ@MIT-AI.ARPA>
        R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
        R: 250-<fred@BBN-UNIX.ARPA>
        R: 250 <xyz@bar-unix.ARPA>

        S: QUIT
        R: 221 MIT-MC.ARPA Service closing transmission channel

Postel [Page 59]


August 1982 RFC 821 Simple Mail Transfer Protocol

     Step 3  --  Mailing to All via a Relay Host

        R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
        S: HELO SU-SCORE.ARPA
        R: 250 USC-ISIE.ARPA

        S: MAIL FROM:<Account.Person@SU-SCORE.ARPA>
        R: 250 OK
        S: RCPT TO:<@USC-ISIE.ARPA:ABC@MIT-MC.ARPA>
        R: 250 OK
        S: RCPT TO:<@USC-ISIE.ARPA:Fonebone@USC-ISIQA.ARPA>
        R: 250 OK
        S: RCPT TO:<@USC-ISIE.ARPA:XYZ@MIT-AI.ARPA>
        R: 250 OK
        S: RCPT
            TO:<@USC-ISIE.ARPA,@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
        R: 250 OK
        S: RCPT TO:<@USC-ISIE.ARPA:joe@FOO-UNIX.ARPA>
        R: 250 OK
        S: RCPT TO:<@USC-ISIE.ARPA:xyz@BAR-UNIX.ARPA>
        R: 250 OK
        S: RCPT TO:<@USC-ISIE.ARPA:fred@BBN-UNIX.ARPA>
        R: 250 OK

        S: DATA
        R: 354 Start mail input; end with <CRLF>.<CRLF>
        S: Blah blah blah...
        S: ...etc. etc. etc.
        S: .
        R: 250 OK

        S: QUIT
        R: 221 USC-ISIE.ARPA Service closing transmission channel

                           Scenario 7

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

[Page 60] Postel


RFC 821 August 1982 Simple Mail Transfer Protocol

Forwarding Scenarios

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

     R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready
     S: HELO LBL-UNIX.ARPA
     R: 250 USC-ISIF.ARPA

     S: MAIL FROM:<mo@LBL-UNIX.ARPA>
     R: 250 OK

     S: RCPT TO:<fred@USC-ISIF.ARPA>
     R: 251 User not local; will forward to <Jones@USC-ISI.ARPA>

     S: DATA
     R: 354 Start mail input; end with <CRLF>.<CRLF>
     S: Blah blah blah...
     S: ...etc. etc. etc.
     S: .
     R: 250 OK

     S: QUIT
     R: 221 USC-ISIF.ARPA Service closing transmission channel

                           Scenario 8

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

Postel [Page 61]


August 1982 RFC 821 Simple Mail Transfer Protocol

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

     Step 1  --  Trying the Mailbox at the First Host

        R: 220 USC-ISIF.ARPA Simple Mail Transfer Service Ready
        S: HELO LBL-UNIX.ARPA
        R: 250 USC-ISIF.ARPA

        S: MAIL FROM:<mo@LBL-UNIX.ARPA>
        R: 250 OK

        S: RCPT TO:<fred@USC-ISIF.ARPA>
        R: 251 User not local; will forward to <Jones@USC-ISI.ARPA>

        S: RSET
        R: 250 OK

        S: QUIT
        R: 221 USC-ISIF.ARPA Service closing transmission channel

     Step 2  --  Delivering the Mail at the Second Host

        R: 220 USC-ISI.ARPA Simple Mail Transfer Service Ready
        S: HELO LBL-UNIX.ARPA
        R: 250 USC-ISI.ARPA

        S: MAIL FROM:<mo@LBL-UNIX.ARPA>
        R: 250 OK

        S: RCPT TO:<Jones@USC-ISI.ARPA>
        R: OK

        S: DATA
        R: 354 Start mail input; end with <CRLF>.<CRLF>
        S: Blah blah blah...
        S: ...etc. etc. etc.
        S: .
        R: 250 OK

        S: QUIT
        R: 221 USC-ISI.ARPA Service closing transmission channel

                           Scenario 9

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

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RFC 821 August 1982 Simple Mail Transfer Protocol

Too Many Recipients Scenario

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

     R: 220 BERKELEY.ARPA Simple Mail Transfer Service Ready
     S: HELO USC-ISIF.ARPA
     R: 250 BERKELEY.ARPA

     S: MAIL FROM:<Postel@USC-ISIF.ARPA>
     R: 250 OK

     S: RCPT TO:<fabry@BERKELEY.ARPA>
     R: 250 OK

     S: RCPT TO:<eric@BERKELEY.ARPA>
     R: 552 Recipient storage full, try again in another transaction

     S: DATA
     R: 354 Start mail input; end with <CRLF>.<CRLF>
     S: Blah blah blah...
     S: ...etc. etc. etc.
     S: .
     R: 250 OK

     S: MAIL FROM:<Postel@USC-ISIF.ARPA>
     R: 250 OK

     S: RCPT TO:<eric@BERKELEY.ARPA>
     R: 250 OK

     S: DATA
     R: 354 Start mail input; end with <CRLF>.<CRLF>
     S: Blah blah blah...
     S: ...etc. etc. etc.
     S: .
     R: 250 OK

     S: QUIT
     R: 221 BERKELEY.ARPA Service closing transmission channel

                          Scenario 10

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

  Note that a real implementation must handle many recipients as
  specified in [Section 4.5.3](#section-4.5.3).

Postel [Page 63]


August 1982 RFC 821 Simple Mail Transfer Protocol

GLOSSARY

ASCII

  American Standard Code for Information Interchange [[1](#ref-1)].

command

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

domain

  The hierarchially structured global character string address of a
  host computer in the mail system.

end of mail data indication

  A special sequence of characters that indicates the end of the
  mail data.  In particular, the five characters carriage return,
  line feed, period, carriage return, line feed, in that order.

host

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

line

  A a sequence of ASCII characters ending with a <CRLF>.

mail data

  A sequence of ASCII characters of arbitrary length, which conforms
  to the standard set in the Standard for the Format of ARPA
  Internet Text Messages ([RFC 822](./rfc822) [[2](#ref-2)]).

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@domain".  Additionally, the "container" in which mail
  is stored.

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RFC 821 August 1982 Simple Mail Transfer Protocol

receiver-SMTP process

  A process which transfers mail in cooperation with a sender-SMTP
  process.  It waits for a connection to be established via the
  transport service.  It receives SMTP commands from the
  sender-SMTP, sends replies, and performs the specified operations.

reply

  A reply is an acknowledgment (positive or negative) sent from
  receiver to sender via the transmission channel in response to a
  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-SMTP process

  A process which transfers mail in cooperation with a receiver-SMTP
  process.  A local language may be used in the user interface
  command/reply dialogue.  The sender-SMTP initiates the transport
  service connection.  It initiates SMTP commands, receives replies,
  and governs the transfer of mail.

session

  The set of exchanges that occur while the transmission channel is
  open.

transaction

  The set of exchanges required for one message to be transmitted
  for one or more recipients.

transmission channel

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

transport service

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

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August 1982 RFC 821 Simple Mail Transfer Protocol

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 sequence of printing characters.
  The characters carriage return and line feed (in that order).
  The space character.

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RFC 821 August 1982 Simple Mail Transfer Protocol

REFERENCES

[1] 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.

[2] RFC 822

  Crocker, D., "Standard for the Format of ARPA Internet Text
  Messages," [RFC 822](./rfc822), Department of Electrical Engineering,
  University of Delaware, August 1982.

[3] TCP

  Postel, J., ed., "Transmission Control Protocol - DARPA Internet
  Program Protocol Specification", [RFC 793](./rfc793), USC/Information Sciences
  Institute, NTIS AD Number A111091, September 1981.  Also in:
  Feinler, E. and J. Postel, eds., "Internet Protocol Transition
  Workbook", SRI International, Menlo Park, California, March 1982.

[4] 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.

[5] 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.

[6] 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.

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August 1982 RFC 821 Simple Mail Transfer Protocol

[7] 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 68] Postel