rfc821_smtp.txt

邮件编码格式说明。

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   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
      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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            Example Undeliverable Mail Notification Message

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

                               Example 7

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   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.


   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.




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.

         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.


         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.

               For example, mail received at relay host A with arguments

                  FROM:<[e-mail]>
                  TO:<@HOSTA.ARPA,@HOSTB.ARPA:[e-mail]>

               will be relayed on to host B with arguments

                  FROM:<@HOSTA.ARPA:[e-mail]>
                  TO:<@HOSTB.ARPA:[e-mail]>.

            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 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
            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].  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