rfc821_smtp.txt

邮件编码格式说明。

            <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>

            <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.

         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.

            <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]).

            

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

                          Return Path Example

         Return-Path: <@CHARLIE.ARPA,@BAKER.ARPA:[e-mail]>

                               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 [e-mail] ; 22 OCT 81 09:23:59 PDT

                               Example 10

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


   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 on Sequencing and 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.  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).  Only the EXPN and HELP commands are expected to
      result in multiline replies in normal circumstances, however
      multiline replies are allowed for any command.

      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
         

      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
         


   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.

            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
            RCPT
               S: 250, 251
               F: 550, 551, 552, 553, 450, 451, 452
               E: 500, 501, 503, 421