4.2 SMTP Replies

   Replies to SMTP commands serve to ensure the synchronization of
   requests and actions in the process of mail transfer and to guarantee
   that the SMTP client always knows the state of the SMTP server.
   Every command MUST generate exactly one reply.

   The details of the command-reply sequence are described in section
   4.3.

   An SMTP reply consists of a three digit number (transmitted as three
   numeric characters) followed by some text unless specified otherwise
   in this document.  The number is for use by automata to determine
   what state to enter next; the text is for the human user.  The three
   digits contain enough encoded information that the SMTP client need
   not examine the text and may either discard it or pass it on to the
   user, as appropriate.  Exceptions are as noted elsewhere in this
   document.  In particular, the 220, 221, 251, 421, and 551 reply codes
   are associated with message text that must be parsed and interpreted
   by machines.  In the general case, 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 section 4.2.1.  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 section 4.2.1).  Since, in violation of this
   specification, the text is sometimes not sent, clients which do not
   receive it SHOULD be prepared to process the code alone (with or
   without a trailing space character).  Only the EHLO, EXPN, and HELP
   commands are expected to result in multiline replies in normal
   circumstances, however, multiline replies are allowed for any
   command.

   In ABNF, server responses are:

      Greeting = "220 " Domain [ SP text ] CRLF
      Reply-line = Reply-code [ SP text ] CRLF

   where "Greeting" appears only in the 220 response that announces that
   the server is opening its part of the connection.

   An SMTP server SHOULD send only the reply codes listed in this
   document.  An SMTP server SHOULD use the text shown in the examples
   whenever appropriate.

   An SMTP client MUST determine its actions only by the reply code, not
   by the text (except for the "change of address" 251 and 551 and, if
   necessary, 220, 221, and 421 replies); in the general case, any text,
   including no text at all (although senders SHOULD NOT send bare
   codes), MUST be acceptable.  The space (blank) following the reply
   code is considered part of the text.  Whenever possible, a receiver-
   SMTP SHOULD test the first digit (severity indication) of the reply
   code.

   The list of codes that appears below MUST NOT be construed as
   permanent.  While the addition of new codes should be a rare and
   significant activity, with supplemental information in the textual
   part of the response being preferred, new codes may be added as the
   result of new Standards or Standards-track specifications.
   Consequently, a sender-SMTP MUST be prepared to handle codes not
   specified in this document and MUST do so by interpreting the first
   digit only.

4.2.1 Reply Code Severities and Theory

   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 SMTP client, or one that receives an unexpected
   code, will be able to determine its next action (proceed as planned,
   redo, retrench, etc.) by examining this first digit.  An SMTP client
   that wants to know approximately what kind of error occurred (e.g.,
   mail system error, command syntax error) may examine the second
   digit.  The third digit and any supplemental information that may be
   present is reserved 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 SMTP client should send another command specifying
      whether to continue or abort the action.  Note: unextended SMTP
      does not have any commands that allow this type of reply, and so
      does not have 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
      SMTP client should send another command specifying this
      information.  This reply is used in command sequence groups (i.e.,
      in DATA).

   4yz   Transient Negative Completion reply
      The command was not accepted, and the requested action did not
      occur.  However, the error condition is temporary and the action
      may be requested again.  The sender should return to the beginning
      of the command sequence (if any).  It is difficult to assign a
      meaning to "transient" when two different sites (receiver- and

      sender-SMTP agents) must agree on the interpretation.  Each reply
      in this category might have a different time value, but the SMTP
      client is encouraged to try again.  A rule of thumb to determine
      whether a reply fits into the 4yz or the 5yz category (see below)
      is that replies are 4yz if they can be successful if repeated
      without any change in command form or in properties of the sender
      or receiver (that is, 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 SMTP client 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 SMTP client 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 do not 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.

   x4z   Unspecified.

   x5z   Mail system: These replies indicate the status of the receiver
      mail system vis-a-vis the requested transfer or other mail system
      action.

   The third digit gives a finer gradation of meaning in each category
   specified by the second digit.  The list of replies illustrates this.
   Each reply text is recommended rather than mandatory, and may even
   change according to the command with which it is associated.  On the
   other hand, the reply codes must strictly follow the specifications
   in this section.  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 SMTP client any new information, will return a 250
   reply.  The reply 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 SMTP client 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>.  As noted above, servers SHOULD send the <SP>
   if subsequent text is not sent, but clients MUST be prepared for it
   to be omitted.

   For example:

      123-First line
      123-Second line
      123-234 text beginning with numbers
      123 The last line

   In many cases the SMTP client then simply needs to search for a line
   beginning with the reply code followed by <SP> or <CRLF> and ignore
   all preceding lines.  In a few cases, there is important data for the
   client in the reply "text".  The client will be able to identify
   these cases from the current context.

4.2.2 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  (see section 4.2.4)
      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>
         (See section 3.4)
      252 Cannot VRFY user, but will accept message and attempt
          delivery
         (See section 3.5.3)
      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, or command rejected
         for policy reasons)
      451 Requested action aborted: error in processing
      551 User not local; please try <forward-path>
         (See section 3.4)
      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 (Or, in the case of a connection-opening
          response, "No SMTP service here")

4.2.3  Reply Codes in Numeric Order

      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>
         (See section 3.4)
      252 Cannot VRFY user, but will accept message and attempt
         delivery
         (See section 3.5.3)

      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 (see section 4.2.4)
      503 Bad sequence of commands
      504 Command parameter not implemented
      550 Requested action not taken: mailbox unavailable
         (e.g., mailbox not found, no access, or command rejected
         for policy reasons)
      551 User not local; please try <forward-path>
         (See section 3.4)
      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  (Or, in the case of a connection-opening
          response, "No SMTP service here")

4.2.4 Reply Code 502

   Questions have been raised as to when reply code 502 (Command not
   implemented) SHOULD be returned in preference to other codes.  502
   SHOULD be used when the command is actually recognized by the SMTP
   server, but not implemented.  If the command is not recognized, code
   500 SHOULD be returned.  Extended SMTP systems MUST NOT list
   capabilities in response to EHLO for which they will return 502 (or
   500) replies.

4.2.5 Reply Codes After DATA and the Subsequent <CRLF>.<CRLF>

   When an SMTP server returns a positive completion status (2yz code)
   after the DATA command is completed with <CRLF>.<CRLF>, it accepts
   responsibility for:

   -  delivering the message (if the recipient mailbox exists), or

   -  if attempts to deliver the message fail due to transient
      conditions, retrying delivery some reasonable number of times at
      intervals as specified in section 4.5.4.

   -  if attempts to deliver the message fail due to permanent
      conditions, or if repeated attempts to deliver the message fail
      due to transient conditions, returning appropriate notification to
      the sender of the original message (using the address in the SMTP
      MAIL command).

   When an SMTP server returns a permanent error status (5yz) code after
   the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make
   any subsequent attempt to deliver that message.  The SMTP client
   retains responsibility for delivery of that message and may either
   return it to the user or requeue it for a subsequent attempt (see
   section 4.5.4.1).

   The user who originated the message SHOULD be able to interpret the
   return of a transient failure status (by mail message or otherwise)
   as a non-delivery indication, just as a permanent failure would be
   interpreted.  I.e., if the client SMTP successfully handles these
   conditions, the user will not receive such a reply.

   When an SMTP server returns a permanent error status (5yz) code after
   the DATA command is completely with <CRLF>.<CRLF>, it MUST NOT make
   any subsequent attempt to deliver the message.  As with temporary
   error status codes, the SMTP client retains responsibility for the
   message, but SHOULD not again attempt delivery to the same server
   without user review and intervention of the message.

4.3 Sequencing of Commands and Replies

4.3.1 Sequencing Overview

   The communication between the sender and receiver is an alternating
   dialogue, controlled by the sender.  As such, the sender issues a
   command and the receiver responds with a reply.  Unless other
   arrangements are negotiated through service extensions, 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 (the
   fully-qualified primary domain name) of the server host as the first
   word following the reply code.  Sometimes the host will have no
   meaningful name.  See 4.1.3 for a discussion of alternatives in these
   situations.

   For example,

      220 ISIF.USC.EDU Service ready
   or
      220 mail.foo.com SuperSMTP v 6.1.2 Service ready
   or
      220 [10.0.0.1] Clueless host service ready

   The table below lists alternative success and failure replies for
   each command.  These SHOULD 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.

4.3.2 Command-Reply Sequences

   Each command is listed with its usual possible replies.  The prefixes
   used before the possible replies are "I" for intermediate, "S" for
   success, and "E" for error.  Since some servers may generate other
   replies under special circumstances, and to allow for future
   extension, SMTP clients SHOULD, when possible, interpret only the
   first digit of the reply and MUST be prepared to deal with
   unrecognized reply codes by interpreting the first digit only.
   Unless extended using the mechanisms described in section 2.2, SMTP
   servers MUST NOT transmit reply codes to an SMTP client that are
   other than three digits or that do not start in a digit between 2 and
   5 inclusive.

   These sequencing rules and, in principle, the codes themselves, can
   be extended or modified by SMTP extensions offered by the server and
   accepted (requested) by the client.

   In addition to the codes listed below, any SMTP command can return
   any of the following codes if the corresponding unusual circumstances
   are encountered:

   500  For the "command line too long" case or if the command name was
      not recognized.  Note that producing a "command not recognized"
      error in response to the required subset of these commands is a
      violation of this specification.

   501  Syntax error in command or arguments.  In order to provide for
      future extensions, commands that are specified in this document as
      not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501
      message if arguments are supplied in the absence of EHLO-
      advertised extensions.

   421  Service shutting down and closing transmission channel

   Specific sequences are:

   CONNECTION ESTABLISHMENT
      S: 220
      E: 554
   EHLO or HELO
      S: 250
      E: 504, 550
   MAIL
      S: 250
      E: 552, 451, 452, 550, 553, 503
   RCPT
      S: 250, 251 (but see section 3.4 for discussion of 251 and 551)
      E: 550, 551, 552, 553, 450, 451, 452, 503, 550
   DATA
      I: 354 -> data -> S: 250
                        E: 552, 554, 451, 452
      E: 451, 554, 503
   RSET
      S: 250
   VRFY
      S: 250, 251, 252
      E: 550, 551, 553, 502, 504
   EXPN
      S: 250, 252
      E: 550, 500, 502, 504
   HELP
      S: 211, 214
      E: 502, 504
   NOOP
      S: 250
   QUIT
      S: 221

4.4 Trace Information

   When an SMTP server receives a message for delivery or further
   processing, it MUST insert trace ("time stamp" or "Received")
   information at the beginning of the message content, as discussed in
   section 4.1.1.4.

   This line MUST be structured as follows:

   -  The FROM field, which MUST be supplied in an SMTP environment,
      SHOULD contain both (1) the name of the source host as presented
      in the EHLO command and (2) an address literal containing the IP
      address of the source, determined from the TCP connection.

   -  The ID field MAY contain an "@" as suggested in RFC 822, but this
      is not required.

   -  The FOR field MAY contain a list of <path> entries when multiple
      RCPT commands have been given.  This may raise some security
      issues and is usually not desirable; see section 7.2.

   An Internet mail program MUST NOT change a Received: line that was
   previously added to the message header.  SMTP servers MUST prepend
   Received lines to messages; they MUST NOT change the order of
   existing lines or insert Received lines in any other location.

   As the Internet grows, comparability of Received fields is important
   for detecting problems, especially slow relays.  SMTP servers that
   create Received fields SHOULD use explicit offsets in the dates
   (e.g., -0800), rather than time zone names of any type.  Local time
   (with an offset) is preferred to UT when feasible.  This formulation
   allows slightly more information about local circumstances to be
   specified.  If UT is needed, the receiver need merely do some simple
   arithmetic to convert the values.  Use of UT loses information about
   the time zone-location of the server.  If it is desired to supply a
   time zone name, it SHOULD be included in a comment.

   When the delivery SMTP server makes the "final delivery" of a
   message, it inserts a return-path line at the beginning of the mail
   data.  This use of return-path is required; mail systems MUST support
   it.  The return-path line preserves the information in the <reverse-
   path> from the MAIL command.  Here, final delivery means the message
   has left the SMTP environment.  Normally, this would mean it had been
   delivered to the destination user or an associated mail drop, 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 to be different
   from the actual sender's mailbox, for example, if error responses are
   to be delivered to a special error handling mailbox rather than to
   the message sender.  When mailing lists are involved, this
   arrangement is common and useful as a means of directing errors to
   the list maintainer rather than the message originator.

   The text above implies 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 headers and body [32].

   It is sometimes difficult for an SMTP server to determine whether or
   not it is making final delivery since forwarding or other operations
   may occur after the message is accepted for delivery.  Consequently,

   any further (forwarding, gateway, or relay) systems MAY remove the
   return path and rebuild the MAIL command as needed to ensure that
   exactly one such line appears in a delivered message.

   A message-originating SMTP system SHOULD NOT send a message that
   already contains a Return-path header.  SMTP servers performing a
   relay function MUST NOT inspect the message data, and especially not
   to the extent needed to determine if Return-path headers are present.
   SMTP servers making final delivery MAY remove Return-path headers
   before adding their own.

   The primary purpose of the Return-path is to designate the address to
   which messages indicating non-delivery or other mail system failures
   are to be sent.  For this to be unambiguous, exactly one return path
   SHOULD be present when the message is delivered.  Systems using RFC
   822 syntax with non-SMTP transports SHOULD designate an unambiguous
   address, associated with the transport envelope, to which error
   reports (e.g., non-delivery messages) should be sent.

   Historical note: Text in RFC 822 that appears to contradict the use
   of the Return-path header (or the envelope reverse path address from
   the MAIL command) as the destination for error messages is not
   applicable on the Internet.  The reverse path address (as copied into
   the Return-path) MUST be used as the target of any mail containing
   delivery error messages.

   In particular:

   -  a gateway from SMTP->elsewhere SHOULD insert a return-path header,
      unless it is known that the "elsewhere" transport also uses
      Internet domain addresses and maintains the envelope sender
      address separately.

   -  a gateway from elsewhere->SMTP SHOULD delete any return-path
      header present in the message, and either copy that information to
      the SMTP envelope or combine it with information present in the
      envelope of the other transport system to construct the reverse
      path argument to the MAIL command in the SMTP envelope.

   The server must give special treatment to cases in which the
   processing following the end of mail data indication is only
   partially successful.  This could happen if, after accepting several
   recipients and the mail data, the SMTP server finds that the mail
   data could be successfully delivered to some, but not all, of the
   recipients.  In such cases, the response to the DATA command MUST be
   an OK reply.  However, the SMTP server MUST compose and send an
   "undeliverable mail" notification message to the originator of the
   message.

   A single notification listing all of the failed recipients or
   separate notification messages MUST be sent for each failed
   recipient.  For economy of processing by the sender, the former is
   preferred when possible.  All undeliverable mail notification
   messages are sent using the MAIL command (even if they result from
   processing the obsolete SEND, SOML, or SAML commands) and use a null
   return path as discussed in section 3.7.

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

Return-path-line = "Return-Path:" FWS Reverse-path <CRLF>

Time-stamp-line = "Received:" FWS Stamp <CRLF>

Stamp = From-domain By-domain Opt-info ";"  FWS date-time

      ; where "date-time" is as defined in [32]
      ; but the "obs-" forms, especially two-digit
      ; years, are prohibited in SMTP and MUST NOT be used.

From-domain = "FROM" FWS Extended-Domain CFWS

By-domain = "BY" FWS Extended-Domain CFWS

Extended-Domain = Domain /
           ( Domain FWS "(" TCP-info ")" ) /
           ( Address-literal FWS "(" TCP-info ")" )

TCP-info = Address-literal / ( Domain FWS Address-literal )
      ; Information derived by server from TCP connection
      ; not client EHLO.

Opt-info = [Via] [With] [ID] [For]

Via = "VIA" FWS Link CFWS

With = "WITH" FWS Protocol CFWS

ID = "ID" FWS String / msg-id CFWS

For = "FOR" FWS 1*( Path / Mailbox ) CFWS

Link = "TCP" / Addtl-Link
Addtl-Link = Atom
      ; Additional standard names for links are registered with the
         ; Internet Assigned Numbers Authority (IANA).  "Via" is
         ; primarily of value with non-Internet transports.  SMTP

         ; servers SHOULD NOT use unregistered names.
Protocol = "ESMTP" / "SMTP" / Attdl-Protocol
Attdl-Protocol = Atom
      ; Additional standard names for protocols are registered with the
         ; Internet Assigned Numbers Authority (IANA).  SMTP servers
         ; SHOULD NOT use unregistered names.

4.5 Additional Implementation Issues

4.5.1 Minimum Implementation

   In order to make SMTP workable, the following minimum implementation
   is required for all receivers.  The following commands MUST be
   supported to conform to this specification:

      EHLO
      HELO
      MAIL
      RCPT
      DATA
      RSET
      NOOP
      QUIT
      VRFY

   Any system that includes an SMTP server supporting mail relaying or
   delivery MUST support the reserved mailbox "postmaster" as a case-
   insensitive local name.  This postmaster address is not strictly
   necessary if the server always returns 554 on connection opening (as
   described in section 3.1).  The requirement to accept mail for
   postmaster implies that RCPT commands which specify a mailbox for
   postmaster at any of the domains for which the SMTP server provides
   mail service, as well as the special case of "RCPT TO:<Postmaster>"
   (with no domain specification), MUST be supported.

   SMTP systems are expected to make every reasonable effort to accept
   mail directed to Postmaster from any other system on the Internet.
   In extreme cases --such as to contain a denial of service attack or
   other breach of security-- an SMTP server may block mail directed to
   Postmaster.  However, such arrangements SHOULD be narrowly tailored
   so as to avoid blocking messages which are not part of such attacks.

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:

   -  Before sending a line of mail text, the SMTP client checks the
      first character of the line.  If it is a period, one additional
      period is inserted at the beginning of the line.

   -  When a line of mail text is received by the SMTP server, it checks
      the line.  If the line is composed of a single period, it is
      treated as the end of mail indicator.  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
   spaces, vertical and horizontal tabs, and other control characters.
   If the transmission channel provides an 8-bit byte (octet) data
   stream, the 7-bit ASCII codes are transmitted right justified in the
   octets, with the high order bits cleared to zero.  See 3.7 for
   special treatment of these conditions in SMTP systems serving a relay
   function.

   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, that
   store data in records rather than strings, or which use special
   character sequences as delimiters inside mailboxes.  If such
   transformations are necessary, they MUST be reversible, especially if
   they are applied to mail being relayed.

4.5.3 Sizes and Timeouts

4.5.3.1 Size limits and minimums

   There are several objects that have required minimum/maximum sizes.
   Every implementation MUST be able to receive objects of at least
   these sizes.  Objects larger than these sizes SHOULD be avoided when
   possible.  However, some Internet mail constructs such as encoded
   X.400 addresses [16] will often require larger objects: clients MAY
   attempt to transmit these, but MUST be prepared for a server to
   reject them if they cannot be handled by it.  To the maximum extent
   possible, implementation techniques which impose no limits on the
   length of these objects should be used.

   local-part
      The maximum total length of a user name or other local-part is 64
      characters.

   domain
      The maximum total length of a domain name or number is 255
      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.  SMTP extensions may be
      used to increase this limit.

   reply line
      The maximum total length of a reply line including the reply code
      and the <CRLF> is 512 characters.  More information may be
      conveyed through multiple-line replies.

   text line
      The maximum total length of a text line including the <CRLF> is
      1000 characters (not counting the leading dot duplicated for
      transparency).  This number may be increased by the use of SMTP
      Service Extensions.

   message content
      The maximum total length of a message content (including any
      message headers as well as the message body) MUST BE at least 64K
      octets.  Since the introduction of Internet standards for
      multimedia mail [12], message lengths on the Internet have grown
      dramatically, and message size restrictions should be avoided if
      at all possible.  SMTP server systems that must impose
      restrictions SHOULD implement the "SIZE" service extension [18],
      and SMTP client systems that will send large messages SHOULD
      utilize it when possible.

   recipients buffer
      The minimum total number of recipients that must be buffered is
      100 recipients.  Rejection of messages (for excessive recipients)
      with fewer than 100 RCPT commands is a violation of this
      specification.  The general principle that relaying SMTP servers
      MUST NOT, and delivery SMTP servers SHOULD NOT, perform validation
      tests on message headers suggests that rejecting a message based
      on the total number of recipients shown in header fields is to be
      discouraged.  A server which imposes a limit on the number of
      recipients MUST behave in an orderly fashion,  such as to reject
      additional addresses over its limit rather than silently
      discarding addresses previously accepted.  A client that needs to

      deliver a message containing over 100 RCPT commands SHOULD be
      prepared to transmit in 100-recipient "chunks" if the server
      declines to accept more than 100 recipients in a single message.

   Errors due to exceeding these limits may be reported by using the
   reply codes.  Some examples of reply codes are:

      500 Line too long.
   or
      501 Path too long
   or
      452 Too many recipients  (see below)
   or
      552 Too much mail data.

   RFC 821 [30] incorrectly listed the error where an SMTP server
   exhausts its implementation limit on the number of RCPT commands
   ("too many recipients") as having reply code 552.  The correct reply
   code for this condition is 452.  Clients SHOULD treat a 552 code in
   this case as a temporary, rather than permanent, failure so the logic
   below works.

   When a conforming SMTP server encounters this condition, it has at
   least 100 successful RCPT commands in its recipients buffer.  If the
   server is able to accept the message, then at least these 100
   addresses will be removed from the SMTP client's queue.  When the
   client attempts retransmission of those addresses which received 452
   responses, at least 100 of these will be able to fit in the SMTP
   server's recipients buffer.  Each retransmission attempt which is
   able to deliver anything will be able to dispose of at least 100 of
   these recipients.

   If an SMTP server has an implementation limit on the number of RCPT
   commands and this limit is exhausted, it MUST use a response code of
   452 (but the client SHOULD also be prepared for a 552, as noted
   above).  If the server has a configured site-policy limitation on the
   number of RCPT commands, it MAY instead use a 5XX response code.
   This would be most appropriate if the policy limitation was intended
   to apply if the total recipient count for a particular message body
   were enforced even if that message body was sent in multiple mail
   transactions.

4.5.3.2 Timeouts

   An SMTP client MUST provide a timeout mechanism.  It MUST use per-
   command timeouts rather than somehow trying to time the entire mail
   transaction.  Timeouts SHOULD be easily reconfigurable, preferably
   without recompiling the SMTP code.  To implement this, a timer is set

   for each SMTP command and for each buffer of the data transfer.  The
   latter means that the overall timeout is inherently proportional to
   the size of the message.

   Based on extensive experience with busy mail-relay hosts, the minimum
   per-command timeout values SHOULD be as follows:

   Initial 220 Message: 5 minutes
      An SMTP client process needs to distinguish between a failed TCP
      connection and a delay in receiving the initial 220 greeting
      message.  Many SMTP servers accept a TCP connection but delay
      delivery of the 220 message until their system load permits more
      mail to be processed.

   MAIL Command: 5 minutes

   RCPT Command: 5 minutes
      A longer timeout is required if processing of mailing lists and
      aliases is not deferred until after the message was accepted.

   DATA Initiation: 2 minutes
      This is while awaiting the "354 Start Input" reply to a DATA
      command.

   Data Block: 3 minutes
      This is while awaiting the completion of each TCP SEND call
      transmitting a chunk of data.

   DATA Termination: 10 minutes.
      This is while awaiting the "250 OK" reply.  When the receiver gets
      the final period terminating the message data, it typically
      performs processing to deliver the message to a user mailbox.  A
      spurious timeout at this point would be very wasteful and would
      typically result in delivery of multiple copies of the message,
      since it has been successfully sent and the server has accepted
      responsibility for delivery.  See section 6.1 for additional
      discussion.

   An SMTP server SHOULD have a timeout of at least 5 minutes while it
   is awaiting the next command from the sender.

4.5.4 Retry Strategies

   The common structure of a host SMTP implementation includes user
   mailboxes, one or more areas for queuing messages in transit, and one
   or more daemon processes for sending and receiving mail.  The exact
   structure will vary depending on the needs of the users on the host

   and the number and size of mailing lists supported by the host.  We
   describe several optimizations that have proved helpful, particularly
   for mailers supporting high traffic levels.

   Any queuing strategy MUST include timeouts on all activities on a
   per-command basis.  A queuing strategy MUST NOT send error messages
   in response to error messages under any circumstances.

4.5.4.1 Sending Strategy

   The general model for an SMTP client is one or more processes that
   periodically attempt to transmit outgoing mail.  In a typical system,
   the program that composes a message has some method for requesting
   immediate attention for a new piece of outgoing mail, while mail that
   cannot be transmitted immediately MUST be queued and periodically
   retried by the sender.  A mail queue entry will include not only the
   message itself but also the envelope information.

   The sender MUST delay retrying a particular destination after one
   attempt has failed.  In general, the retry interval SHOULD be at
   least 30 minutes; however, more sophisticated and variable strategies
   will be beneficial when the SMTP client can determine the reason for
   non-delivery.

   Retries continue until the message is transmitted or the sender gives
   up; the give-up time generally needs to be at least 4-5 days.  The
   parameters to the retry algorithm MUST be configurable.

   A client SHOULD keep a list of hosts it cannot reach and
   corresponding connection timeouts, rather than just retrying queued
   mail items.

   Experience suggests that failures are typically transient (the target
   system or its connection has crashed), favoring a policy of two
   connection attempts in the first hour the message is in the queue,
   and then backing off to one every two or three hours.

   The SMTP client can shorten the queuing delay in cooperation with the
   SMTP server.  For example, if mail is received from a particular
   address, it is likely that mail queued for that host can now be sent.
   Application of this principle may, in many cases, eliminate the
   requirement for an explicit "send queues now" function such as ETRN
   [9].

   The strategy may be further modified as a result of multiple
   addresses per host (see below) to optimize delivery time vs. resource
   usage.

   An SMTP client may have a large queue of messages for each
   unavailable destination host.  If all of these messages were retried
   in every retry cycle, there would be excessive Internet overhead and
   the sending system would be blocked for a long period.  Note that an
   SMTP client can generally determine that a delivery attempt has
   failed only after a timeout of several minutes and even a one-minute
   timeout per connection will result in a very large delay if retries
   are repeated for dozens, or even hundreds, of queued messages to the
   same host.

   At the same time, SMTP clients SHOULD use great care in caching
   negative responses from servers.  In an extreme case, if EHLO is
   issued multiple times during the same SMTP connection, different
   answers may be returned by the server.  More significantly, 5yz
   responses to the MAIL command MUST NOT be cached.

   When a mail message is to be delivered to multiple recipients, and
   the SMTP server to which a copy of the message is to be sent is the
   same for multiple recipients, then only one copy of the message
   SHOULD be transmitted.  That is, the SMTP client SHOULD use the
   command sequence:  MAIL, RCPT, RCPT,... RCPT, DATA instead of the
   sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA.  However, if there
   are very many addresses, a limit on the number of RCPT commands per
   MAIL command MAY be imposed.  Implementation of this efficiency
   feature is strongly encouraged.

   Similarly, to achieve timely delivery, the SMTP client MAY support
   multiple concurrent outgoing mail transactions.  However, some limit
   may be appropriate to protect the host from devoting all its
   resources to mail.

4.5.4.2 Receiving Strategy

   The SMTP server SHOULD attempt to keep a pending listen on the SMTP
   port at all times.  This requires the support of multiple incoming
   TCP connections for SMTP.  Some limit MAY be imposed but servers that
   cannot handle more than one SMTP transaction at a time are not in
   conformance with the intent of this specification.

   As discussed above, when the SMTP server receives mail from a
   particular host address, it could activate its own SMTP queuing
   mechanisms to retry any mail pending for that host address.

4.5.5   Messages with a null reverse-path

   There are several types of notification messages which are required
   by existing and proposed standards to be sent with a null reverse
   path, namely non-delivery notifications as discussed in section 3.7,

   other kinds of Delivery Status Notifications (DSNs) [24], and also
   Message Disposition Notifications (MDNs) [10].  All of these kinds of
   messages are notifications about a previous message, and they are
   sent to the reverse-path of the previous mail message.  (If the
   delivery of such a notification message fails, that usually indicates
   a problem with the mail system of the host to which the notification
   message is addressed.  For this reason, at some hosts the MTA is set
   up to forward such failed notification messages to someone who is
   able to fix problems with the mail system, e.g., via the postmaster
   alias.)

   All other types of messages (i.e., any message which is not required
   by a standards-track RFC to have a null reverse-path) SHOULD be sent
   with with a valid, non-null reverse-path.

   Implementors of automated email processors should be careful to make
   sure that the various kinds of messages with null reverse-path are
   handled correctly, in particular such systems SHOULD NOT reply to
   messages with null reverse-path.

5. Address Resolution and Mail Handling

   Once an SMTP client lexically identifies a domain to which mail will
   be delivered for processing (as described in sections 3.6 and 3.7), a
   DNS lookup MUST be performed to resolve the domain name [22].  The
   names are expected to be fully-qualified domain names (FQDNs):
   mechanisms for inferring FQDNs from partial names or local aliases
   are outside of this specification and, due to a history of problems,
   are generally discouraged.  The lookup first attempts to locate an MX
   record associated with the name.  If a CNAME record is found instead,
   the resulting name is processed as if it were the initial name.  If
   no MX records are found, but an A RR is found, the A RR is treated as
   if it was associated with an implicit MX RR, with a preference of 0,
   pointing to that host.  If one or more MX RRs are found for a given
   name, SMTP systems MUST NOT utilize any A RRs associated with that
   name unless they are located using the MX RRs; the "implicit MX" rule
   above applies only if there are no MX records present.  If MX records
   are present, but none of them are usable, this situation MUST be
   reported as an error.

   When the lookup succeeds, the mapping can result in a list of
   alternative delivery addresses rather than a single address, because
   of multiple MX records, multihoming, or both.  To provide reliable
   mail transmission, the SMTP client MUST be able to try (and retry)
   each of the relevant addresses in this list in order, until a
   delivery attempt succeeds.  However, there MAY also be a configurable
   limit on the number of alternate addresses that can be tried.  In any
   case, the SMTP client SHOULD try at least two addresses.

   Two types of information is used to rank the host addresses: multiple
   MX records, and multihomed hosts.

   Multiple MX records contain a preference indication that MUST be used
   in sorting (see below).  Lower numbers are more preferred than higher
   ones.  If there are multiple destinations with the same preference
   and there is no clear reason to favor one (e.g., by recognition of an
   easily-reached address), then the sender-SMTP MUST randomize them to
   spread the load across multiple mail exchangers for a specific
   organization.

   The destination host (perhaps taken from the preferred MX record) may
   be multihomed, in which case the domain name resolver will return a
   list of alternative IP addresses.  It is the responsibility of the
   domain name resolver interface to have ordered this list by
   decreasing preference if necessary, and SMTP MUST try them in the
   order presented.

   Although the capability to try multiple alternative addresses is
   required, specific installations may want to limit or disable the use
   of alternative addresses.  The question of whether a sender should
   attempt retries using the different addresses of a multihomed host
   has been controversial.  The main argument for using the multiple
   addresses is that it maximizes the probability of timely delivery,
   and indeed sometimes the probability of any delivery; the counter-
   argument is that it may result in unnecessary resource use.  Note
   that resource use is also strongly determined by the sending strategy
   discussed in section 4.5.4.1.

   If an SMTP server receives a message with a destination for which it
   is a designated Mail eXchanger, it MAY relay the message (potentially
   after having rewritten the MAIL FROM and/or RCPT TO addresses), make
   final delivery of the message, or hand it off using some mechanism
   outside the SMTP-provided transport environment.  Of course, neither
   of the latter require that the list of MX records be examined
   further.

   If it determines that it should relay the message without rewriting
   the address, it MUST sort the MX records to determine candidates for
   delivery.  The records are first ordered by preference, with the
   lowest-numbered records being most preferred.  The relay host MUST
   then inspect the list for any of the names or addresses by which it
   might be known in mail transactions.  If a matching record is found,
   all records at that preference level and higher-numbered ones MUST be
   discarded from consideration.  If there are no records left at that
   point, it is an error condition, and the message MUST be returned as
   undeliverable.  If records do remain, they SHOULD be tried, best
   preference first, as described above.

6. Problem Detection and Handling

6.1 Reliable Delivery and Replies by Email

   When the receiver-SMTP accepts a piece of mail (by sending a "250 OK"
   message in response to DATA), it is accepting responsibility for
   delivering or relaying the message.  It must take this responsibility
   seriously.  It MUST NOT lose the message for frivolous reasons, such
   as because the host later crashes or because of a predictable
   resource shortage.

   If there is a delivery failure after acceptance of a message, the
   receiver-SMTP MUST formulate and mail a notification message.  This
   notification MUST be sent using a null ("<>") reverse path in the
   envelope.  The recipient of this notification MUST be the address
   from the envelope return path (or the Return-Path: line).  However,
   if this address is null ("<>"), the receiver-SMTP MUST NOT send a
   notification.  Obviously, nothing in this section can or should
   prohibit local decisions (i.e., as part of the same system
   environment as the receiver-SMTP) to log or otherwise transmit
   information about null address events locally if that is desired.  If
   the address is an explicit source route, it MUST be stripped down to
   its final hop.

   For example, suppose that an error notification must be sent for a
   message that arrived with:

      MAIL FROM:<@a,@b:user@d>

   The notification message MUST be sent using:

      RCPT TO:<user@d>

   Some delivery failures after the message is accepted by SMTP will be
   unavoidable.  For example, it may be impossible for the receiving
   SMTP server to validate all the delivery addresses in RCPT command(s)
   due to a "soft" domain system error, because the target is a mailing
   list (see earlier discussion of RCPT), or because the server is
   acting as a relay and has no immediate access to the delivering
   system.

   To avoid receiving duplicate messages as the result of timeouts, a
   receiver-SMTP MUST seek to minimize the time required to respond to
   the final <CRLF>.<CRLF> end of data indicator.  See RFC 1047 [28] for
   a discussion of this problem.

6.2 Loop Detection

   Simple counting of the number of "Received:" headers in a message has
   proven to be an effective, although rarely optimal, method of
   detecting loops in mail systems.  SMTP servers using this technique
   SHOULD use a large rejection threshold, normally at least 100
   Received entries.  Whatever mechanisms are used, servers MUST contain
   provisions for detecting and stopping trivial loops.

6.3 Compensating for Irregularities

   Unfortunately, variations, creative interpretations, and outright
   violations of Internet mail protocols do occur; some would suggest
   that they occur quite frequently.  The debate as to whether a well-
   behaved SMTP receiver or relay should reject a malformed message,
   attempt to pass it on unchanged, or attempt to repair it to increase
   the odds of successful delivery (or subsequent reply) began almost
   with the dawn of structured network mail and shows no signs of
   abating.  Advocates of rejection claim that attempted repairs are
   rarely completely adequate and that rejection of bad messages is the
   only way to get the offending software repaired.  Advocates of
   "repair" or "deliver no matter what" argue that users prefer that
   mail go through it if at all possible and that there are significant
   market pressures in that direction.  In practice, these market
   pressures may be more important to particular vendors than strict
   conformance to the standards, regardless of the preference of the
   actual developers.

   The problems associated with ill-formed messages were exacerbated by
   the introduction of the split-UA mail reading protocols [3, 26, 5,
   21].  These protocols have encouraged the use of SMTP as a posting
   protocol, and SMTP servers as relay systems for these client hosts
   (which are often only intermittently connected to the Internet).
   Historically, many of those client machines lacked some of the
   mechanisms and information assumed by SMTP (and indeed, by the mail
   format protocol [7]).  Some could not keep adequate track of time;
   others had no concept of time zones; still others could not identify
   their own names or addresses; and, of course, none could satisfy the
   assumptions that underlay RFC 822's conception of authenticated
   addresses.

   In response to these weak SMTP clients, many SMTP systems now
   complete messages that are delivered to them in incomplete or
   incorrect form.  This strategy is generally considered appropriate
   when the server can identify or authenticate the client, and there
   are prior agreements between them.  By contrast, there is at best
   great concern about fixes applied by a relay or delivery SMTP server
   that has little or no knowledge of the user or client machine.

   The following changes to a message being processed MAY be applied
   when necessary by an originating SMTP server, or one used as the
   target of SMTP as an initial posting protocol:

   -  Addition of a message-id field when none appears

   -  Addition of a date, time or time zone when none appears

   -  Correction of addresses to proper FQDN format

   The less information the server has about the client, the less likely
   these changes are to be correct and the more caution and conservatism
   should be applied when considering whether or not to perform fixes
   and how.  These changes MUST NOT be applied by an SMTP server that
   provides an intermediate relay function.

   In all cases, properly-operating clients supplying correct
   information are preferred to corrections by the SMTP server.  In all
   cases, documentation of actions performed by the servers (in trace
   fields and/or header comments) is strongly encouraged.