Network Working Group                                        J. Yao, Ed.
Internet-Draft                                               W. Mao, Ed.
Updates: RFC4952                                                   CNNIC
(if approved)                                          November 17, 2007
Intended status: Experimental                                      CNNIC
Expires: March 6, May 20, 2008                                 September 3, 2007

           SMTP extension for internationalized email address
                     draft-ietf-eai-smtpext-08.txt
                     draft-ietf-eai-smtpext-09.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document specifies the use of an SMTP extension for transport and delivery
   of email messages with internationalized email address delivery. addresses or header
   information.  Communication with systems that do not implement this
   specification is specified in another document.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Role of this specification . . . . . . . . . . . . . . . .  3
     1.2.  Proposal Context . . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Mail Transport-level Protocol  . . . . . . . . . . . . . . . .  4
     2.1.  Framework for the Internationalization Extension . . . . .  4
     2.2.  The UTF8SMTP Extension . . . . . . . . . . . . . . . . . .  5
     2.3.  Extended Mailbox Address Syntax  . . . . . . . . . . . . .  6
     2.4.  The ALT-ADDRESS parameter Parameter  . . . . . . . . . . . . . . . .  8
     2.5.  ALT-ADDRESS parameter usage Parameter Usage and response codes Response Codes . . . . . .  9
     2.6.  Body Parts and SMTP Extensions . . . . . . . . . . . . . .  9 10
     2.7.  Additional ESMTP Changes and Clarifications  . . . . . . . 10
       2.7.1.  The Initial SMTP Exchange  . . . . . . . . . . . . . . 10 11
       2.7.2.  Mail eXchangers  . . . . . . . . . . . . . . . . . . . 10 11
       2.7.3.  Trace Information  . . . . . . . . . . . . . . . . . . 10 11
       2.7.4.  UTF-8 Reply  . . . . . . Strings in Replies . . . . . . . . . . . . . . . 11 12
   3.  Issues with Other Parts of the Email System  . . . . . . . . . 13
     3.1.  LMTP . . . . . . . . . . . . . . . . . . . . . .  IANA Considerations  . . . . . 13
     3.2.  SMTP Service Extension for DSNs . . . . . . . . . . . . . 13
     3.3.  POP and IMAP . . . 14
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   5.  Acknowledgements . 13
   4.  Potential problems . . . . . . . . . . . . . . . . . . . . . . 14
     4.1.  Impact many email related RFC  . .
   6.  Change History . . . . . . . . . . . . 14
   5.  Implementation Advice . . . . . . . . . . . . 15
     6.1.  draft-ietf-eai-smtpext: Version 00 . . . . . . . . 14
   6.  IANA Considerations . . . . 15
     6.2.  draft-ietf-eai-smtpext: Version 01 . . . . . . . . . . . . 15
     6.3.  draft-ietf-eai-smtpext: Version 02 . . . . . 14
   7.  Security considerations . . . . . . . 15
     6.4.  draft-ietf-eai-smtpext: Version 03 . . . . . . . . . . . . 15
   8.  Acknowledgements . . . . . . . . . . .
     6.5.  draft-ietf-eai-smtpext: Version 04 . . . . . . . . . . . . 15
   9.  Change History 16
     6.6.  draft-ietf-eai-smtpext: Version 05 . . . . . . . . . . . . 16
     6.7.  draft-ietf-eai-smtpext: Version 06 . . . . . . . . . . . . 15
     9.1. 16
     6.8.  draft-ietf-eai-smtpext: Version 00 07 . . . . . . . . . . . . 15
     9.2. 16
     6.9.  draft-ietf-eai-smtpext: Version 01 08 . . . . . . . . . . . . 15
     9.3. 16
     6.10. draft-ietf-eai-smtpext: Version 02 09 . . . . . . . . . . . . 16
     9.4.  draft-ietf-eai-smtpext: Version 03
   7.  References . . . . . . . . . . . . . 16
     9.5.  draft-ietf-eai-smtpext: Version 04 . . . . . . . . . . . . 16
     9.6.  draft-ietf-eai-smtpext: Version 05 . 17
     7.1.  Normative References . . . . . . . . . . . 16
     9.7.  draft-ietf-eai-smtpext: Version 06 . . . . . . . . 17
     7.2.  Informative References . . . . 16
     9.8.  draft-ietf-eai-smtpext: Version 07 . . . . . . . . . . . . 16
     9.9.  draft-ietf-eai-smtpext: Version 08 . . 18
   Appendix A.  Material Updating RFC 4952  . . . . . . . . . . 17
   10. References . . . 18
     A.1.  Conventional Message and Internationalized Message . . . . 19
     A.2.  LMTP . . . . . . . . . . . . . . . . . . . 17
     10.1. Normative References . . . . . . . . 19
     A.3.  SMTP Service Extension for DSNs  . . . . . . . . . . . . 17
     10.2. Informative References . 19
     A.4.  Implementation Advice  . . . . . . . . . . . . . . . . . 18 . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
   Intellectual Property and Copyright Statements . . . . . . . . . . 20 21

1.  Introduction

   Internationalized

   An internationalized email address includes two parts, the local part
   and the domain part.  The ways email addresses are used by protocols
   are different from the ways domain names are used.  The most critical
   difference is that emails are delivered through a chain of peering clients
   and servers while domain names are resolved by name servers
   by looking
   up those names in their own tables.  In addition to this, the
   extended email transport protocol ESMTP[RFC1869] [RFC2821] provides a negotiation
   mechanism through with which clients can make decisions discover server capabilities and
   make decisions for further processing; please see
   more in [EAI-framework].  Email addresses can exploit processing.  An extended overview of the SMTP
   extension negotiation mechanism while Internationalized Domain
   Name(IDN) does not have such a facility.  This is also more
   architecturally desirable approach. model for internationalized addresses and headers appears
   in [EAI-framework], referred to as "the framework document" or just
   as "Framework" elsewhere in this specification.  This document
   specifies an SMTP extension to permit internationalized email
   addresses in envelopes, and UTF-8 UNICODE characters (encoded in UTF-8) in
   headers.  The protocol described here is an MTA solution
   which is feasible, architecturally elegant, and not difficult to
   deploy.

1.1.  Role of this specification

   The framework document [EAI-framework] specifies the requirements for, and describes
   components of, full internationalization of electronic mail.
   To understand and implement this specification,  A
   thorough understanding of the
   context presented information in [EAI-framework] that document and in the
   base Internet email specifications [RFC2821] [RFC2822] is necessary. necessary
   to understand and implement this specification.

   This document specifies an element of that the email internationalization
   work, specifically the definition of an SMTP extension [RFC1869] [RFC2821] for the
   internationalized email address transport delivery.

1.2.  Proposal Context

   This specification describes a change an optional extension to the email
   transport mechanism that permits non-ASCII [ASCII] characters in both
   the envelope and header fields of messages while the messages.  The EAI-utf8header
   specification in [EAI-utf8header]
   specifies provides the details of how and where
   non-ASCII characters are permitted in the header fields of messages.
   The context for the change is described in [EAI-framework]. the framework document.

1.3.  Terminology

   The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED",
   and "MAY" in this document specification are to be interpreted as described in
   RFC 2119 [RFC2119].

   The terms "conventional message" and "internationalized message" are
   defined in an appendix to this specification.  The terms "UTF-8
   string" or "UTF-8 character" are used informally to refer to Unicode
   characters encoded in UTF-8 [RFC3629].  All other specialized terms
   used in this specification are defined in the
   EAI framework [EAI-framework] document or
   in the base Internet email specifications [RFC2821] and [RFC2822].  The  In
   particular, the terms "ASCII address", "internationalized email
   address", "non-ASCII address", "i18mail address", "UTF8SMTP",
   "message" and "mailing list" are used with in this document according to
   the definitions from in the [EAI-framework] document. framework one.

   This document specification defines only those ABNF [RFC4234] syntax rules
   that are different from those of the base email specifications
   [RFC2821][RFC2822] and, where the earlier rules are upgraded or
   extended, gives them new names.  When the new rule is a small upgrade
   modification to the older one, it is typically given a name starting
   with "u".  Rules that are undefined here may be found in the base
   email
   documents specifications under the same names.

   [[anchor4: NOTE TO RFC EDITOR'S NOTE: The EDITOR: Please remove the following text should be deleted
   before publication.]]
   This document specification is being discussed on the EAI mailing list.  See
   https://www1.ietf.org/mailman/listinfo/ima for information about
   subscribing.  The list's archive is at
   http://www1.ietf.org/mail-archive/web/ima/index.html.

2.  Mail Transport-level Protocol

2.1.  Framework for the Internationalization Extension

   The following service extension is defined:

   1.   The name of the SMTP service extension is "Email Address
        Internationalization";
        Internationalization".
   2.   The EHLO keyword value associated with this extension is
        "UTF8SMTP";
        "UTF8SMTP".
   3.   No parameter values are defined for this EHLO keyword value.  In
        order to permit future (although unanticipated) extensions, the
        EHLO response MUST NOT contain any parameters for that keyword.
        Clients MUST ignore any parameters, that is, clients MUST behave
        as if the parameters do not appear.  If a server includes
        UTF8SMTP in its EHLO response, it MUST be fully compliant with
        this version of this specification.
   4.   One optional parameter, ALT-ADDRESS, is added to the SMTP MAIL and
        RCPT commands. commands of SMTP.  ALT-ADDRESS specifies an all-ASCII
        address which can be used as a substitute for the i18mail addresses that
        we call the corresponding
        primary address; you can learn more (i18mail) address when downgrading.  More discussion of
        the use of this parameter appears in [EAI-framework] or and
        [EAI-downgrading].

   5.   One optional parameter "UTF8REPLY" is added to the VRFY and EXPN
        commands.  The parameter UTF8REPLY has no value.  The parameter
        indicates that the SMTP client can accept Unicode characters in
        UTF-8 on encoding in replies of from the VRFY and EXPN commands.
   6.   No additional SMTP verbs are defined by this extension.
   7.   Servers offering this extension MUST provide support for, and
        announce, the 8BITMIME extension [RFC1652].
   8.   The reverse-path and forward-path of the SMTP MAIL and RCPT
        commands are extended to allow UTF-8 Unicode characters encoded in
        UTF-8 in the specified mailbox
        address. names (addresses).
   9.   The mail datum message body is extended as specified in compliance with [EAI-utf8header]
        [EAI-utf8header].
   10.  The maximum length of a MAIL and RCPT command lines is increased
        by 460 characters by the possible addition of the ALT-ADDRESS
        keyword and value.
   11.  The UTF8SMTP extension is valid on the submission port
        [RFC4409].

2.2.  The UTF8SMTP Extension

   An SMTP Server that announces this extension MUST be prepared to
   accept a UTF-8 string [RFC3629] in any position in which RFC 2821
   specifies that a "mailbox" MAY mailbox can appear.  That string MUST be parsed only
   as specified in RFC 2821, i.e., by separating the mailbox into source
   route, local part and domain part, using only the characters colon
   (U+003A), comma (U+002C), and at-sign (U+0040) as specified there.
   Once isolated by this parsing process, the local part MUST be treated
   as opaque unless the SMTP Server is the final delivery MTA.  Any
   domain names that are to be looked up in the DNS MUST first be
   processed into the form specified in IDNA [RFC3490] by means of the
   ToASCII() operation unless they are already in that form.  Any domain
   names that are to be compared to local strings SHOULD be checked for
   validity and then MUST be compared as specified in section 3.4 of
   IDNA.

   The UTF8SMTP extension is valid on the submission port [RFC4409].

   An SMTP Client that receives the UTF8SMTP extension keyword in
   response to the "EHLO" command MAY transmit a mailbox name names within SMTP
   commands as an internationalized string strings in UTF-8 form and form.  It MAY send an a
   UTF-8 header
   [EAI-utf8header]. [EAI-utf8header] (which may also include mailbox names
   in UTF-8).  It MAY transmit the domain part parts of that string mailbox names within
   SMTP commands or the message header in either the form of ACE labels
   as specified in IDNA [RFC3490] or as UTF-8 form.
   In the strings.  All labels in
   domain part parts of the mailbox string, if any of the labels names which are
   intended to IDNs (either UTF-8 or ACE
   strings) MUST be interpreted as non-ASCII (i.e., are IDNs), then valid.  If the original client submits a message to
   a Message Submission Server ("MSA") [RFC4409] MUST take [RFC4409], it is the
   responsibility
   for ensuring that of the MSA that all domain labels are valid (whether they appear in native
   character or ACE form). valid; otherwise
   it is the original client's responsibility.  The presence of the
   UTF8SMTP extension does not change the requirement of RFC 2821 that
   servers relaying mail MUST not attempt to parse, evaluate, or
   transform the local part in any way.

   If the UTF8SMTP SMTP extension is not offered by the Server, the SMTP
   client MUST NOT transmit an internationalized address and MUST NOT
   transmit a mail message which contains containing internationalized mail headers as
   described in [EAI-utf8header] at any level within its MIME structure.
   Instead, if an SMTP client (SMTP sender) attempts to transfer a UTF8SMTP
   internationalized message and encounters a server that does not
   support the extension, it MUST make one of the following four
   choices:

   1.  If and only if the SMTP client (sender) is a Message Submission
       Server[RFC4409],
       Server ("MSA") [RFC4409], it MAY, consistent with the general
       provisions for changes by such servers, rewrite the envelope,
       headers, or message material to make them entirely ASCII and
       consistent with the provisions of RFC 2821 [RFC2821] and RFC 2822
       [RFC2822].
   2.  Reject  Either reject the message during the SMTP transaction or accept
       the message and then generate and transmit a notification of non-deliverability, non-
       deliverability.  Such notification MUST be done as specified in
       RFC 2821
       [RFC2821] and [RFC2821], RFC 3464 [RFC3464].  If the message content can be
       returned without alteration, content should be returned as
       specified in 2821 but, if a server is encountered along the
       return path that cannot accept UTF8SMTP traffic, [RFC3464], and the content
       should simply be abridged or dropped. EAI DSN
       specification [EAI-dsn].
   3.  Find an alternate route to the destination that permits UTF8SMTP.
       That route may be discovered by trying alternate MX hosts (using
       preference rules as specified in RFC 2821) or using other means
       available to the SMTP-sender.
   4.  If and only if ASCII addresses are available for all addresses
       that appear in the return path and the specific forward paths
       being attempted, downgrade the message to an all-ASCII form as
       specified in [EAI-downgrading].  An ASCII address is considered
       to be "available" for a particular address if the original
       address in the envelope is in ASCII or if an ALT-Address ALT-ADDRESS
       parameter is specified for a UTF8SMTP address.

2.3.  Extended Mailbox Address Syntax

   RFC 2821, section 4.1.2, defines the syntax of a mailbox entirely in
   terms of ASCII characters, using the production for "Mailbox" a mailbox and
   those on which it depends.

   The key changes made by this specification are, informally, to

   o  Change the definition of "sub-domain" to permit either the
      definition above or a UTF-8 string representing a DNS label that
      is conformant with IDNA [RFC3490].

   o  Change the definition of "Atom" to permit either the definition
      above or a UTF-8 string.  That string MUST NOT contain any of the
      ASCII characters (either graphics or controls) that are not
      permitted in "atext"; it is otherwise unrestricted.

   According to the description above, define the syntax of an
   internationalized email mailbox with name (address) is defined in ABNF
   [RFC4234] as

         uMailbox = uLocal-part "@" uDomain
           ; Replace Mailbox in RFC 2821, section 4.1.2

         uLocal-part = uDot-string / uQuoted-string
           ; MAY be case-sensitive
           ; Replace Local-part in RFC 2821, section 4.1.2

         uDot-string = uAtom *("." uAtom)
           ; Replace Dot-string in RFC 2821, section 4.1.2

         uAtom = 1*ucharacter
               ; Replace Atom in RFC 2821, section 4.1.2

         ucharacter = atext / UTF8-xtra-char
           ; Replace character in RFC 2821, section 4.1.2
           ; atext is defined in RFC 2822

         uQuoted-string = DQUOTE *uqcontent DQUOTE
           ; Replace Quoted-string in RFC 2821, section 4.1.2
           ; DQUOTE is Double Quote defined in RFC 4234

         uqcontent = qcontent / UTF8-xtra-char
           ; qcontent is defined in RFC 2822, section 3.2.5

         uDomain = (sub-udomain 1*("." sub-udomain)) / address-literal
           ; Replace Domain in RFC 2821, section 4.1.2
           ; address-literal is defined in RFC2821 section 4.1.2

         sub-udomain = uLet-dig [uLdh-str]
           ; Replace sub-domain in RFC 2821, section 4.1.2
         uLet-dig = Let-dig / UTF8-xtra-char
           ; Let-dig is defined in RFC 2821, section 4.1.3

         uLdh-str = *( ALPHA / DIGIT / "-" / UTF8-xtra-char) uLet-dig
           ; Replace Ldh-str in RFC 2821, section 4.1.3

         UTF8-xtra-char = UTF8-2 / UTF8-3 / UTF8-4
               ; UTF8-2, UTF8-3 and UTF8-4 are two, three, or four
               ; octet UTF-8 characters, as defined in RFC 3629

   The value of "udomain" SHOULD be verified with by applying the tests
   specified as part of IDNA [RFC3490]; [RFC3490].  If
   failed, that verification fails, the
   email address with that udomain can not MUST NOT be regarded as
   the a valid email
   address.

2.4.  The ALT-ADDRESS parameter Parameter

   If the UTF8SMTP extension is offered, the syntax of the SMTP MAIL and
   RCPT commands is extended to support the optional esmtp-keyword "ALT-
   ADDRESS", which
   ADDRESS".  That keyword specifies an alternate all-ASCII address
   which may be used when downgrading.  If the ALT-ADDRESS esmtp-keyword
   is used, it MUST have an associated esmtp-value (ALT-ADDRESS-esmtp-value (ALT-ADDRESS-esmtp-
   value, which is defined below).

   Based on the definition of mail-parameters in [RFC2821], the ALT-
   ADDRESS parameter usage in the commands of "MAIL" and "RCPT" is
   defined below. as follows.  The following definitions are given in the same
   format as used in RFC 2821.

        "MAIL FROM:" ("<>" / uReverse-path) [ SP Mail-parameters ] CRLF
           ; Update mail the MAIL command in RFC 2821, section 4.1.1.2.
           ; A new parameter defined by the ABNF non-terminal
           ; <ALT-ADDRESS-parameter> is added. It complies
           ; with the syntax specified by <esmtp-param>. for <esmtp-param> in RFC 2821.

        "RCPT TO:" ("<Postmaster@" domain uDomain ">" / "<Postmaster>" /
              uForward-Path) [ SP Rcpt-parameters ] CRLF
               ; Update rcpt RCPT command in RFC 2821, section 4.1.1.3.
               ; A new parameter defined by the ABNF non-terminal
               ; <ALT-ADDRESS-parameter> is added. It complies
               ; with the syntax specified by for <esmtp-param>.
               ; uDomain is defined in section 2.3 of this document
        uReverse-path = uPath
           ; Replace Reverse-path in RFC 2821, section 4.1.2

        uForward-path = uPath
           ; Replace Forward-path in RFC 2821, section 4.1.2

        uPath = "<" [ A-d-l ":" ] uMailbox ">"
           ; Replace Path in RFC 2821, section 4.1.2
           ; A-d-l is defined in RFC 2821, section 4.1.2
           ; uMailbox is defined in section 2.3 of this document

        ALT-ADDRESS-parameter="ALT-ADDRESS=" ALT-ADDRESS-esmtp-value

        ALT-ADDRESS-esmtp-value=xtext ALT-ADDRESS-value

        ALT-ADDRESS-value=xtext
               ; Mailbox The value is a mailbox name encoded as xtext.
               ; xtext is defined in RFC 3461 [RFC3461], 3461, section 4.2

   The ALT-ADDRESS-parameter MUST NOT appear more than once in any MAIL
   or RCPT command.  ALT-ADDRESS-esmtp-value MUST be an all-ASCII email
   address before xtext encoding.

2.5.  ALT-ADDRESS parameter usage Parameter Usage and response codes

   [EAI-utf8header] specifies "UTF8SMTP Response Codes

   An "internationalized message" which requires UTF8SMTP
   support.  Such a message as defined in the appendix of this
   specification MUST NOT be sent to an SMTP server which that does not
   support UTF8SMTP.  Such a message MAY be rejected due to
   lack of the ALT-ADDRESS by a server if it
   lacks one or more ALT-ADDRESSes as discussed in section Section 2.2 of this document.
   specification.

   The three-digit reply codes used in this section are consistent with
   their meanings as defined in RFC 2821.

   When messages are rejected because they require UTF8SMTP, the RCPT command requires an ALT-
   ADDRESS, the response code "550" 553 is used, defined in [RFC2821], used with the meaning "mailbox name
   not allowed".  When messages are rejected for other reasons, such as
   the MAIL command requiring an ALT-ADDRESS, the response code 550 is
   used with the meaning "mailbox unavailable".  If enhanced mail system
   status codes [RFC3463] are used, the response code should be "5.6.x"
   [SMTP-codes], meaning that "The alt-address ALT-ADDRESS is required but not
   specified".

   If the response code is issued after the final "." of the DATA
   command, the response code "554" is used, defined in [RFC2821], used with the meaning
   "Transaction failed".  If enhanced mail system status codes [RFC3463]
   are used, the response code should be "5.6.z" [SMTP-codes], meaning
   that "UTF8SMTP downgrade failed".

   [[anchor8: REMOVE THIS: IANA

   [[anchor7: RFC Editor: please assign insert the proper error codes for
   "5.6.x" and "5.6.z".]] "5.6.z" after IANA has made the relevant assignments.]]

2.6.  Body Parts and SMTP Extensions

   Since there is no ESMTP parameter which tells whether the message is
   UTF8SMTP
   an internationalized message, an SMTP server needs that requires accurate
   knowledge of whether a message is internationalized is required to
   parse all message header fields and MIME header fields in the message body to discover which
   messages are UTF8SMTP.
   body.  While this specification requires that servers support the
   8BITMIME extension [RFC1652] to ensure that servers have adequate
   handling capability for 8-bit data and to avoid a number of complex
   encoding problems, the use of internationalized addresses obviously
   does not require non-ASCII body parts in the MIME message.  The
   UTF8SMTP extension MAY be used with the BODY=8BITMIME parameter if
   that is appropriate given the body content or, if the server
   advertises it BINARYMIME [RFC3030] and it the BODY=BINARYMIME is
   appropriate, with the BODY=BINARYMIME
   parameter specified in [RFC3030]. parameter.

   Assuming that the server advertises UTF8SMTP and 8BITMIME, and
   receives at least one non-ASCII address, with or without ALT-ADDRESS,
   the precise interpretation of "No 'Body' parameter", "BODY=
   8BITMIME", and "BODY= BINARYMIME" in the MAIL command is:
   1.  For No  If there is no "Body" parameter, headers are in UTF-8, the header contains UTF-8
       characters but all the body parts are in
       ASCII. ASCII (possibly as the
       result of a Content-transfer-encoding).
   2.  For  If a BODY=8BITMIME parameter, headers are in UTF-8, parameter is present, the header contains
       UTF-8 characters and some or all of the body parts contain 8-bit
       line-oriented data.
   3.  For  If a BODY=BINARYMIME parameter, headers are in UTF-8, parameter is present, the header contains
       UTF-8 characters and some or all body parts contain binary data
       without restriction as to line lengths or delimiters.

2.7.  Additional ESMTP Changes and Clarifications

   The information carried in the mail transport process involves
   addresses ("mailboxes") and domain names in contexts in addition to
   the MAIL and RCPT commands and extended alternatives to them.  In
   general, the rule is that, when RFC 2821 specifies a mailbox, this document
   specification expects UTF-8 to be used for the entire string; when
   RFC 2821 specifies a domain name, the name SHOULD be in the form of
   ACE labels if its raw form is non-
   ASCII. non-ASCII.

   The following subsections list and discuss all of the relevant cases.

   Support and use of this extension requires support for 8BITMIME.  It
   means that 8BITMIME MUST be advertised by the UTF8SMTP capable SMTP
   server.

2.7.1.  The Initial SMTP Exchange

   When an SMTP or ESMTP connection is opened, the server normally sends
   a "greeting" response consisting of the '220' reply code and some
   information.  The client then sends the EHLO command.  Since the
   client cannot know whether the server supports UTF8SMTP until after
   it receives the response from EHLO, any domain names that appear in
   this dialogue, or in responses to EHLO, MUST be in the hostname form,
   i.e., internationalized ones MUST be in the form of ACE labels.

2.7.2.  Mail eXchangers

   Commonly, organizations

   Organizations often authorize multiple servers to accept mail
   addressed to them.  For example, the organization may itself operate
   more than one server, and may also or instead have an agreement with
   other organizations to accept mail as a backup.  Authorized servers
   are generally listed in MX records [RFC2821]. as described in RFC2821.  When
   more than one server accepts mail for the domain-part of a Mailbox, mailbox,
   it is strongly advised that either all or none of them support the
   UTF8SMTP extension.  Otherwise, surprising downgrades can happen
   during temporary failures, which is not a good thing.

2.7.3.  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.  Time stamp  "Time stamp" or
   "Received" appears in the form of "Received: lines".  The most
   important use of Received: lines is for debugging mail faults.  When
   the delivery SMTP server makes the "final delivery" of a message, it
   inserts a return-
   path return-path line at the beginning of the mail data.  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 the trace information, we update this memo updates the
   time stamp line and the return path line [RFC2821] formally defined
   as follows:

   uReturn-path-line = "Return-Path:" FWS uReverse-path <CRLF>
       ; Replaces Return-path-line in the section 4.4 of [RFC2821] RFC2821
       ; uReverse-path is defined in Section 2.3 of this document

   uTime-stamp-line = "Received:" FWS uStamp <CRLF>
       ; Replaces Time-stamp-line in the section 4.4 of [RFC2821] RFC2821

   uStamp = From-domain By-domain uOpt-info ";"  FWS date-time
       ; Replaces Stamp in the section 4.4 of [RFC2821] RFC2821
    uOpt-info = [Via] [With] [ID] [uFor]
       ; Replaces Opt-info in the section 4.4 of [RFC2821] RFC2821
       ; [With]'s The protocol value for With will allow a UTF8SMTP value

      uFor = "FOR" 1*( ( FWS (uPath / uMailbox) ) CFWS
       ; Replaces For in the section 4.4 of [RFC2821] RFC2821
       ; uPath is and uMailbox are defined in section Sections 2.4 of this document and
       ; uMailbox is defined in section 2.3 2.3, respectively, of this document

   [[anchor12:

   [[anchor11: Note: Whether the The FOR parameter is permitted has been changed to accept
   more than match the
   definition in RFC2821bis, permitting only one address is now under discussion as part of in the
   rfc2821bis effort. For
   clause.  The multiple-address construction was introduced
   with RFC 2821; it is not clear that it has been widely implemented or group working on that it is wise.  Whatever decision is document reached about RFC2821bis will
   be reflected in mailing list
   consensus that the syntax of in RFC 2821 that permitted more than one
   address was simply a future version of this document.]] mistake.]]
   Except in the 'uFor' and 'uReverse-path' line where non-ASCII domain
   name
   names may be used, internationalized domain names in Received fields
   MUST be transmitted in the form of ACE labels.  The protocol value of
   the WITH clause is UTF8SMTP when this extension is used.  More
   information is in the "IANA Considerations" section of this document.
   specification.

2.7.4.  UTF-8 Reply Strings in Replies

2.7.4.1.  MAIL and RCPT Commands

   If the client issues the RCPT command which contains containing non-ASCII
   characters, the SMTP server is permitted to use UTF-8 characters in
   the email address within associated with 251 and 551 response codes.

   If an SMTP client follows this specification and sends any RCPT
   commands containing non-ASCII addresses, it MUST be able to accept
   and process 251 or 551 replies containing UTF-8 email addresses.  If
   a given RCPT command does not include a non-ASCII envelope addresses, address,
   the server MUST not return a 251 or 551 response containing a non-
   ASCII mailbox.  Instead, it MUST transform such responses into 250 or
   550 responses that do not contain addresses.

2.7.4.2.  VRFY and EXPN Commands and the UTF8REPLY Parameter

   If the VRFY and EXPN commands have are transmitted the optional parameter
   "UTF8REPLY", it indicates the client can accept UTF-8 on strings in
   replies of
   the VRFY and EXPN from those commands.  Specially this  This allows the server to use UTF-8 on
   mailboxes
   strings in mailbox names and full names which occur on replies.  The in replies
   without concern that the client might be confused by them.  An SMTP
   client
   following that conforms to this specification MUST accept UTF-8 on and correctly
   process replies of from the VRFY and EXPN commands. commands that contain UTF-8
   strings.  However the SMTP server MUST not NOT use UTF-8 on
   replies, strings in
   replies if the SMTP client does not ask UTF-8 replies.  Some specifically allow such replies
   include the mailbox, but usually most of
   by transmitting this parameter.  Most replies do not require that
   the a
   mailbox is name be included in it the returned text and therefore UTF-8 is
   not needed.  The
   UTF8REPLY parameter on needed in them.  Some replies, notably those resulting from
   successful execution of the VRFY and EXPN commands tells commands, do include the SMTP
   server that
   mailbox, making the client is prepared for UTF-8 on SMTP replies. provisions of this section important.

   VERIFY (VRFY) and EXPAND (EXPN)command syntaxes are changed to:

       "VRFY" SP (uLocal-part / uMailbox) [SP "UTF8REPLY"] CRLF;
              ;uLocal-part is defined in section 2.3 of this document
              ;uMailbox is CRLF
              ; uLocal-part and uMailbox are defined in section
                      : Section 2.3 of this document

       "EXPN" SP (uLocal-part ( uLocal-part / uMailbox) [SP "UTF8REPLY"] CRLF;
              ;uLocal-part is uMailbox ) [ SP "UTF8REPLY" ] CRLF
                  ; uLocal-part and uMailbox are defined in section
                      ; Section 2.3 of this document
              ;uMailbox

   There is defined in section 2.3 of this document

   This parameter no value associated with the "UTF8REPLY" does not have value. parameter.  If SMTP
   reply requires UTF-8, but SMTP client does not use "UTF8REPLY"
   parameter in the VERIFY (VRFY) and EXPAND (EXPN) commands, the
   response code 252 is used, defined in [RFC2821], meaning "Cannot VRFY
   user, but will accept the message and attempt the delivery".  Also
   response code 550 may be used, meaning "Requested action not taken:
   mailbox unavailable".  If enhanced mail system status code [RFC3463]
   is used, response codes given on below is used.  "UTF8REPLY" on the
   VERIFY (VRFY) or EXPAND (EXPN) commands enables UTF-8 for that
   command only.

   If a normal success response (i.e., 250) response is returned, the response
   MAY include the full name of the user and MUST include the mailbox of
   the user.  It MUST be in either of the following forms:

         User Name <uMailbox>
            ; uMailbox is defined in section 2.3 of this document
            ; User Name allows the can contain non-ASCII character. characters.

         uMailbox
            ; uMailbox is defined in section 2.3 of this document

   If the SMTP reply requires UTF-8, UTF-8 strings, but UTF-8 is not allowed on in
   the reply, and enhanced mail system status code codes [RFC3463] is are used,
   the response code should be "5.6.y" or "2.6.y" [SMTP-codes], meaning
   that "The
   UTF-8 "A reply required, containing a UTF-8 string is required to show the
   mailbox name, but that form of response is not allowed.". [[anchor13: REMOVE THIS:
   IANA please assign permitted by the proper error codes for "5.6.y" and "2.6.y".]]
   client.".

   If the SMTP Client lack of does not support the UTF8SMTP support service extension,
   but receives a the UTF-8
   message on string in a reply, it may not be able to
   properly report the reply to the user or even crash.  UTF-8
   Internationalized messages on reply in replies are only allowed in the
   commands under the situations described above.  Under any other
   circumstances, UTF-8 messages on the reply text MUST NOT be
   used. appear in the reply.

   Although UTF-8 is needed to represent email addresses in responses
   under the rules specified in this section, this extension does not
   permit the use of UTF-8 for any other purposes.  SMTP servers MUST
   NOT include non-ASCII characters in replies except in the limited
   cases specifically permitted in this section.

3.  Issues with Other Parts of  IANA Considerations

   IANA is requested to add "UTF8SMTP" to the Email System

3.1.  LMTP

   LMTP [RFC2033] may be used as SMTP extensions registry
   with the final delivery agent.  In such
   cases, LMTP may be arranged entry pointing to deliver the mail this specification for its definition.

   IANA is requested to assign the mail store. proper error codes for "5.6.x",
   "5.6.z", "5.6.y" and "2.6.y", following the guidance in Section 2.5,
   and based on [SMTP-codes] and enter them in the appropriate registry.

   The mail store may not have UTF8SMTP capability.  LMTP need "Mail Transmission Types" registry is requested to be updated to deal
   include the following new entries:

   +---------------+----------------------------+----------------------+
   | WITH protocol | Description                | Reference            |
   | types         |                            |                      |
   +---------------+----------------------------+----------------------+
   | UTF8SMTP      | UTF8SMTP with these situations.

3.2.  SMTP Service Extension for DSNs

   The existing draft standard Delivery status notifications
   (DSNs)[RFC3461] is presently limited to US-ASCII      | [RFCXXXX]            |
   |               | Extensions                 |                      |
   | UTF8SMTPA     | UTF8SMTP with SMTP AUTH    | [RFC4954] [RFCXXXX]  |
   | UTF8SMTPS     | UTF8SMTP with STARTTLS     | [RFC3207] [RFCXXXX]  |
   | UTF8SMTPSA    | UTF8SMTP with both         | [RFC3207] [RFC4954]  |
   |               | STARTTLS and SMTP AUTH     | [RFCXXXX]            |
   +---------------+----------------------------+----------------------+

4.  Security Considerations

   See the extended security considerations discussion in the framework
   document [EAI-framework].

5.  Acknowledgements

   Much of the text in the machine
   readable portions initial version of this specification was
   derived or copied from [Klensin-emailaddr] with the protocol.  "International Delivery permission of the
   author.  Significant comments and
   Disposition Notifications" [EAI-dsn] adds a new address type for
   international email addresses so an original recipient address with
   non-US-ASCII characters can be correctly preserved even after
   downgrading.  If an SMTP server advertises both suggestions were received from
   Xiaodong LEE, Nai-Wen Hsu, Yangwoo KO, Yoshiro YONEYA, and other
   members of the UTF8SMTP JET team and were incorporated into the
   DSN extension, that server MUST implement EAI-dsn [EAI-dsn] including
   support for specification.
   Additional important comments and suggestions, and often specific
   text, were contributed by many members of the ORCPT parameter.

3.3.  POP WG and IMAP

   The [EAI-framework] has introduced two documents [EAI-pop] design team.
   Those contributions include material from John C Klensin, Charles
   Lindsey, Dave Crocker, Harald Tveit Alvestrand, Marcos Sanz, Chris
   Newman, Martin Duerst, Edmon Chung, Tony Finch, Kari Hurtta, Randall
   Gellens, Frank Ellermann, Alexey Melnikov, Pete Resnick, S.M., and
   [EAI-imap] to how to use internationalized user names based on UTF-8
   characters
   Soobok Lee. Of course, none of the individuals are necessarily
   responsible for the retrieval combination of messages from a mail server.

4.  Potential problems

4.1.  Impact many email related ideas represented here.

6.  Change History

   [[anchor17: RFC

   Internationalized email has implications for all processes and
   protocols which examine, handle, generate, or otherwise deal with
   mail.  In particular, address parsing or validity checks, message
   parsing or handling, etc.

5.  Implementation Advice

   In Editor: Please remove this section.]]

6.1.  draft-ietf-eai-smtpext: Version 00

   This version supercedes draft-yao-ima-smtpext-03.txt.  It refines the absence
   ABNF definition of this extension, SMTP clients and servers are
   constrained the internationalized email address.  It
   represents as the EAI working group document.

6.2.  draft-ietf-eai-smtpext: Version 01

   o  Upgraded to using only those addresses permitted by RFC 2821.  The
   local parts of those addresses MAY be made up reflect discussions during IETF 66.
   o  Remove the atomic parameter.
   o  Add the new section of any ASCII
   characters, although some "the Suggestion of them MUST be quoted as specified there.
   It is notable in an internationalization context that there is a long
   history on some systems the value of using overstruck ASCII characters (a
   character, a backspace, and another character) within a quoted string the ALT-
      ADDRESS parameter".

6.3.  draft-ietf-eai-smtpext: Version 02

   o  Upgraded to approximate non-ASCII characters.  This form reflect the recent discussion of
   internationalization SHOULD be phased out as this extension becomes
   widely deployed but backward-compatibility considerations require
   that it continue to be supported.

6.  IANA Considerations

   IANA is requested to add "UTF8SMTP" to the ima@ietf.org
      mailing list.
   o  Add the section of "Body Parts and SMTP Extensions".
   o  Add the new section of "Change History".
   o  Add the subsection about SMTP extensions registry
   with for DSN.

6.4.  draft-ietf-eai-smtpext: Version 03

   o  Update the entry pointing syntax related to this specification mailbox.
   o  Update the trace field section.
   o  Add the new section about message retry.
   o  Update the subsection about SMTP extensions for its definition.

   IANA is requested DSN.

6.5.  draft-ietf-eai-smtpext: Version 04

   o  Refine some syntax.
   o  Delete "Message Header Label" section.
   o  Change "bounce" to assign "reject".

6.6.  draft-ietf-eai-smtpext: Version 05

   o  Refine the proper error codes "5.6.x", "5.6.z",
   "5.6.y" abstract.
   o  Delete "The Suggestion of the Value of the ALT-ADDRESS parameter"
      section.
   o  Move original section 2.7.4 and "2.6.y" for this specification based on [SMTP-codes].

   The "Mail Transmission Types" registry is requested to be updated 2.7.5 to
   include the following new entries:

  WITH protocol types  Description                          Reference
  -------------------  ----------------------------         ---------
  UTF8SMTP             UTF8SMTP with Service Extensions     [RFCxxxx]
  UTF8SMTPA            UTF8SMTP section 3 with SMTP AUTH              [RFC2554bis]
                                                            [RFCxxxx]
  UTF8SMTPS            UTF8SMTP the name
      "Issues with STARTTLS               [RFC3207]
                                                            [RFCxxxx]
  UTF8SMTPSA           UTF8SMTP with both STARTTLS and      [RFC3207]
                       SMTP AUTH                            [RFC2554bis]
                                                            [RFCxxxx]
   [[anchor22: REMOVE THIS: where RFCxxxx represents the future RFC N0. other parts of this document.  When this document is published as RFC and
   assigned with a RFC No., "xxxx" should be replaced with 4-digits No..
   "RFC2554bis" should be replaced with the email system".
   o  Add the new RFC No. when section "LMTP".
   o  Refine some text according to suggestions from the
   "RFC2554bis" document is assigned with EAI mailing
      list discussion
   o  Remove the section "Mailing List Question"

6.7.  draft-ietf-eai-smtpext: Version 06

   o  Delete the section about message retry.
   o  Add the new RFC No.]]

7.  Security considerations

   See subsection about Mail eXchangers
   o  Add the extended security considerations discussion in
   [EAI-framework]

8.  Acknowledgements

   Much of new section about "UTF-8 Reply"
   o  Refine some response code for the text in section "Using the initial version of this document was derived
   or copied from [Klensin-emailaddr] with ALT-ADDRESS
      parameter"

6.8.  draft-ietf-eai-smtpext: Version 07

   o  Rename the permission of section 2.5
   o  Refine sthe section 2.7

6.9.  draft-ietf-eai-smtpext: Version 08

   o  Refine some texts and update some references

6.10.  draft-ietf-eai-smtpext: Version 09

   o  Add the author.
   Significant comments appendix
   o  Move section 3.1, 3.2 and suggestions were received from Xiaodong LEE,
   Nai-Wen Hsu, Yangwoo KO, Yoshiro YONEYA, section 5 to Appendix
   o  Remove section 3.3 and other members of section 4
   o  Add the JET
   team new term definitions of conventional message and were incorporated into
      international message in the document.  Special thanks appendix
   o  Refine some texts according to
   those contributors suggestions from the EAI mailing
      list discussion during WG Last call
   o  Use the same reference for this ASCII as RFC 2821.
   o  General editorial revision and cleanup, including extensive
      modifications to the XML to produce a version that has better odds
      of getting through the document, those include
   (but not limited to) John C Klensin, Charles Lindsey, Dave Crocker,
   Harald Tveit Alvestrand, Marcos Sanz, Chris Newman, Martin Duerst,
   Edmon Chung, Tony Finch, Kari Hurtta, Randall Gellens, Frank
   Ellermann, Alexey Melnikov.  Of course, none of the individuals are
   necessarily responsible for the combination of ideas represented
   here.

9.  Change History

   [[anchor25: REMOVE THIS: This section is used for tracking the update
   of this document.  It may be useful to retain parts of it to
   facilitate establishing dates and documents for the history of this
   work.]]

9.1.  draft-ietf-eai-smtpext: Version 00

   This version supercedes draft-yao-ima-smtpext-03.txt.  It refines the
   ABNF definition of the internationalized email address.  It
   represents as the EAI working group document.

9.2.  draft-ietf-eai-smtpext: Version 01

   o  Upgraded to reflect discussions during IETF 66.
   o  Remove the atomic parameter.

   o  Add the new section of "the Suggestion of the value of the ALT-
      ADDRESS parameter".

9.3.  draft-ietf-eai-smtpext: Version 02

   o  Upgraded to reflect the recent discussion of the ima@ietf.org
      mailing list.
   o  Add the section of "Body Parts and SMTP Extensions".
   o  Add the new section of "Change History".
   o  Add the subsection about SMTP extensions for DSN.

9.4.  draft-ietf-eai-smtpext: Version 03

   o  Update the syntax related to mailbox.
   o  Update the trace field section.
   o  Add the new section about message retry.
   o  Update the subsection about SMTP extensions for DSN.

9.5.  draft-ietf-eai-smtpext: Version 04

   o  Refine some syntax.
   o  Delete "Message Header Label" section.
   o  Change "bounce" to "reject".

9.6.  draft-ietf-eai-smtpext: Version 05

   o  Refine the abstract.
   o  Delete "The Suggestion of the Value of the ALT-ADDRESS parameter"
      section.
   o  Move original section 2.7.4 and 2.7.5 to section 3 with the name
      "Issues with other parts of the email system".
   o  Add the new section "LMTP".
   o  Refine some text according to suggestions from the EAI mailing
      list discussion
   o  Remove the section "Mailing List Question"

9.7.  draft-ietf-eai-smtpext: Version 06

   o  Delete the section about message retry.
   o  Add the new subsection about Mail eXchangers
   o  Add the new section about "UTF-8 Reply"
   o  Refine some response code for the section "Using the ALT-ADDRESS
      parameter"

9.8.  draft-ietf-eai-smtpext: Version 07
   o  Rename the section 2.5
   o  Refine sthe section 2.7

9.9.  draft-ietf-eai-smtpext: Version 08

   o  Refine some texts various checkers and update some references

10. validators.

7.  References

10.1.

7.1.  Normative References

   [ASCII]    Cerf, V., "ASCII format    American National Standards Institute (formerly United
              States of America Standards Institute), "USA Code for network interchange", RFC 20,
              October 1969.
              Information Interchange", ANSI X3.4-1968, 1968.

   [EAI-dsn]  Newman, C. and A. Melnikov, "SMTP extensions for DSNs",
              draft-ietf-eai-dsn-03.txt (work in progress),
              September 2007.

   [EAI-framework]
              Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 4952, July 2007.

   [EAI-utf8header]
              Abel, Y., "Transmission of Email Headers in UTF-8
              Encoding", draft-ietf-eai-utf8headers-07.txt (work in
              progress), September 2007.

   [RFC1652]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
              Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
              RFC 1652, July 1994.

   [RFC1869]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
              Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
              November 1995.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

   [RFC3461]  Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
              Extension for Delivery Status Notifications (DSNs)",
              RFC 3461, January 2003.

   [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes",
              RFC 3463, January 2003.

   [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format
              for Delivery Status Notifications", RFC 3464,
              January 2003.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", RFC 3629, November 2003.

   [RFC4234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

10.2.  Informative References

   [EAI-downgrading]
              YONEYA, Y., Ed. and K. Fujiwara, Ed., "Downgrading
              mechanism for Internationalized eMail Address",
              draft-ietf-eai-downgrade-04 (work in progress), 7 2007.

   [EAI-dsn]  Newman, C. and A. Melnikov, "SMTP extensions for DSNs",
              draft-ietf-eai-dsn-03.txt (work in progress), 9 2007.

   [EAI-imap]
              Resnick, P. and C. Newman, "Considerations for IMAP in
              Conjunction with Email Address Internationalization",
              draft-ietf-eai-imap-utf8-01 (work in progress),
              March 2007.

   [EAI-mailing list] for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

   [RFC4409]  Gellens, R. and E. Chung, "Mailing Lists J. Klensin, "Message Submission for Mail",
              RFC 4409, April 2006.

7.2.  Informative References

   [EAI-downgrading]
              YONEYA, Y., Ed. and
              Internationalized Email Addresses",
              draft-ietf-eai-mailinglist-01.txt (work in progress),
              January 2007.

   [EAI-pop]  Newman, C., "POP3 Support K. Fujiwara, Ed., "Downgrading
              mechanism for UTF-8",
              draft-ietf-eai-pop-01.txt Internationalized eMail Address",
              draft-ietf-eai-downgrade-04 (work in progress),
              January 7 2007.

   [Klensin-emailaddr]
              Klensin, J., "Internationalization of Email Addresses",
              draft-klensin-emailaddr-i18n-03 (work in progress),
              July 2005.

   [RFC2033]  Myers, J., "Local Mail Transfer Protocol", RFC 2033,
              October 1996.

   [RFC2554bis]

   [RFC3030]  Vaudreuil, G., "SMTP Service Extensions for Transmission
              of Large and Binary MIME Messages", RFC 3030,
              December 2000.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.

   [RFC4954]  Siemborski, R. and A. Melnikov, "SMTP Service Extension
              for Authentication", draft-siemborski-rfc2554bis-09 (work Authentication", RFC 4954, July 2007.

   [SMTP-codes]
              KLensin, J., "An IANA Registry for Extended SMTP Status
              Codes", draft-klensin-smtp-code-registry-00 (work in
              progress), April 2007.

Appendix A.  Material Updating RFC 4952

   RFC 4952, the Overview and Framework document covering this set of
   extensions for internationalized email [EAI-framework], was completed
   before this specification, which specifies a particular part of the
   protocol set.  This appendix, which is normative, contains material
   that would have been incorporated into RFC 4952 had it been delayed
   until the work described in the rest of this specification was
   completed and that should be included in any update to RFC 4952.

A.1.  Conventional Message and Internationalized Message

   o  A conventional message is one that does not use any extension
      defined in this document or in the UTF8header specification
      [EAI-utf8header], and strictly conformant to RFC 2822 [RFC2822].
   o  An internationalized message is a message utilizing one or more of
      the extensions defined in this specification or in the UTF8header
      specification [EAI-utf8header], so that it is no longer conformant
      to the RFC 2822 specification of a message.

A.2.  LMTP

   LMTP [RFC2033] may be used as the final delivery agent.  In such
   cases, LMTP may be arranged to deliver the mail to the mail store.
   The mail store may not have UTF8SMTP capability.  LMTP need to be
   updated to deal with these situations.

A.3.  SMTP Service Extension for DSNs

   The existing draft standard Delivery status notifications
   (DSNs)[RFC3461] is limited to ASCII text in progress), April 2007.

   [RFC3030]  Vaudreuil, G., "SMTP Service Extensions for Transmission the machine readable
   portions of Large the protocol.  "International Delivery and Binary MIME Messages", RFC 3030,
              December 2000.

   [RFC3207]  Hoffman, P., "SMTP Service Extension Disposition
   Notifications" [EAI-dsn] adds a new address type for Secure international
   email addresses so an original recipient address with non-ASCII
   characters can be correctly preserved even after downgrading.  If an
   SMTP over
              Transport Layer Security", RFC 3207, February 2002.

   [RFC4409]  Gellens, R. server advertises both the UTF8SMTP and J. Klensin, "Message Submission for Mail",
              RFC 4409, April 2006.

   [SMTP-codes]
              KLensin, J., "An IANA Registry the DSN extension, that
   server MUST implement EAI-dsn [EAI-dsn] including support for Extended the
   ORCPT parameter.

A.4.  Implementation Advice

   In the absence of this extension, SMTP Status
              Codes", draft-klensin-smtp-code-registry-00 (work clients and servers are
   constrained to using only those addresses permitted by RFC 2821.  The
   local parts of those addresses MAY be made up of any ASCII
   characters, although some of them MUST be quoted as specified there.
   It is notable in
              progress), April 2007. an internationalization context that there is a long
   history on some systems of using overstruck ASCII characters (a
   character, a backspace, and another character) within a quoted string
   to approximate non-ASCII characters.  This form of
   internationalization SHOULD be phased out as this extension becomes
   widely deployed but backward-compatibility considerations require
   that it continue to be supported.

Authors' Addresses

   Jiankang YAO (editor)
   CNNIC
   No.4 South 4th Street, Zhongguancun
   Beijing

   Phone: +86 10 58813007
   Email: yaojk@cnnic.cn

   Wei MAO (editor)
   CNNIC
   No.4 South 4th Street, Zhongguancun
   Beijing

   Phone: +86 10 58813055
   Email: maowei_ietf@cnnic.cn

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