Network Working Group                                        J. Yao, Ed.
Internet-Draft                                               W. Mao, Ed.
Intended status: Experimental                                      CNNIC
Expires: March 6, 2008                                 September 3, 2007

           SMTP extension for internationalized email address

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document specifies the use of SMTP extension for
   internationalized email address delivery.  Communication with systems
   that do not implement this specification is specified in another

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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  . . . . . . . . . . . . . . . .  8
     2.5.  ALT-ADDRESS parameter usage and response codes . . . . . .  9
     2.6.  Body Parts and SMTP Extensions . . . . . . . . . . . . . .  9
     2.7.  Additional ESMTP Changes and Clarifications  . . . . . . . 10
       2.7.1.  The Initial SMTP Exchange  . . . . . . . . . . . . . . 10
       2.7.2.  Mail eXchangers  . . . . . . . . . . . . . . . . . . . 10
       2.7.3.  Trace Information  . . . . . . . . . . . . . . . . . . 10
       2.7.4.  UTF-8 Reply  . . . . . . . . . . . . . . . . . . . . . 11
   3.  Issues with Other Parts of the Email System  . . . . . . . . . 13
     3.1.  LMTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     3.2.  SMTP Service Extension for DSNs  . . . . . . . . . . . . . 13
     3.3.  POP and IMAP . . . . . . . . . . . . . . . . . . . . . . . 13
   4.  Potential problems . . . . . . . . . . . . . . . . . . . . . . 14
     4.1.  Impact many email related RFC  . . . . . . . . . . . . . . 14
   5.  Implementation Advice  . . . . . . . . . . . . . . . . . . . . 14
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   7.  Security considerations  . . . . . . . . . . . . . . . . . . . 15
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  Change History . . . . . . . . . . . . . . . . . . . . . . . . 15
     9.1.  draft-ietf-eai-smtpext: Version 00 . . . . . . . . . . . . 15
     9.2.  draft-ietf-eai-smtpext: Version 01 . . . . . . . . . . . . 15
     9.3.  draft-ietf-eai-smtpext: Version 02 . . . . . . . . . . . . 16
     9.4.  draft-ietf-eai-smtpext: Version 03 . . . . . . . . . . . . 16
     9.5.  draft-ietf-eai-smtpext: Version 04 . . . . . . . . . . . . 16
     9.6.  draft-ietf-eai-smtpext: Version 05 . . . . . . . . . . . . 16
     9.7.  draft-ietf-eai-smtpext: Version 06 . . . . . . . . . . . . 16
     9.8.  draft-ietf-eai-smtpext: Version 07 . . . . . . . . . . . . 16
     9.9.  draft-ietf-eai-smtpext: Version 08 . . . . . . . . . . . . 17
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     10.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
   Intellectual Property and Copyright Statements . . . . . . . . . . 20

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1.  Introduction

   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 their own tables.  In addition to this, email transport
   protocol ESMTP[RFC1869] provides a negotiation mechanism through
   which clients can make decisions for further processing; please see
   more in [EAI-framework].  Email addresses can exploit the SMTP
   extension negotiation mechanism while Internationalized Domain
   Name(IDN) does not have such a facility.  This is also more
   architecturally desirable approach.  This document specifies an SMTP
   extension to permit internationalized email addresses in envelopes,
   and UTF-8 in headers.  The protocol described here is an MTA solution
   which is feasible, architecturally elegant, and not difficult to

1.1.  Role of this specification

   The framework document [EAI-framework] specifies the requirements
   for, and components of, full internationalization of electronic mail.
   To understand and implement this specification, understanding the
   context presented in [EAI-framework] is necessary.

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

1.2.  Proposal Context

   This specification describes a change to the email transport
   mechanism that permits non-ASCII characters in both the envelope and
   header fields of messages while the specification in [EAI-utf8header]
   specifies 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].

1.3.  Terminology

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

   All specialized terms used in this specification are defined in the
   EAI framework [EAI-framework] or in [RFC2821] and [RFC2822].  The
   terms "ASCII address", "internationalized email address", "non-ASCII

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   address", "i18mail address", "UTF8SMTP", "message" and "mailing list"
   are used with the definitions from the [EAI-framework] document.

   This document 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
   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 under the same names.

   [[anchor4: RFC EDITOR'S NOTE: The following text should be deleted
   before publication.]]  This document is being discussed on the EAI
   mailing list.  See for
   information about subscribing.  The list's archive is at

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
   2.   The EHLO keyword value associated with this extension is
   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.  ALT-ADDRESS specifies an all-ASCII address
        which can be used as a substitute for the i18mail addresses that
        we call the primary address; you can learn more in
        [EAI-framework] or [EAI-downgrading].
   5.   One optional parameter "UTF8REPLY" is added to the VRFY and EXPN
        commands.  The parameter UTF8REPLY has no value.  The parameter
        indicates the SMTP client can accept UTF-8 on replies of 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].

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   8.   The reverse-path and forward-path of SMTP MAIL and RCPT commands
        are extended to allow UTF-8 characters in the specified mailbox
   9.   The mail datum is extended in compliance with [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.

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 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

   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 as an
   internationalized string in UTF-8 form and MAY send an UTF-8 header
   [EAI-utf8header].  It MAY transmit the domain part of that string in
   either the form of ACE labels specified in [RFC3490] or UTF-8 form.
   In the domain part of the mailbox string, if any of the labels are
   intended to be interpreted as non-ASCII (i.e., are IDNs), then the
   Message Submission Server ("MSA") [RFC4409] MUST take responsibility
   for ensuring that the labels are valid (whether they appear in native
   character or ACE form).  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 internationalized mail headers
   [EAI-utf8header] at any level within its MIME structure.  Instead, if
   an SMTP client (SMTP sender) attempts to transfer a UTF8SMTP message
   and encounters a server that does not support the extension, it MUST
   make one of the following four choices:

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   1.  If and only if the SMTP client (sender) is a Message Submission
       Server[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 the message during the SMTP transaction or generate a
       notification of non-deliverability, as specified in RFC 2821
       [RFC2821] and 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, the content
       should simply be abridged or dropped.
   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
       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" 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 ABNF [RFC4234] as

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         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 defined in RFC 3629

   The value of "udomain" SHOULD be verified with IDNA [RFC3490]; If
   failed, the email address with that udomain can not be regarded as
   the valid email address.

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2.4.  The ALT-ADDRESS 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 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 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.

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

        "RCPT TO:" ("<Postmaster@" domain ">" / "<Postmaster>" /
                  uForward-Path) [ SP Rcpt-parameters ] CRLF
               ; Update rcpt command in RFC 2821, section
               ; A new parameter defined by the ABNF non-terminal
               ; <ALT-ADDRESS-parameter> is added. It complies
               ; with the syntax specified by <esmtp-param>.

        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

               ; Mailbox encoded as xtext.
               ; xtext is defined in RFC 3461 [RFC3461], 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

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   address before xtext encoding.

2.5.  ALT-ADDRESS parameter usage and response codes

   [EAI-utf8header] specifies "UTF8SMTP message" which requires UTF8SMTP
   support.  Such a message MUST NOT be sent to an SMTP server which
   does not support UTF8SMTP.  Such a message MAY be rejected due to
   lack of the ALT-ADDRESS as discussed in section 2.2 of this document.

   When messages are rejected because they require UTF8SMTP, the
   response code "550" is used, defined in [RFC2821], 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 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],
   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 please assign the proper error codes for
   "5.6.x" and "5.6.z".]]

2.6.  Body Parts and SMTP Extensions

   Since there is no ESMTP parameter which tells whether the message is
   UTF8SMTP message, SMTP server needs to parse all message header
   fields and MIME header fields in the message body to discover which
   messages are UTF8SMTP.  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 and it is appropriate, with the BODY=BINARYMIME
   parameter specified in [RFC3030].

   Assuming that the server advertises UTF8SMTP and 8BITMIME, and 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 "Body" parameter, headers are in UTF-8, body parts are in
   2.  For BODY=8BITMIME parameter, headers are in UTF-8, some or all
       body parts contain 8-bit line-oriented data.

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   3.  For BODY=BINARYMIME parameter, headers are in UTF-8, some or all
       body parts contain binary data without restriction as to line
       lengths or delimiters.

2.7.  Additional ESMTP Changes and Clarifications

   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 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-

   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

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 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].  When more than one
   server accepts mail for the domain-part of a 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
   appears in the form of "Received: lines".  The most important use of

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   Received: lines is for debugging mail faults.  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.  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 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]
           ; uReverse-path is defined in Section 2.3

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

   uStamp = From-domain By-domain uOpt-info ";"  FWS date-time
           ; Replaces Stamp in the section 4.4 of [RFC2821]

   uOpt-info = [Via] [With] [ID] [uFor]
           ; Replaces Opt-info in the section 4.4 of [RFC2821]
           ; [With]'s protocol value will allow UTF8SMTP value

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

   [[anchor12: Note: Whether the FOR parameter is permitted to accept
   more than one address is now under discussion as part of the
   rfc2821bis effort.  The multiple-address construction was introduced
   with RFC 2821; it is not clear that it has been widely implemented or
   that it is wise.  Whatever decision is reached about RFC2821bis will
   be reflected in the syntax of a future version of this document.]]
   Except in the 'uFor' and 'uReverse-path' line where non-ASCII domain
   name 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.

2.7.4.  UTF-8 Reply

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

   If an SMTP client follows this specification and sends any RCPT

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   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 non-ASCII envelope addresses,
   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.

   If the VRFY and EXPN commands have the optional parameter
   "UTF8REPLY", it indicates the client can accept UTF-8 on replies of
   the VRFY and EXPN commands.  Specially this allows to use UTF-8 on
   mailboxes and full names which occur on replies.  The SMTP client
   following this specification MUST accept UTF-8 on replies of the VRFY
   and EXPN commands.  However the SMTP server MUST not use UTF-8 on
   replies, if the SMTP client does not ask UTF-8 replies.  Some replies
   include the mailbox, but usually most of replies do not require that
   the mailbox is included in it and therefore UTF-8 is not needed.  The
   UTF8REPLY parameter on the VRFY and EXPN commands tells the SMTP
   server that the client is prepared for UTF-8 on SMTP replies.

   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 defined in section 2.3 of this document

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

   This parameter "UTF8REPLY" does not have value.  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 (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 non-ASCII character.


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                ; uMailbox is defined in section 2.3 of this document

   If the SMTP reply requires UTF-8, but UTF-8 is not allowed on reply,
   and enhanced mail system status code [RFC3463] is used, the response
   code should be "5.6.y" or "2.6.y" [SMTP-codes], meaning that "The
   UTF-8 reply required, but not allowed.". [[anchor13: REMOVE THIS:
   IANA please assign the proper error codes for "5.6.y" and "2.6.y".]]

   If the SMTP Client lack of the UTF8SMTP support receives the UTF-8
   message on reply, it may crash.  UTF-8 messages on reply are only
   allowed in the commands under the situations described above.  Under
   any other circumstances, UTF-8 messages on the reply MUST NOT be

   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 except in the limited cases
   specifically permitted in this section.

3.  Issues with Other Parts of the Email System

3.1.  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.

3.2.  SMTP Service Extension for DSNs

   The existing draft standard Delivery status notifications
   (DSNs)[RFC3461] is presently limited to US-ASCII text in the machine
   readable portions of the protocol.  "International Delivery 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 the UTF8SMTP and the
   DSN extension, that server MUST implement EAI-dsn [EAI-dsn] including
   support for the ORCPT parameter.

3.3.  POP and IMAP

   The [EAI-framework] has introduced two documents [EAI-pop] and
   [EAI-imap] to how to use internationalized user names based on UTF-8
   characters for the retrieval of messages from a mail server.

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4.  Potential problems

4.1.  Impact many email related 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 the absence of this extension, SMTP 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 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.

6.  IANA Considerations

   IANA is requested to add "UTF8SMTP" to the SMTP extensions registry
   with the entry pointing to this specification for its definition.

   IANA is requested to assign the proper error codes "5.6.x", "5.6.z",
   "5.6.y" and "2.6.y" for this specification based on [SMTP-codes].

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

  WITH protocol types  Description                          Reference
  -------------------  ----------------------------         ---------
  UTF8SMTP             UTF8SMTP with Service Extensions     [RFCxxxx]
  UTF8SMTPA            UTF8SMTP with SMTP AUTH              [RFC2554bis]
  UTF8SMTPS            UTF8SMTP with STARTTLS               [RFC3207]
  UTF8SMTPSA           UTF8SMTP with both STARTTLS and      [RFC3207]
                       SMTP AUTH                            [RFC2554bis]

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   [[anchor22: REMOVE THIS: where RFCxxxx represents the future RFC N0.
   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 new RFC No. when the
   "RFC2554bis" document is assigned with the new RFC No.]]

7.  Security considerations

   See the extended security considerations discussion in

8.  Acknowledgements

   Much of the text in the initial version of this document was derived
   or copied from [Klensin-emailaddr] with the permission of the author.
   Significant comments and suggestions were received from Xiaodong LEE,
   Nai-Wen Hsu, Yangwoo KO, Yoshiro YONEYA, and other members of the JET
   team and were incorporated into the document.  Special thanks to
   those contributors for this version of 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

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

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.

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   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
      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"
   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

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

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   o  Rename the section 2.5
   o  Refine sthe section 2.7

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

   o  Refine some texts and update some references

10.  References

10.1.  Normative References

   [ASCII]    Cerf, V., "ASCII format for network interchange", RFC 20,
              October 1969.

              Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 4952, July 2007.

              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.

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   [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

              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.

              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]
              Gellens, R. and E. Chung, "Mailing Lists and
              Internationalized Email Addresses",
              draft-ietf-eai-mailinglist-01.txt (work in progress),
              January 2007.

   [EAI-pop]  Newman, C., "POP3 Support for UTF-8",
              draft-ietf-eai-pop-01.txt (work in progress),
              January 2007.

              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.

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              Siemborski, R. and A. Melnikov, "SMTP Service Extension
              for Authentication", draft-siemborski-rfc2554bis-09 (work
              in progress), April 2007.

   [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.

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

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

Authors' Addresses

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

   Phone: +86 10 58813007

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

   Phone: +86 10 58813055

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Full Copyright Statement

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