Network Working Group                                        J. Yao, Ed.
Internet-Draft                                               W. Mao, Ed.
Expires: August 16, September 4, 2007                                         CNNIC
                                                       February 12,
                                                           March 3, 2007

           SMTP extension for internationalized email address

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   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
   protocols SMTP and ESMTP provide a negotiation mechanism through
   which clients can make decisions for further processing.  This
   document specifies the use of SMTP extension for internationalized
   email address delivery.  It also mentions the backward compatible
   mechanism for downgrade procedure, as specified in an associated

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Role of this specification . . . . . . . . . . . . . . . .  3
     1.2.  Proposal Context . . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Mail Transport-level Protocol  . . . . . . . . . . . . . . . .  4
     2.1.  Framework for the Internationalization Extension . . . . .  4
     2.2.  The Address Internationalization Service Extension . . . .  5
     2.3.  Extended Mailbox Address Syntax  . . . . . . . . . . . . .  5
     2.4.  The ALT-ADDRESS parameter  . . . . . . . . . . . . . . . .  8
     2.5.  The Suggestion of the Value of the ALT-ADDRESS
           parameter  . . . . . . . . . . . . . . . . . . . . . . . .  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.  Message Retry  . . . . . . . . . . . . . . . . . . . . 10
       2.7.3.  Trace Information  . . . . . . . . . . . . . . . . . . 11
       2.7.4.  Mailing List Question  . . . . . . . . . . . . . . . . 12
       2.7.5.  Message Header Label . . . . . . . . . . . . . . . . . 12
       2.7.6.  POP and IMAP . . . . . . . . . . . . . . . . . . . . . 12
       2.7.6.  SMTP Service Extension for DSNs  . . . . . . . . . . . 13 12
   3.  Potential problems . . . . . . . . . . . . . . . . . . . . . . 13 12
     3.1.  Impact to IRI  . . . . . . . . . . . . . . . . . . . . . . 13
     3.2.  Impact to RFC 2476 4409 and many email related RFC  . . . . . . 13
   4.  Implementation Advice  . . . . . . . . . . . . . . . . . . . . 13
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   6.  Security considerations  . . . . . . . . . . . . . . . . . . . 14 13
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   8.  Change History . . . . . . . . . . . . . . . . . . . . . . . . 14
     8.1.  draft-ietf-eai-smtpext: Version 00 . . . . . . . . . . . . 14
     8.2.  draft-ietf-eai-smtpext: Version 01 . . . . . . . . . . . . 14
     8.3.  draft-ietf-eai-smtpext: Version 02 . . . . . . . . . . . . 15 14
     8.4.  draft-ietf-eai-smtpext: Version 03 . . . . . . . . . . . . 14
     8.5.  draft-ietf-eai-smtpext: Version 04 . . . . . . . . . . . . 15
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 19 18

1.  Introduction

   Internationalized email address is different from the
   internationalized domain name (IDN).  It can be solved by exploiting
   the negotiation mechanism while IDN can not use the negotiation
   mechanism.  So internationalized email address SHOULD be solved in
   the mail transport-level using the negotiation mechanism, which is an
   architecturally desirable approach.  This document specifies a
   protocol to solve the problem of internationalized email address
   based on ESMTP.  The protocol proposed here is MTA-level solution
   which is feasible, architecturally elegant, and not as difficult to
   be deployed in relevant communities.

1.1.  Role of this specification

   An overview document [EAI-overview] specifies the requirements for,
   and components of, full internationalization of electronic mail.
   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

   In order to use internationalized email addresses, we need to
   internationalize both the domain part and the local part of the email
   address.  The domain part of the email address has been
   internationalized through IDNA RFC 3490 [RFC3490].  But the local
   part of the email address still remains as non-internationalized.

   The syntax of Internet email addresses is restricted to a subset of
   7-bit ASCII for the domain-part, with a less-restricted subset for
   the local-part.  These restrictions are specified in RFC 2821
   [RFC2821].  To be able to deliver internationalized email through
   SMTP servers, we need to upgrade SMTP server to be able to carry the
   internationalized email address.  Since the older SMTP servers, the
   mail-reading clients and other systems that are downstream from them
   might not be prepared to handle these extended addresses, an SMTP
   extension is specified to identify and protect the addressing

   This specification describes a change to the email transport
   mechanism that permits non-ASCII address in both the envelope and
   header fields of messages.  The context for the change is described
   in [EAI-overview] and the details of the header changes are described
   in [EAI-utf8header].

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 overview [EAI-overview] or in [RFC2821] and [RFC2822].  The terms
   "ASCII address", "internationalized email address", "non-ASCII
   address", "i18mail address", "UTF8SMTP", "message" and "mailing list"
   are used with the definitions from the EAI overview document.

   This document defines only those 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.

   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.
       If a parameter appears, the SMTP client that is conformant to
       this version of this specification MUST treat the ESMTP response
       as if the "UTF8SMTP" keyword did not appear.
   4.  An optional parameter is added to the SMTP MAIL and RCPT
       commands.  The parameter is named as ALT-ADDRESS.  The "ALT-
       ADDRESS" requires an all-ASCII address as a substitute for the
       i18mail addresses that we call the primary address; you can learn
       more in [EAI-overview] or [EAI-downgrading].  The value of "ALT-
       ADDRESS" is set by the sender when MUA and the Submission server
       have a communication.

   5.  No additional SMTP verbs are defined by this extension.
   6.  Servers offering this extension MUST provide support for, and
       announce, the 8BITMIME extension [RFC1652].

2.2.  The Address Internationalization Service 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 as 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.

   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
   internationalized mail header [EAI-utf8header].  It MAY transmit the
   domain part of that string in either punycode (derived from the IDNA
   process) or UTF-8 form.  If it sends the domain in UTF-8 form, the
   original SMTP client SHOULD first verify that the string is valid for
   a domain name according to IDNA rules.  As required by RFC 2821, it
   MUST not attempt to parse, evaluate, or transform the local part in
   any way if the UTF8SMTP SMTP extension is offered by the server.  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 body message which contains internationalized mail headers
   [EAI-utf8header].  Instead, it MUST either return the message to the
   user as undeliverable or replace it with the alternate ASCII address.
   If it is replaced, the replacement MUST be the ASCII-only address
   specified with the ALT-ADDRESS parameter.[EAI-downgrading].

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].  That label MUST NOT contain
      the characters "@" or ".", even though those characters can
      normally be inserted into a DNS label.
   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
         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 / Non-ASCII UTF8-non-ASCII
                   ; Replace character in RFC 2821, section 4.1.2
                   ; atext is defined in RFC 2822
                   ; UTF8-2, UTF8-3 and UTF8-4 are defined in RFC 3629

         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 / Non-ASCII UTF8-non-ASCII
                   ; 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 / Non-ASCII UTF8-non-ASCII
                   ; Let-dig in the right of '=' is defined in RFC 2822 2821, section 4.1.3

         uLdh-str = *( ALPHA / DIGIT / "-" / Non-ASCII) UTF8-non-ASCII) uLet-dig
                   ; Replace Ldh-str in RFC 2821, section 4.1.2

         Non-ASCII 4.1.3

         UTF8-non-ASCII = 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 [RFC3490]; If failed,
   the email address with that udomain can not be regarded as the valid
   email address.

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", to specify the conditions under which the SMTP server may
   use ALT-ADDRESS for the possible downgrading.  If the ALT-ADDRESS
   esmtp-keyword is used, it MUST have an associated esmtp-value (ALT-
   ADDRESS-esmtp-value which is defined below) which requires an all-
   ASCII email address.

   Based on the definition of mail-parameters in [RFC2821], the ALT-
   ADDRESS parameter usage in the commands of "mail from" and "rcpt to"
   is defined below.

        "MAIL FROM:" SP <uReverse-path> [ SP <mail-parameters> ]<CRLF>
                   ; Update mail command in RFC 2821, section 3.3

            "RCPT TO:" SP <uForward-path> [ SP <rcpt-parameters> ]<CRLF>
                   ; Update rcpt command in RFC 2821, section 3.3

        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 = "<" [ uA-d-l A-d-l ":" ] uMailbox ">"
               ; Replace Path in RFC 2821, section 4.1.2

        uA-d-l = uAt-domain *( "," uA-d-l )
                   ; Replace A-d-l in RFC 2821, section 4.1.2

        uAt-domain = "@" udomain
               ; Replace At-domain is defined in RFC 2821, section 4.1.2
                   ; udomain uMailbox is defined in section 2.3 of this document

           ; Mailbox is defined in RFC 2821, section 4.1.2

   The use of the ALT-ADDRESS is specified below: If some involved SMTP
   servers can not support UTF8SMTP capability and if the sender has
   already set the ALT-ADDRESS value, the client SMTP server will use
   this address as the email address when the SMTP server does the
   subsequent operations.  If the ALT-ADDRESS value is not set by the
   sender, the email must be bounced rejected to the original sender.  If the
   email is bounced rejected due to the incapability of supporting UTF8SMTP, the
   relative server should issue the response error code "5.3.3" defined
   in [RFC3463] which means that System is not capable of selected
   features, permanent failure.

2.5.  The Suggestion of the Value of the ALT-ADDRESS parameter

   The "ALT-ADDRESS" requires an all-ASCII address.  There are two
   alternative ways to set ALT-ADDRESS value: one is set by the sender
   using the all-ASCII address, the other is set using the transformed
   email address.

   Some may prefer transforming the non-ASCII address to the ASCII
   Compatible Encoding(ACE) address as the value of the ALT-ADDRESS.  A
   significant obstacle with applying an ACE to all local-parts is that
   the sending or converting system doesn't know if there are some
   specific data or instructions embedded in the address that the ACE
   process would hide.  Some SMTP servers may depend on these specific
   data or instructions to do some operations while the local parts
   applied with ACE will lose or hide these data or instructions.  SMTP
   [RFC2821] prohibits SMTP relays from converting local parts because
   the level of SMTP relays' knowledge on the structure of local parts
   is assumed to be zero.  However, we can raise the knowledge level by
   supplying additional information.  Many human users' email addresses
   do not have any embedded structure processed by the final delivery
   MTA.  In that case, the sender can specify that these email addresses
   are safe to be converted in the predefined way.  The final delivery
   SMTP server can revert the addresses even though they are as in all
   ASCII form.  Unless the MUA or the submission server clearly knows
   that the non-ASCII address can be safely transformed into the all-
   ASCII address, the non-ASCII address should not be transformed
   because transformed email address may cause some potential problems.

   This document suggests that the ALT-ADDRESS is set directly by the
   sender; In default, the all-ASCII address should not be gotten from
   the transformation of the non-ASCII address.

2.6.  Body Parts and SMTP Extensions

   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 these parameters on the MAIL command is:

   1.  Headers are in UTF-8, body parts are in ASCII.
   2.  Headers are in UTF-8, some or all body parts contain 8-bit line-
       oriented data.
   3.  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 punycode form if its raw form is 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 capability
   SMTP server.

2.7.1.  The Initial SMTP Exchange

   When an SMTP or ESMTP connection is opened, the server sends a
   "banner" 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 hostname form,
   i.e., internationalized ones MUST be in punycode form.

2.7.2.  Message Retry

   When an MSA or MTA encounters a server that doesn't support UTF8SMTP
   while relaying a message that requires such support, it is
   RECOMMENDED that an alternate MX be tried, and/or the message is
   requeued for a later attempt, rather than immediately downgrading or
   bouncing.  If the message is requeued, the total elapsed time before
   bouncing or downgrading SHOULD be smaller than the value used for
   other SMTP error conditions such as host unreachable or persistent
   4xx response codes.

   This alternate-MX-or-retry-later technique SHOULD NOT be used when
   the message's return path is a non-ASCII address and the specific
   forward path being attempted is an ASCII address (because the
   implication that the delivery path normally supports UTF8SMTP does
   not hold in this case).

   The selection of submission servers is presumably under the control
   of the sender's client, while the selection of potential intermediate
   relays is under the control of the administration of the final
   delivery server.  Hence, there is a presumption, at least when the
   recipient address is non-ASCII, that the delivery path servers
   normally support UTF8SMTP (if the sender's client or MSA didn't
   support UTF8SMTP, the message would not have been accepted for
   delivery in the first place).  Thus, a lack of UTF8SMTP support is
   likely to be a temporary situation, such as a normal inbound server
   being down and a cooperating site acting as a backup MX.  If the lack
   of UTF8SMTP in the delivery path of a message is a temporary
   situation, and the message is sent successfully after retrying, then
   it was a good thing to do.  Of course, if there is always an ASCII-
   only SMTP server in the path, then retrying only adds delay to the
   failure (bounce (reject or downgrade).

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.  The most
   important use of Received: lines is for debugging mail faults.  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 protocl value will allow UTF8SMTP value

   uFor = "FOR" FWS 1*( Path uPath / uMailbox ) CFWS
           ; Replaces For in the section 4.4 of [RFC2821]
       ; uReverse-path is defined in Section 2.4

   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 punycode form.  The [With]'s protocl value
   will have the value of 'UTF8SMTP' for UTF8SMTP extension.  We will
   give more informaiton about this in "IANA consideration" section of
   this document.  If a "for" clause containing non-ASCII is encountered
   when downgrading a message, it is better to just drop the "for"
   clause rather than figure out some creative way to encode it.  When
   only the domain portion of a "for" clause address contains non-ASCII,
   this document suggests using the punycode form of the domain portion.
   For more detailed information, you may see it in [EAI-utf8header].

2.7.4.  Mailing List Question

   How a mixture of traditional and internationalized addresses on a
   mailing list will impact message flows, error reports, and delivery
   notifications in all plausible combinations of UTF8SMTP capability
   and un-capability servers is discussed and specified in the
   [EAI-mailing list].

2.7.5.  Message Header Label

   Today it is routine that many MTAs scan every message for spam, virus
   or other reasons.  It seems that few MTAs depend on "Header-Type"
   fields or marker to decide the message's type.  The better choice is
   to rely on scanning the message to decide the message's type:
   UTF8SMTP or ASCII, instead of the header label "Header-Type" fields
   or marker.  The message header label "Header-Type" SHOULD NOT be used
   to identify and distinguish the i18mail message from the normal
   message when SMTP messages are transmitted on wire.  This issue is
   discussed and specified in [EAI-utf8header].

2.7.6.  POP and IMAP

   While SMTP mainly takes care of the transportation of messages and
   the header fields on wire, POP essentially handles the retrieval of
   mail objects from the server by a client.  In order to use
   internationalized user names based on i18mail for the retrieval of
   messages from a mail server using the POP protocol, a new capability
   SHOULD be introduced following the POP3 extension mechanism
   [RFC2449].  This is discussed and specified in the [EAI-pop].

   IMAP [RFC3501] uses the traditional user name which is based on
   ASCII.  IMAP SHOULD be updated to support the internationalized user
   names based on i18mail for the retrieval of messages from a mail
   server.  This is discussed and specified in the [EAI-imap].


2.7.6.  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.  Potential problems

3.1.  Impact to IRI

   The mailto: schema in IRI [RFC3987] MAY need to be modified when EAI
   is standardized.

3.2.  Impact to RFC 2476 4409 and many email related RFC

   The EAI internationalized email address protocols will impact on many
   email related RFC documents such as Message Submission [RFC2476].  These [RFC4409].
   Those protocols SHOULD be considered when implementing the EAI protocol.
   internationalized email address protocols.

4.  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.

5.  IANA Considerations

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

   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                 [RFCxxxx]
  UTF8SMTPS            UTF8SMTP with STARTTLS                  [RFCxxxx]
  UTF8SMTPSA           UTF8SMTP with both STARTTLS and         [RFCxxxx]
                       SMTP AUTH

6.  Security considerations

   See the extended security considerations discussion in [EAI-overview]

7.  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 document, those includes (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. Gellens, Frank Ellermann.

8.  Change History


   [[anchor19: 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

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

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

8.2.  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".

8.3.  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.

8.4.  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.

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

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

9.  References

9.1.  Normative References

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

              YONEYA, Y., Ed. and K. Fujiwara, Ed., "Downgrading
              mechanism for Internationalized eMail Address (IMA)",
              draft-ietf-eai-downgrade-02 (work in progress),
              August 2006.

   [EAI-dsn]  Newman, C., "SMTP extensions for DSNs",
              draft-ietf-eai-dsn-00.txt (work in progress), 1 2007.

              Resnick, P. and C. Newman, "Considerations for IMAP in
              Conjunction with Email Address Internationalization",
              draft-ietf-eai-imap-utf8-00 (work in progress), May 2006.

   [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.

              Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", draft-ietf-eai-framework-04.txt
              (work in progress), 12 2006.

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

              Yeh, J., "Transmission of Email Headers in UTF-8
              Encoding", draft-ietf-eai-utf8headers-01.txt (work in
              progress), August 2006.

   [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.

   [RFC2449]  Gellens, R., Newman, C., and L. Lundblade, "POP3 Extension
              Mechanism", RFC 2449, November 1998.

   [RFC2476]  Gellens, R. and J. Klensin, "Message Submission",
              RFC 2476, December 1998.

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

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

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

   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

   [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.

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

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.

              4rev1", RFC 3501, March 2003.

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

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

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

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

9.2.  Informative References

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

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

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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