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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 5336

Network Working Group                                        J. Yao, Ed.
Internet-Draft                                               W. Mao, Ed.
Intended status: Informational                                     CNNIC
Expires: April 26, 2007                                 October 23, 2006


           SMTP extension for internationalized email address
                     draft-ietf-eai-smtpext-02.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   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.  So



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   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 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 specification.  The protocol
   proposed here is MTA-level solution which is feasible,
   architecturally more elegant, and not as difficult to deploy in
   relevant communities.







































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Role of this specification . . . . . . . . . . . . . . . .  4
     1.2.  Proposal Context . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Mail Transport-level Protocol  . . . . . . . . . . . . . . . .  5
     2.1.  Framework for the Internationalization Extension . . . . .  5
     2.2.  The Address Internationalization Service Extension . . . .  5
     2.3.  Extended Mailbox Address Syntax  . . . . . . . . . . . . .  6
     2.4.  The ALT-ADDRESS parameter  . . . . . . . . . . . . . . . .  7
     2.5.  The Suggestion of the Value of the ALT-ADDRESS
           parameter  . . . . . . . . . . . . . . . . . . . . . . . .  8
     2.6.  Body Parts and SMTP Extensions . . . . . . . . . . . . . .  9
     2.7.  Additional ESMTP Changes and Clarifications  . . . . . . .  9
       2.7.1.  The Initial SMTP Exchange  . . . . . . . . . . . . . . 10
       2.7.2.  Trace Fields . . . . . . . . . . . . . . . . . . . . . 10
       2.7.3.  Mailing List Question  . . . . . . . . . . . . . . . . 10
       2.7.4.  Message Header Label . . . . . . . . . . . . . . . . . 10
       2.7.5.  POP and IMAP . . . . . . . . . . . . . . . . . . . . . 10
       2.7.6.  SMTP Service Extension for DSNs  . . . . . . . . . . . 11
   3.  Potential problems . . . . . . . . . . . . . . . . . . . . . . 11
     3.1.  Impact to IRI  . . . . . . . . . . . . . . . . . . . . . . 11
     3.2.  Impact to RFC 2476 and many email related RFC  . . . . . . 11
   4.  Implementation Advice  . . . . . . . . . . . . . . . . . . . . 11
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   6.  Security considerations  . . . . . . . . . . . . . . . . . . . 12
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  Change History . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  draft-ietf-eai-smtpext: Version 00 . . . . . . . . . . . . 12
     8.2.  draft-ietf-eai-smtpext: Version 01 . . . . . . . . . . . . 12
     8.3.  draft-ietf-eai-smtpext: Version 02 . . . . . . . . . . . . 12
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
   Intellectual Property and Copyright Statements . . . . . . . . . . 16














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1.  Introduction

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.  Domain part of the email address has been internationalized
   through IDNA [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 older SMTP servers and the
   mail-reading clients and other systems that are downstream from them
   MAY not be prepared to handle these extended addresses, an SMTP
   extension is specified to identify and protect the addressing
   mechanism.

   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 is being discussed on the EAI mailing list.  See



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   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";
   2.  The EHLO keyword value associated with this extension is
       "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.
       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.



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   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 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 as


         Mailbox = Local-part "@" Domain

         Local-part = Dot-string / Quoted-string
               ; MAY be case-sensitive

         Dot-string = Atom *("." Atom)

         Atom = 1*atext

         Quoted-string = DQUOTE *qcontent DQUOTE

         Domain = (sub-domain 1*("." sub-domain)) / address-literal
         sub-domain = Let-dig [Ldh-str]


   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



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      permitted in "atext"; it is otherwise unrestricted.

   According to the description above, define the syntax of an
   internationalized email mailbox with ABNF [RFC4234] as


         Mailbox = Local-part "@" Domain

         Local-part = Dot-string / Quoted-string
               ; MAY be case-sensitive

         Dot-string = Atom *("." Atom)

         Atom = 1*Ucharacter
         Ucharacter = atext / UTF8-2 / UTF8-3 / UTF8-4

         Quoted-string = DQUOTE *qcontent DQUOTE

         Domain = (sub-domain 1*("." sub-domain)) / address-literal
         sub-domain = ULet-dig [ULdh-str]

         ULet-dig = Let-dig / Non-ASCII

         ULdh-str = *( ALPHA / DIGIT / "-" / Non-ASCII) ULet-dig

         Non-ASCII = UTF8-2 / UTF8-3 / UTF8-4



   Where "atext", "qcontent" and "DQUOTE" are defined in [RFC2822],
   "Let-dig", "Ldh-str" and "address-literal" are defined in [RFC2821]
   and UTF8-2, UTF8-3 and UTF8-4 are defined in [RFC3629].  The value of
   "domain" SHOULD be verified with [RFC3490]; If failed, the email
   address with that domain 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 "ALT-ADDRESS"
   parameter.

   The "ALT-ADDRESS" requires an all-ASCII address.

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




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         MAIL FROM:<reverse-path> [ SP <mail-parameters> ] <CRLF>
         RCPT TO:<forward-path> [ SP <rcpt-parameters> ] <CRLF>
         Mail-parameters = esmtp-param *(SP esmtp-param)
         Rcpt-parameters = esmtp-param *(SP esmtp-param)
         esmtp-param     = esmtp-keyword ["=" esmtp-value]
         esmtp-keyword   = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
         esmtp-value     = 1*(%d33-60 / %d62-127)
               ; any CHAR excluding "=", SP, and control characters
         Reverse-path = Path
         Forward-path = Path
         Path = "<" [ A-d-l ":" ] Mailbox ">"
         A-d-l = At-domain *( "," A-d-l )
            ; Note that this form, the so-called "source route",
            ; MUST BE accepted, SHOULD NOT be generated, and SHOULD be
            ; ignored.
         At-domain = "@" domain


   where the value of esmtp-keyword is "ALT-ADDRESS" and the value of
   esmtp-value is all-ASCII email address, and where the domain and
   Mailbox are defined at section 2.3 of this document.

   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 to the original sender.  If the
   email is bounced 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 transformed the non-ASCII address to the ASCII
   Compatible Encoding(ACE) address as the value of the ALT-ADDRESS.
   The big problem 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]



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   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.



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   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.  Trace Fields

   Internationalized domain names in Received fields MUST be transmitted
   in the punycode form.  Addresses in "for" clauses need further
   examination and might be treated differently depending on
   [EAI-utf8header].  The reasoning in the introductory portion of
   [EAI-overview] strongly suggests that these addresses be in UTF-8
   form, rather than some specialized encoding.

2.7.3.  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].

2.7.4.  Message Header Label

   The message header label MAY 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].

2.7.5.  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].




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   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

   How to facilitate the use of SMTP Service Extension for DSNs
   [RFC3461] in the work of EAI will be addressed in the [EAI-dsn].


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 and many email related RFC

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


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 certain 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.







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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.


8.  Change History

   [[anchor20: 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.]]

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 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.  References



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9.1.  Normative References

   [ASCII]    American National Standards Institute (formerly United
              States of America Standards Institute), "USA Code for
              Information Interchange", ANSI X3.4-1968, 1968.

              ANSI X3.4-1968 has been replaced by newer versions with
              slight modifications, but the 1968 version remains
              definitive for the Internet.

   [EAI-downgrading]
              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", 12 2006, <http://
              www.ietf.org/internet-drafts/draft-ietf-eai-dsn-00.txt>.

   [EAI-imap]
              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]
              Chung, E., "Mailing Lists and Internationalized Email
              Addresses", June 2006.

              Forthcoming

   [EAI-overview]
              Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", draft-ietf-eai-framework-02.txt
              (work in progress), October 2006.

   [EAI-pop]  Newman, C., "POP3 Support for UTF-8", June 2006, <http://
              www.ietf.org/internet-drafts/draft-ietf-eai-pop-00.txt>.

   [EAI-utf8header]
              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.



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              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.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              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



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              Identifiers (IRIs)", RFC 3987, January 2005.

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

9.2.  Informative References

   [Klensin-emailaddr]
              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)
   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: mao@cnnic.cn
















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