Network Working Group                                        J. Yao, Ed.
Internet-Draft                                               W. Mao, Ed.
Expires: November 13, 2006 January 27, 2007                                          CNNIC
                                                            May 12,
                                                           July 26, 2006

           SMTP extension for internationalized email address

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   Internationalized eMail Address (IMA) email address includes two parts, the local part
   and the domain part.  The way email addresses are used by protocols
   are different from the way 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
   internationalized email address is different from the
   internationalized domain name (IDN).  IMA  It can be solved by exploiting
   the negotiation mechanism while IDN can not use the negotiation
   mechanism.  So IMA should 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 IMA 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.

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 Address Internationalization Service Extension . . . .  4
     2.3.  Extended Mailbox Address Syntax  . . . . . . . . . . . . .  5
     2.4.  The ALT-ADDRESS and ATOMIC parameter  . . . . . . . . . . . . . . . .  6
     2.5.  The Suggestion of the Value of the ALT-ADDRESS
           parameter  . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.6.  Additional ESMTP Changes and Clarifications  . . . . . . .  8
       2.6.1.  The Initial SMTP Exchange  . . . . . . . . . . . . . .  8
       2.6.2.  Trace Fields . . . . . . . . . . . . . . . . . . . . .  8
       2.6.3.  Mailing List Question  . . . . . . . . . . . . . . . .  8
       2.5.4.  9
       2.6.4.  Message Header Label . . . . . . . . . . . . . . . . .  8
   3.  Potential problems .  9
       2.6.5.  POP and IMAP . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Impact to IRI
   3.  Potential problems . . . . . . . . . . . . . . . . . . . . . .  9
     3.2.  POP and IMAP .
     3.1.  Impact to IRI  . . . . . . . . . . . . . . . . . . . . . .  9
     3.2.  Impact to RFC 2476 and many email related RFC  . . . . . .  9
   4.  Implementation Advice  . . . . . . . . . . . . . . . . . . . .  9 10
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   6.  Security considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13 14
   Intellectual Property and Copyright Statements . . . . . . . . . . 14 15

1.  Introduction

1.1.  Role of this specification

   An overview document [IMA-overview] [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 IMA 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 IMA. the
   internationalized email address.  Since older SMTP servers and the
   mail-reading clients and other systems that are downstream from them may
   MAY 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 IMA non-ASCII address in both the envelope and
   header fields of messages.  The context for the change is described
   in [IMA-overview] [EAI-overview] and the details of the header changes are described
   in [IMA-
   utf8header]. [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 [IMA-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 IMA 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 "Internationalized
       Email and Extensions";
   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 IMA "UTF8SMTP" keyword did not appear.
   4.  Two  An optional parameters are is added to the SMTP MAIL and RCPT
       commands.  The first parameter is named as ALT-ADDRESS.  The
       second is ATOMIC.  The "ALT-ADDRESS" "ALT-
       ADDRESS" requires an all-ASCII address as a substitute for the internationalized (UTF-8 coded)
       i18mail addresses that we call the primary address; you can learn
       more in
       [IMA-overview] [EAI-overview] or [IMA-downgrading]. [EAI-downgrading].  The value of "ALT-ADDRESS"
       may be "ALT-
       ADDRESS" is set by sender or be gotten by using some algorithmic
       transformation according to the value of "ATOMIC".  The "ATOMIC"
       has one of two values: y or n.  The parameter "ATOMIC" is
       designed to assert whether the address is atomic, which means
       that the primary address(IMA) can be safely transformed or
       converted to the respect ASCII email address via ACE (ASCII
       Compatible Encoding) if the value is 'y' or not if sender when MUA and the value is
       'n'. submit ion 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 MAY appear.  That string must 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 IMA 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 [IMA-utf8header]. [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 IMA UTF8SMTP SMTP extension is offered by the server.  If
   the IMA 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 [IMA-utf8header].
   [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 either the ASCII-only address
   specified with the ALT-ADDRESS
   parameter or with an address obtained from some algorithmic
   conversions of the primary address that conforms to the syntax rules
   of RFC 2821, which is defined in [IMA-downgrading]. 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
      permitted in "atext"; it is otherwise unrestricted.

   According to the description above, define the syntax of an IMA
   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
                       ; UTF-8 characters prohibited by nameprep
                       ; MUST NOT be used.

   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
   "Local-part" should pass Stringprep [RFC3454]; The value of
   should SHOULD be verified with [RFC3490]; If failed, The value of "Local-
   part" and "domain", the email
   address with that domain can not be regarded as the valid email

2.4.  The ALT-ADDRESS and ATOMIC parameter

   If the IMA UTF8SMTP extension is offered, the syntax of the SMTP MAIL and
   RCPT commands is extended to support both the optional "ALT-ADDRESS" and

   The "ALT-ADDRESS" requires an all-ASCII address, which may set by address.

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

         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 algorithmic transformation. 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]
   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 predefined way.  The final delivery SMTP server
   can revert the addresses even though they are as in all ASCII form.
   In such cases, a potential recipient might be able to tell someone to
   Unless the address is given "it is ok, there is no embedded information
   here and you can convert it to an ACE address without danger".  If
   the recipient says that, then if the sender can pass that assertion
   along to his MUA or her own (originator) MTA and the MTA can pass it down
   the line, then an MTA that needs to do downgrading would know submission server clearly knows that
   ACE-encoding is safe.  The "ATOMIC" parameter is designed for the
   above aim.  Transmission of local-parts of UTF-8 avoids having to
   deal with the problem.

   The use of the ALT-ADDRESS will be according to the following
   priority if SMTP servers non-
   ASCII address can not support IMA capability.  If the
   sender has already set the ALT-ADDRESS value in spite of the value of
   ATOMIC, be safely transformed into the client SMTP server will use this address as all-ASCII address,
   the email non-ASCII address when the SMTP server does the subsequent operations.  If the
   ALT-ADDRESS value is not set by the sender but the value of ATOMIC is
   'y', the sender SMTP server should apply some algorithmic
   transformation such as punycode to the entire local part of IMA; IDNA should also not be applied to the domain part of IMA; these operations
   will get an ASCII transformed because transformed
   email address for the subsequent SMTP operations
   related to the email address.  If may cause some potential problems.

   This document suggests that the ALT-ADDRESS value is not set directly by the sender and the value of ATOMIC is 'n' which means that the local
   part of IMA can not be converted to
   sender; In default, the ASCII email all-ASCII address safely,
   the email must be bounced to the original sender.

   The suggested algorithmic transformation is punycode if the value of
   ALT-ADDRESS is not set by sender and the value of ATOMIC is 'y' when
   SMTP servers can not support IMA.  Since the prefix "xn--" had been
   used for IDNA, it is better that other prefix such as "bq--" is used
   for the local part of converted version of the primary address to
   avoid the potential confusion.

2.5. be transformed.

2.6.  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 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 MUST be advertised by the IMA UTF8SMTP capability
   SMTP server.


2.6.1.  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 IMA UTF8SMTP until after
   it receives the response from EHLO, any domain names that appear in
   this dialogue, or in responses to EHLO, must MUST be in hostname form,
   i.e., internationalized ones must MUST be in punycode form.


2.6.2.  Trace Fields

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


2.6.3.  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 IMA UTF8SMTP capability
   and un-
   capability un-capability servers is still not clear.  This is an issue, which we
   can delve into in detail in the future discussion.  We will proposed
   the detail solution to it in another document, discussed and do some
   experiments to find specified in the best solution to it.

2.5.4. [EAI-
   mailing list].

2.6.4.  Message Header Label

   There is a hot discussion about

   The message header label when SMTP
   messages are transmitted on wire.  How MAY be used to identify them and distinguish them the
   i18mail message from the normal message.  Many referred the famous
   "MIME-Version:1.0" as the example.  In order to get the robustness in
   the absence of context, we should consider the issue whether or not
   we need a mechanism(such as self-label) or some indicator to
   distinguish or recognize the format of a "stored" message: new
   format(i.e.  IMA compliant) or old one (i.e.  RFC 822 compliant).
   [Note in draft: The detail discussion of this issue will be available
   in [IMA-utf8header].]

3.  Potential problems

3.1.  Impact to IRI

   The mailto: schema in IRI [RFC3987] may need to be modified message when IMA SMTP messages are
   transmitted on wire.  This issue is standardized.

3.2. discussed and specified in [EAI-

2.6.5.  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 IMA 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 SHOULD be updated to support the internationalized user
   names based on IMA i18mail for the retrieval of messages from a mail

3.3.  This is discussed and specified in the [EAI-imap].

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 IMA EAI protocol will impact on many email related RFC such as
   Message Submission [RFC2476] and SMTP Service Extension for DSNs
   [RFC3461].  These protocol should 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 MAY be made up of any ASCII
   characters, although certain of them must 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 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 "IEmail" "UTF8SMTP" to the SMTP extensions registry
   with the entry pointing to this specification for its definition.

6.  Security considerations

   See the extended security considerations discussion in [IMA-overview] [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.

8.  References

8.1.  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.


              YONEYA, Y., Ed. and K. Fujiwara, Ed., "Downgrading
              mechanism for Internationalized eMail Address (IMA)",
              draft-ietf-eai-downgrade-00 (work in progress),
              October 2005.

              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.


              Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", draft-klensin-ima-framework-01 draft-ietf-eai-framework-01.txt
              (work in progress), February June 2006.

              Klensin, J. and J.

   [EAI-pop]  Newman, C., "POP3 Support for UTF-8", June 2006, <http://

              Yeh, J., "Transmission of Email Headers in UTF-8
              Encoding", draft-yeh-utf8headers-00 draft-ietf-eai-utf8headers-00.txt (work in
              progress), October 2005. June 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.

   [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.

8.2.  Informative References

              YONEYA, Y. and K. Fujiwara, "Downgrade Mechanism for
              Internationalized Email Address (IMA)",
              draft-yoneya-ima-downgrade-00 (work in progress),
              October 2005.

              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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