Network Working Group                                        J. Yao, Ed.
Internet-Draft                                                     CNNIC
Expires: March 30, 2006                               September 26, 2005

           SMTP extension for internationalized email address

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   The internationalized email address (IMA) should be solved in the
   transport-level, which is an architecturally desirable approach.
   This document specifies the use of SMTP extension for
   internationalized email address (IMA) delivery.  And also mention
   about the backward compatible mechanism for downgrade procedure, as
   specified in an associated specification.  The protocol propoesed
   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 . . . . . . . . . . . . . . . . . . . . . . . . .  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 parameter  . . . . . . . . . . . . . . . .  5
     2.5.  Additional ESMTP Changes and Clarifications  . . . . . . .  6
       2.5.1.  The Initial SMTP Exchange  . . . . . . . . . . . . . .  6
       2.5.2.  Trace Fields . . . . . . . . . . . . . . . . . . . . .  6
   3.  Implementation Advice  . . . . . . . . . . . . . . . . . . . .  6
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   5.  Security considerations  . . . . . . . . . . . . . . . . . . .  7
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     7.1.  Normative References . . . . . . . . . . . . . . . . . . .  7
     7.2.  Informative References . . . . . . . . . . . . . . . . . .  8
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 10

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1.  Introduction

1.1.  Role of this specification

   An overview document [IMA-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 internationalized email
   address (IMA) 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 email
   address.  Domain part of email addresses may be internationalized
   through IDNA [RFC3490].  But the local part of 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 able to carry
   internationalized email address.  Since older servers and the mail-
   reading clients and other systems that are downstream from them will
   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 internationalized addresses in both the
   envelope and header fields of messages.  The context for the change
   is described in [IMA-overview] and the details of the header changes
   are described in [IMA-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
   IMA overview [IMA-overview] or in [RFC2821] and [RFC2822].

   This document is being discussed on the ima mailing list.  See for information about
   subscribing.  The list's archive is at

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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 Address";
   2.  The EHLO keyword value associated with this extension is "IMA";
   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 keyword did not appear.
   4.  An optional parameter is added to the SMTP MAIL and RCPT
       commands.  This parameter is named ALT-ADDRESS.  It requires an
       argument as a substitute for the internationalized (UTF-8 coded)
       address, which is discussed in [IMA-downgrading].  This all-ASCII
       address MAY incorporate the IDNA "punycode" form if the domain
       name is internationalized.  No algorithmic transformation is
       specified for the local-part; in the general case, it may
       identify a completely separate mailbox from the one identified in
       the primary command argument.  The domain part of the ALT-ADDRESS
       may either be the same one as in the primary address (or its
       punycode equivalent) or may be completely different.
   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 be
   processed into punycode form as specified in IDNA [RFC3490] 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 IDNA.

   An SMTP Client that receives the IMA extension keyword MAY transmit a
   mailbox name as an internationalized string in UTF-8 form.  It MAY

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   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 SMTP extension is
   not offered by the Server, the SMTP Client MUST not transmit an
   internationalized address.  Instead, it MUST either return the
   message to the user as undeliverable or replace it.  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 another source that conforms to the syntax rules of RFC 2821.

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.

2.4.  The ALT-ADDRESS parameter

   If the IMA extension is offered, the syntax of the SMTP MAIL and RCPT

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   commands is extended to support the optional "ALT-ADDRESS" parameter,
   which is specified in an associated document [IMA-downgrading].

2.5.  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.

2.5.1.  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 internationalized
   addresses 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

2.5.2.  Trace Fields

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

3.  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

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   widely deployed but backward-compatibility considerations require
   that it continue to be supported.

4.  IANA Considerations

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

5.  Security considerations

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

6.  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.

7.  References

7.1.  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.

              Klensin, J. and Y. Ko, "Overview and Framework of
              Internationalized Email Address Delivery",
              draft-klensin-ima-framework-00 (work in progress),
              October 2005.

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

   [RFC1652]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.

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              Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
              RFC 1652, July 1994.

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

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

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

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

   [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.

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

7.2.  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.

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Author's Address

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

   Phone: +86 10 58813007

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