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Versions: 00 01                                                         
Network Working Group                                         J. Klensin
Expires: April 2, 2006                                             Y. Ko
                                                            MOCOCO, Inc.
                                                      September 29, 2005

           Overview and Framework for Internationalized Email

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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Copyright Notice

   Copyright (C) The Internet Society (2005).


   Full use of electronic mail throughout the world requires that people
   be able to use their own names, written correctly in their own
   languages and scripts, as mailbox names in email addresses.  This
   document introduces a series of specifications and operational
   suggestions that define mechanisms and protocol extensions needed to
   fully support internationalized email addresses.  These changes
   include an SMTP extension and extension of email header syntax to

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   accommodate UTF-8 data.  The document set also will include
   discussion of key assumptions and issues in deploying fully
   internationalized email.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Role of This Specification . . . . . . . . . . . . . . . .  3
     1.2.  Problem statement  . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Overview of the Approach . . . . . . . . . . . . . . . . . . .  5
   3.  Document Roadmap . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Overview of Protocol Extensions and Changes  . . . . . . . . .  6
     4.1.  SMTP Extension for Internationalized eMail Address . . . .  6
     4.2.  Transmission of Email Header in UTF-8 Encoding . . . . . .  6
     4.3.  Downgrading Mechanism for Backward Compatibility . . . . .  7
   5.  Advice to Designers and Operators of Mail-receiving Systems  .  7
   6.  Internationalization Considerations  . . . . . . . . . . . . .  8
   7.  Additional Issues  . . . . . . . . . . . . . . . . . . . . . .  8
     7.1.  Impact to IRI  . . . . . . . . . . . . . . . . . . . . . .  8
     7.2.  POP and IMAP . . . . . . . . . . . . . . . . . . . . . . .  8
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   11. Change History . . . . . . . . . . . . . . . . . . . . . . . . 10
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 10
     12.2. Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 14

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1.  Introduction

   In order to use internationalized email addresses, we need to
   internationalize both domain part and local part of email address.
   The domain part of email addresses is already internationalized
   [RFC3490], while the local part is not.  Without these extensions,
   the mailbox name is restricted to a subset of 7-bit ASCII in
   [RFC2821].  Though MIME enables the transport of non-ASCII data, it
   does not provide a mechanism for internationalized email address.
   [RFC2047] defines an encoding mechanism for some specific message
   header fields to accommodate non-ASCII data.  However, it does not
   address the issue of email addresses that include non-ASCII

1.1.  Role of This Specification

   This document presents the overview and framework for an approach to
   the next stage of email internationalization.  This new stage
   requires not only internationalization of addresses and headers, but
   also associated transport and delivery models.  The history of
   developments and design ideas leading to this specification is
   described in [IMA-history].

   This document describes how the various elements of email
   internationalization fit together and provides a roadmap for
   navigating the various documents involved.

1.2.  Problem statement

   [[anchor1: Note in draft: this section needs very significant
   reworking for both content and presentation.  Changed with -01c, but
   may still not be good enough]]

   Though domain names are already internationalized, the
   internationalized forms are far from general adoption by ordinary
   users.  One of the reasons for this is that we do not yet have fully
   internationalized naming schemes.  Domain names are just one of the
   various names and identifiers that are required to be

   Email addresses are a particularly important example of where
   internationalization of domain names alone is not sufficient.  Unless
   email addresses are presented to the user in familiar characters and
   formats, the user's perception will not be of internationalization
   and behavior that is culturally friendly.  One thing most of us have
   almost certainly learned from the experience with email usage is that
   users strongly prefer email addresses that closely resemble names or
   initials to those involving.  If the names or initials of the names

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   in the email address is expressed in their native languages, which
   will be very good news to those whose native language is not written
   in a subset of a Roman-derived script.

   Internationalization of email addresses is not merely a matter of
   changing the SMTP envelope, or of modifying the From, To, and Cc
   headers, or of permitting upgraded mail user agents (MUA) to decode a
   special coding and display local characters.  To be perceived as
   usable by end users, the addresses must be internationalized, and
   handled consistently, in all of the contexts in which they occur.
   That requirement has far-reaching implications: collections of
   patches and workarounds are not adequate.  Instead, we need to build
   a fully internationalized email environment, focusing on permitting
   efficient communication among those who share a language or other
   community.  That, in turn, implies changes to the mail header
   environment to permit the full range of Unicode characters where that
   makes sense, an SMTP extension to permit UTF-8 mail addressing and
   delivery of those extended headers, and (finally) a requirement for
   support of the 8BITMIME option so that all of this can be transported
   through the mail system without having to overcome the limitation
   that headers not have content-transfer-encodings.

1.3.  Terminology

   This document assumes a reasonable understanding of the protocols and
   terminology of the core email standards as documented in [RFC2821]
   and [RFC2822].

   Much of the description in this document depends on the abstractions
   of "Mail Transfer Agent" ("MTA") and "Mail User Agent" ("MUA").
   However, it is important to understand that those terms and the
   underlying concepts postdate the design of the Internet's email
   architecture and the "protocols on the wire" principle.  That email
   architecture, as it has evolved, and the "wire" principle have
   prevented any strong and standardized distinctions about how MTAs and
   MUAs interact on a given origin or destination host (or even whether
   they are separate).

   In this document, an address is "all-ASCII" if every character in the
   address is in the ASCII character repertoire [ASCII]; an address is
   "non-ASCII" if any character is not in the ASCII character
   repertoire.  The term "all-ASCII" is also applied to other protocol
   elements when the distinction is important, with "non-ASCII" or
   "internationalized" as its opposite.

   The term "internationalized email address", or "IMA", refers to an
   address permitted by this specification. [[anchor3: Note in Draft/
   Placeholder: it appears that the term "IMA" is not used in a precise

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   and consistent way across the document set.  It is sometimes used to
   refer simply to a "non-ASCII" address; sometimes to an address that
   contains non-ASCII characters, even if that address is encoded into
   ASCII characters (i.e., as an ACE); and sometimes as an address that
   may contain non-ASCII characters but may also be a traditional
   adress.  The definition needs to be clarified in an upcoming draft
   and all uses of the term brought into line with the definition.]]

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

2.  Overview of the Approach

   This set of specifications changes both SMTP and the format of email
   headers to permit non-ASCII characters to be represented directly.
   Each important component of the work is described in a separate
   document.  The document set, whose members are described in the next
   section, also contains informational documents whose purpose is to
   provide operational and implementation suggestions and guidance for
   the protocols.

3.  Document Roadmap

   In addition to this document, the following documents make up this
   specification and provide advice and context for it.

   o  SMTP extensions.  This document provides an SMTP extension for
      internationalized addresses, as provided for in RFC 2821 [IMA-
   o  Email headers in UTF-8.  This document essentially updates RFC
      2822 to permit some information in email headers to be expressed
      directly by Unicode characters encoded in UTF-8 when the SMTP
      extension is used [IMA-UTF8].
   o  Downgrading from internationalized addressing with the SMTP
      extension and UTF-8 headers to traditional email formats and
      characters [IMA-downgrade].
   o  Operational guidelines and suggestions for the deployment of
      internationalized email [IMA-ops].
   o  Special considerations for mailing lists and similar distributions
      during the transition to internationalized email [IMA-Exploder].
   o  Design decisions, history, and alternative models for
      internationalized Internet email [IMA-history].

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4.  Overview of Protocol Extensions and Changes

4.1.  SMTP Extension for Internationalized eMail Address

   An SMTP extension, "IMA" is specified that
   o  Permits the use of UTF-8 strings in email addresses, both local
      parts and domain names
   o  Permits the selective use of UTF-8 strings in email headers (see
      the next subsection)
   o  Requires support for the 8BITMIME extension so that header
      information can be transmitted without using a special content-

   Some general principles apply to this work.
   1.  Whatever encoding is used should apply to the whole address and
       be directly compatible with software used at the user interface.
   2.  An SMTP relay must
       *  Either recognize the format explicitly, agreeing to do so via
          an ESMTP option,
       *  Select and use an ASCII-only address, or
       *  Bounce the message so that the sender can make another plan.
   3.  In the interest of interoperability, charsets other than UTF-8
       are prohibited.  There is no practical way to identify them
       properly with an extension like this without introducing great

4.2.  Transmission of Email Header in UTF-8 Encoding

   [[anchor8: Note in Draft: Much better than earlier version and good
   enough for now.  It could still benefit from a further rework in
   -01.]]  There are many places in MUAs or in user presentation in
   which email addresses or domain names appear.  Examples include the
   conventional From, To, or Cc header fields; Message-IDs; In-Reply-To
   fields that may contain addresses or domain names; in message bodies;
   or elsewhere.  We must examine all of them from an
   internationalization perspective.  The user will expect to see
   mailbox and domain names in local characters, and to see them
   consistently.  Variations on that problem will exist with any
   internationalization method, whether transport or MUA-only in
   structure.  Perhaps, if we have to live with it for a short time as a
   transition activity, that is worthwhile.  But the only practical way
   to avoid it, in both the medium and the longer term, is to have the
   encodings used in transport be as nearly as possible the same as the
   encodings used in message headers and message bodies.

   It seems clear that the point at which email local parts are
   internationalized is the point that email headers should simply be
   shifted to a full internationalized form, presumably using UTF-8

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   rather than ASCII as the base character set for other than protocol
   elements such as the header field names themselves.  The transition
   to that model includes support for address, and address-related,
   fields within the headers of legacy systems.  This is done by
   extending the encoding models of [RFC2045] and [RFC2231].  However,
   our target should be fully internationalized headers, as discussed

4.3.  Downgrading Mechanism for Backward Compatibility

   As with any use of the SMTP extension mechanism, there is always a
   possibility of a client that requires the feature encountering a
   server that does not.  In the case of IMA, the risk should be
   minimized by the fact that the selection of submission servers are
   presumably under the control of the client and the selection of
   potential intermediate relays is under the control of the
   administration of the final delivery server.

   For those situations, there are basically two possibilities:
   o  Reject or bounce the message, requiring the sender to resubmit it
      with traditional-format addresses and headers.
   o  Figure out a way to downgrade the envelope or message body in
      transit.  Especially when internationalized addresses are
      involved, downgrading will require either that an all-ASCII
      address be obtained from some source or computed.  An optional
      extension parameter is provided as a way of transmitting an
      alternate address.  Computing an ASCII form of an IMA address
      requires that the sender have some knowledge that is normally
      restricted to final delivery servers, but save extensions may be
      feasible there too.  Downgrade issues and a specification are
      discussed in [IMA-downgrade].

   The first of these two options, that of rejecting or returning the
   message to the sender MAY always be chosen.

   There is also a third case, one in which the client is IMA-capable,
   the server is not, but the message does not require the extended
   capabilities.  In other words, both the addresses in the envelope and
   the entire set of headers of the message are entirely in ASCII
   (perhaps including encoded-words in the headers).  In that case, the
   client SHOULD send the message whether or not the server announces
   the IMA capability.

5.  Advice to Designers and Operators of Mail-receiving Systems

   [[anchor10: Note in draft: The material that follows contains some
   forward-looking, predictive, statements.  Be sure they are true

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   before Last Call.]]

   In addition to the protocol specification materials in this set of
   documents, the working group has had extensive discussions about
   operational considerations in the use of internationalized addresses.
   Those topics include how such addresses should be chosen, how they
   should relate to ASCII alternatives if such alternatives exist, the
   management of mailing lists that might support and contain a mixture
   of all-ASCII and non-ASCII addresses, and so on.  Those issues are
   discussed in [IMA-ops] and [IMA-Exploder].

6.  Internationalization Considerations

   This entire specification addresses issues in internationalization
   and especially the boundaries between internationalization and
   localization and between network protocols and client/user interface

7.  Additional Issues

   This section identifies issues that are not covered as part of this
   set of specifications, but that will need to be considered as part of
   IMA deployment.

7.1.  Impact to IRI

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

7.2.  POP and IMAP

   While SMTP takes care of the transportation of messages, IMAP
   [RFC3501] and POP3 [RFC1939] are among mechanisms used to handle the
   retrieval of mail objects from a mail store by a client.  The use of
   internationalized mail addresses or UTF-8 headers will require
   extensions to POP and IMAP and/or modifications to the design and
   implementation of mail stores and the mechanisms that final delivery
   SMTP servers use to put mail into them.  However, those mechanisms
   are separate from those associated with transport across the network
   and are not discussed in this series of documents.  The general
   issues are covered in [IMA-imap-pop].

8.  IANA Considerations

   This specification does not contemplate any IANA registrations or

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   other actions.

9.  Security Considerations

   Any expansion of permitted characters and encoding forms in email
   addresses raises some risks.  There have been discussions on so
   called "IDN-spoofing".  IDN homograph attacks allow an attacker/
   phisher to spoof the domain/URLs of businesses.  The same kind of
   attack is also possible on the local part of internationalized email
   addresses.  It should be noted that one of the proposed fixes for,
   e.g., URLs, does not work for email local parts since they are case-
   sensitive.  That fix involves forcing all elements that are displayed
   to be in lower-case and normalized,

   Since email addresses are often transcribed from business cards and
   notes on paper, they are subject to problems arising from confusable
   characters.  These problems are somewhat reduced if the domain
   associated with the mailbox is unambiguous and supports a relatively
   small number of mailboxes whose names follow local system
   conventions; they are increased with very large mail systems in which
   users can freely select their own addresses.

   The internationalization of email addresses and headers must not
   leave the Internet less secure than it is that without the required
   extensions.  The requirements and mechanisms documented in this set
   of IMA specifications do not, in general, raise any new security
   issues other than those associated with confusable characters -- a
   topic that is being explored thoroughly elsewhere. [[anchor16: Note
   in Draft: If the IAB-IDN report is completed and published, a
   reference to it should go here.]]  Specific issues are discussed in
   more detail in the other documents in this set.  However, in
   particular, caution should be taken that any "downgrading" mechanism,
   or use of downgraded addresses, does not inappropriately assume
   authenticated bindings between the IMA and ASCII addresses.

   In addition, email addresses are used in many contexts other than
   sending mail, such as for identifiers under various circumstances.
   Each of those contexts will need to be evaluated, in turn, to
   determine whether the use of non-ASCII forms is appropriate and what
   particular issues they raise.

10.  Acknowledgements

   This document, and the related ones, were originally derived from
   drafts by John Klensin and the JET group [Klensin-emailaddr], [JET-
   IMA].  The work drew inspiration from discussions on the "IMAA"

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   mailing list, sponsored by the Internet Mail Consortium and
   especially from an early draft by Paul Hoffman and Adam Costello
   [Hoffman-IMAA] that attempted to define an MUA-only solution to the
   IMA problem. [[anchor18: Note in draft: may want to move some of this
   to "history" or reference it]]

11.  Change History

   [[anchor20: Note to RFC Editor: this section to be removed prior to

   Version 00 This version supercedes draft-lee-jet-ima-00 and
      draft-klensin-emailaddr-i18n-03.  It represents a major rewrite
      and change of architecture from the former and incorporates many
      ideas and some text from the latter.

12.  References

12.1.  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.

              "Placeholder: whatever we call the mailing list document",

              Yao, J., Ed., "SMTP Extension for Internationalized Email
              Address", draft-yao-smtpext-00 (work in progress),
              September 2005.

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

              YONEYA, Y., Ed., "Placeholder: whatever we call the
              downgrading document", October 2005.

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   [IMA-ops]  "Placeholder: whatever we call the operations document",

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

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

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

12.2.  Informative References

              Hoffman, P. and A. Costello, "Internationalizing Mail
              Addresses in Applications (IMAA)", draft-hoffman-imaa-03
              (work in progress), October 2003.

              Klensin, J., "Decisions and Alternatives for
              Internationalization of Email Addresses", Internet-
              Draft forthcoming, September 2005.

              Klensin, J., "Considerations for IMAP and POP in
              Conjunction with Email Address Internationalization",
              draft-klensin-ima-imappop-00a (work in progress),
              October 2005.

   [JET-IMA]  Yao, J. and J. Yeh, "Internationalized eMail Address
              (IMA)", draft-lee-jet-ima-00 (work in progress),
              June 2005.

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

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

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

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)

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              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

   [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
              Word Extensions: Character Sets, Languages, and
              Continuations", RFC 2231, November 1997.

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

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

              4rev1", RFC 3501, March 2003.

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

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Authors' Addresses

   John C Klensin
   1770 Massachusetts Ave, #322
   Cambridge, MA  02140

   Phone: +1 617 491 5735
   Email: john-ietf@jck.com

   YangWoo Ko
   MOCOCO, Inc.
   996-1, 11F, Mirae Asset Venture Tower, Daechi-dong
   Gangnam-gu, Seoul  135-280

   Email: yw@mrko.pe.kr

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Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
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Disclaimer of Validity

   This document and the information contained herein are provided on an

Copyright Statement

   Copyright (C) The Internet Society (2005).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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