Email Address Internationalization                            J. Klensin
Internet-Draft                                                     Y. Ko
Expires: November 25, 2006                                  MOCOCO, Inc.
                                                            May 24, 2006

           Overview and Framework for Internationalized Email

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   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  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Overview of the Approach . . . . . . . . . . . . . . . . . . .  5
   3.  Document Roadmap . . . . . . . . . . . . . . . . . . . . . . .  6
   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 . . . . . .  7
     4.3.  Downgrading Mechanism for Backward Compatibility . . . . .  8
   5.  Downgrading Before and After SMTP Transactions . . . . . . . .  8
     5.1.  Downgrading Before or During Message Submission  . . . . .  9
     5.2.  Downgrading or Other Processing After Final SMTP
           Delivery . . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Advice to Designers and Operators of Mail-receiving Systems  . 10
   7.  Internationalization Considerations  . . . . . . . . . . . . . 10
   8.  Additional Issues  . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Impact on IRIs . . . . . . . . . . . . . . . . . . . . . . 11
     8.2.  POP and IMAP . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   12. Change History . . . . . . . . . . . . . . . . . . . . . . . . 12
     12.1. draft-klensin-ima-framework: Version 00  . . . . . . . . . 13
     12.2. draft-klensin-ima-framework: Version 01  . . . . . . . . . 13
     12.3. draft-ietf-eai-framework: Version 00 . . . . . . . . . . . 13
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 13
     13.2. Informative References . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
   Intellectual Property and Copyright Statements . . . . . . . . . . 19

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1.  Introduction

   [[anchor1: NOTE IN DRAFT: The next version of this document (-01)
   will include references that are updated as appropriate to utilize
   the new names of documents and a list of documents that are
   harmonized with the WG Charter.  This version is transitional and
   those reading it are asked to be tolerant of the transition.]]

   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
   characters.  Without the extensions defined here, or some equivalent
   set, the only way to incorporate non-ASCII characters in email
   addresses is to use RFC2047 coding to embed them in what RFC 2822
   [RFC2822] calls the "display name" (known as a "name phrase" or by
   other terms elsewhere) of the relevant headers.  Of course, that type
   of coding is invisible in the message envelope and would not be
   considered by many to be part of the address at all.

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 [I18Nemail-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

   [[anchor2: 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

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   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 meaningless strings of letters or
   numbers.  If the names or initials of the names in the email address
   can be expressed in the native languages and writing systems of the
   users, the Internet will be perceived as more natural by those whose
   native language is not written in a subset of a Roman-derived script
   (this is the same collection of characters known as "Latin" in
   Unicode Consortium and ISO/IEC JTC1 publications.  In much of the
   linguistic literature, the term "Latin Script" is used exclusively
   for the characters used to write the Latin language at the time of
   the Roman Republic, so its use for all characters constructed from
   that base has been a source of confusion.).

   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 (MUAs) 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.  Even if they were
   adequate, that approach risks an assortment of implementations with
   different sets of patches and workarounds having been applied with
   consequent user confusion about what is actually be run and
   supported.  Instead, we need to build a fully internationalized email
   environment, focusing on permitting efficient communication among
   those who share a language or other community (see [I18Nemail-
   constraints] for an extended discussion of this optimization).  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 [RFC3629] 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 do not have content-transfer-encodings.

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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 "EAI", refers to an
   address permitted by this specification. [[anchor4: Note in Draft/
   Placeholder: it appears that the terms "EAI" and the earlier "IMA"
   are not used in a precise 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 address.  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

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   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
   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 [I18Nemail-UTF8].
   o  In-transit downgrading from internationalized addressing with the
      SMTP extension and UTF-8 headers to traditional email formats and
      characters [I18Nemail-downgrade].  Downgrading either at the point
      of message origination or after the mail has successfully been
      received by a final delivery SMTP server (sometimes called an
      "MDA") involve different constraints and possibilities; see
      Section 4.3 and Section 5, below.
   o  Extensions to the IMAP and POP protocols to support
      internationalized headers [I18Nemail-imap] and [I18Nemail-pop].
   o  Operational guidelines and suggestions for the deployment of
      internationalized email [I18Nemail-ops].
   o  Special considerations for mailing lists and similar distributions
      during the transition to internationalized email [I18Nemail-
   o  Design decisions, history, and alternative models for
      internationalized Internet email [I18Nemail-history].

4.  Overview of Protocol Extensions and Changes

4.1.  SMTP Extension for Internationalized eMail Address

   An SMTP extension, "Email18N" [[anchor8: Extension name should be
   corrected when we make a final decision and synchronized with the
   "I18Nemail-SMTPext" document]] 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 that the server advertise the 8BITMIME extension
      [RFC1652] and that the client support 8-bit transmission so that
      header information can be transmitted without using a special

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   o  Provides information to support downgrading mechanisms.

   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.

       If the message cannot be forwarded because the next-hop system
       cannot accept the extension and insufficient information is
       available to reliably downgrade it, it MUST be bounced.
   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 similar to this without introducing
       great complexity.

   Conformance to the group of standards specified here for email
   transport and delivery requires implementation of the SMTP Extension
   specification, including recognition of the keywords associated with
   alternate and synthesized addresses, and the UTF-8 Header
   specification.  Support for downgrading is not required, but, if
   implemented, MUST be implemented as specified.

4.2.  Transmission of Email Header in UTF-8 Encoding

   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.  If non-obvious
   encodings, such as protocol-specific ACE variants, are used, the user
   will inevitably see them, at least occasionally, rather than "native"
   characters and will find that discomfiting or astonishing.
   Similarly, if different codings are used for mail transport and
   message bodies, the user is particularly likely to be surprised, if
   only as a consequence of the long-established "things leak"
   principle.  But the only practical way to avoid these sources of
   discomfort, 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

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   shifted to a full internationalized form, presumably using UTF-8
   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 email address and header
   internationalization, the risk should be minimized by the fact that
   the selection of submission servers are presumably under the control
   of the sender's 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 all-ASCII form of a non-ASCII
      address requires that the sender have some knowledge.  This
      knowledge is normally restricted to final delivery servers, but
      some extensions may be feasible there too.  Downgrade issues and a
      specification are discussed in [I18Nemail-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 I18Nemail-
   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 capability specified here.

5.  Downgrading Before and After SMTP Transactions

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   In addition to the in-transit downgrades discussed above, downgrading
   may also occur before or during initial message submission or after
   delivery to the final delivery MTA.  Because these cases have a
   different set of available information from in-transit cases, the
   constraints and opportunities may be somewhat different too.  These
   two cases are discussed in the subsections below.

5.1.  Downgrading Before or During Message Submission

   Perhaps obviously, the most convenient time to convert an address or
   message from internationalized to conventional ASCII form is at the
   originating MUA, either before the message is sent or after the
   internationalized form of the message is rejected or bounced by some
   MTA in the path to the presumed destination.  At that point, the user
   has a full range of choices available, including contacting the
   intended recipient out of band for an alternate address, consulting
   appropriate directories, arranging for translation of both addresses
   and message content into a different language, and so on.  While it
   is natural to think of message downgrading as optimally being a
   fully-automated process, we should not underestimate the capabilities
   of a user of at least moderate intelligence who wishes to communicate
   with another such user.

   In this context, one can easily imagine modifications to message
   submission servers (as described in RFC 4409 [RFC4409]) so that they
   would perform downgrading, or perhaps even upgrading, operations,
   receiving messages with one or more of the internationalization
   extensions discussed here and adapting the outgoing message, as
   needed, to respond to the delivery or next-hop environment it

5.2.  Downgrading or Other Processing After Final SMTP Delivery

   When an email message is received by a final delivery SMTP server, it
   is usually stored in some form.  Then it is retrieved by client
   software via some email retrieval mechanisms such as POP, IMAP or

   The SMTP extension described in Section 4.1 provides protection only
   in transport.  It does not prevent MUAs and email retrieval
   mechanisms that have not been upgraded to understand
   internationalized addresses and UTF-8 headers from accessing stored
   internationalized emails.

   Since the final delivery SMTP server (to be more specific, its
   corresponding mail storage agent) cannot safely assume that agents
   accessing email storage will be always be capable of handling the
   extensions proposed here, it MAY either downgrade internationalized

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   emails or specially identify messages that utilize these extensions,
   or both.  If this is the case, the final delivery SMTP server MUST
   include a mechanism to preserve the original internationalized forms
   without information loss to support access by I18Nemail-aware agents.

   The method and format for downgrading at the final delivery SMTP
   server is [[anchor10: will be]] discussed in [I18Nemail-pop] and

   [[anchor11: Note in draft: There are at least four cases.  Both MUA
   and IMAP/POP are compliant.  Both are non compliant.  And only of
   them is compliant.  Do we need to invent different methods for each

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

   [[anchor13: Note in draft: The material that follows contains some
   forward-looking, predictive, statements about discussions to occur
   and documents to be written.  Be sure they are true before Last

   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 [I18Nemail-ops] and [I18Nemail-Exploder].

7.  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

8.  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
   deployment of email address and header internationalization.

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8.1.  Impact on IRIs

   The mailto: schema defined in [RFC2368] and discussed in IRI
   [RFC3987] may need to be modified when this work is completed and

8.2.  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 discussed only minimally in this series of documents.  The
   general issues, and proposed required modifications to the protocols,
   are [[anchor18: will be]] covered in [I18Nemail-pop] and [I18Nemail-
   imap].  Some preliminary discussion appears in in Section 5.2.
   Implementation of internationalized POP and IMAP support is, of
   course, not required for implementation of the transport and in-
   transit header extensions specified in other documents or this set
   (or vica versa).

9.  IANA Considerations

   This overview description and framework document does not contemplate
   any IANA registrations or other actions.  Some of the documents in
   the group have their own IANA considerations sections and

10.  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" or "IDN homograph attacks".  These attacks
   allow an attacker (or "phisher") to spoof the domain or 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

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   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 specifications do not, in general, raise any new security issues.
   They do require a review of issues associated with confusable
   characters -- a topic that is being explored thoroughly elsewhere
   [IDN-nextsteps] -- and, potentially, some issues with UTF-8
   canonicalization, discussed in [RFC3629].  The latter is also part of
   the subject of ongoing work discussed in [Net-Unicode].  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
   internationalized 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.

11.  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"
   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
   address internationalization problem. [[anchor21: Note in draft: may
   want to move some of this to "history" or reference it]]

12.  Change History

   [[anchor23: This section to be restructured prior to publication.  It
   may be useful to retain parts of it to facilitate establishing dates
   and documents for the history of this work.]]

   This document has evolved through several titles as well as the usual
   version numbers.  The list below tries to trace that thread as well

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   as changes within the substance of the document.  The first document
   of the series was posted as draft-klensin-emailaddr-i18n-00.txt in
   October 2003.

12.1.  draft-klensin-ima-framework: 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.

12.2.  draft-klensin-ima-framework: Version 01

   o  Some clarifications of terminology (more to follow) and general
      editorial improvements.
   o  Upgrades to reflect discussions during IETF 64.
   o  Improved treatment of downgrading before and after message

12.3.  draft-ietf-eai-framework: Version 00

   This version supercedes draft-klensin-ima-framework-01; its file name
   should represent the form to be used until the IETF email address and
   header internationalization ("EAI") work concludes.

   o  Changed "display name" terminology to be consistent with RFC 2822.
      Also clarified some other terminology issues.
   o  Added a comment about the possible role of MessageSubmission
      servers in downgrading.
   o  Removed the "IMA" terminology, converting it to either "EAI" or
   o  Per meeting and mailing list discussion, added conformance
      statements about bouncing if neither forwarding nor downgrading
      were possible and about implementation requirements.
   o  Updated several references.  Some documents are still tentative.
   o  Fixed many typographical errors.

13.  References

13.1.  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

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              definitive for the Internet.

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

              This document is expected to be developed by the WG.  The
              date given here is purely arbitrary.

              Yao, J., Ed. and X. Lee, Ed., "SMTP extension for
              internationalized email address", draft-yao-smtpext-01
              (work in progress), January 2006.

              The next version of this document will appear as

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

              The next version of this document will appear as

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

              The next version of this document will appear as

   [RFC1651]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
              Crocker, "SMTP Service Extensions", RFC 1651, July 1994.

   [RFC1652]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
              Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
              RFC 1652, July 1994.

   [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,

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              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

13.2.  Informative References

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

              Klensin, J., "Internationalization in Internet
              Applications: Issues, Tradeoffs, and Email Addresses",
              February 2006.

              Klensin, J., "Decisions and Alternatives for
              Internationalization of Email Addresses", April 2006.

              This document is expected to be developed by the WG.  The
              date given here is purely arbitrary.

              Klensin, J., "Considerations for IMAP in Conjunction with
              Email Address Internationalization", June 2006.

              This document is expected to be developed by the WG.  The
              date and authorship given here are purely arbitrary.

              "Placeholder: whatever we call the operations document",

              This document is expected to be developed by the WG.  The
              date given here is purely arbitrary.

              Newman, C., ""POP3 Support for UTF-8", February 2006, <htt

              The next version of this document will appear as


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              Klensin, J. and P. Faltstrom, "Review and Recommendations
              for Internationalized Domain Names (IDN)", April 2006, <ht

   [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.

              Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", April 2006, <

   [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)
              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.

   [RFC2368]  Hoffman, P., Masinter, L., and J. Zawinski, "The mailto
              URL scheme", RFC 2368, July 1998.

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

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Authors' Addresses

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

   Phone: +1 617 491 5735

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


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