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Versions: (RFC 4952) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 6530

Email Address Internationalization                            J. Klensin
(EAI)                                                              Y. Ko
Internet-Draft                                          October 28, 2011
Obsoletes: 4952, 5504, 5825
(if approved)
Intended status: Standards Track
Expires: April 30, 2012


           Overview and Framework for Internationalized Email
                   draft-ietf-eai-frmwrk-4952bis-12

Abstract

   Full use of electronic mail throughout the world requires that
   (subject to other constraints) people be able to use close variations
   on their own names (written correctly in their own languages and
   scripts) as mailbox names in email addresses.  This document
   introduces a series of specifications 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 accommodate UTF-8 data.  The document set also
   includes discussion of key assumptions and issues in deploying fully
   internationalized email.  This document is a replacement for RFC
   4952; it reflects additional issues identified since that document
   was published.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 30, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   than English.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Role of This Specification . . . . . . . . . . . . . . . . . .  5
   3.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Mail User and Mail Transfer Agents . . . . . . . . . . . .  6
     4.2.  Address Character Sets . . . . . . . . . . . . . . . . . .  7
     4.3.  User Types . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.4.  Messages . . . . . . . . . . . . . . . . . . . . . . . . .  8
     4.5.  Mailing Lists  . . . . . . . . . . . . . . . . . . . . . .  8
     4.6.  Conventional Message and Internationalized Message . . . .  8
     4.7.  Undeliverable Messages, Notification, and Delivery
           Receipts . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Overview of the Approach and Document Plan . . . . . . . . . .  9
   6.  Review of Experimental Results . . . . . . . . . . . . . . . . 10
   7.  Overview of Protocol Extensions and Changes  . . . . . . . . . 10
     7.1.  SMTP Extension for Internationalized Email Address . . . . 10
     7.2.  Transmission of Email Header Fields in UTF-8 Encoding  . . 11
     7.3.  SMTP Service Extension for DSNs  . . . . . . . . . . . . . 12
   8.  Downgrading before and after SMTP Transactions . . . . . . . . 12
     8.1.  Downgrading before or during Message Submission  . . . . . 13
     8.2.  Downgrading or Other Processing After Final SMTP
           Delivery . . . . . . . . . . . . . . . . . . . . . . . . . 14
   9.  Downgrading in Transit . . . . . . . . . . . . . . . . . . . . 15
   10. User Interface and Configuration Issues  . . . . . . . . . . . 15



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     10.1. Choices of Mailbox Names and Unicode Normalization . . . . 16
   11. Additional Issues  . . . . . . . . . . . . . . . . . . . . . . 17
     11.1. Impact on URIs and IRIs  . . . . . . . . . . . . . . . . . 17
     11.2. Use of Email Addresses as Identifiers  . . . . . . . . . . 17
     11.3. Encoded Words, Signed Messages, and Downgrading  . . . . . 18
     11.4. Other Uses of Local Parts  . . . . . . . . . . . . . . . . 18
     11.5. Non-Standard Encapsulation Formats . . . . . . . . . . . . 19
   12. Key Changes From the Experimental Protocols and Framework  . . 19
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   15. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 21
     16.2. Informative References . . . . . . . . . . . . . . . . . . 22
   Appendix A.  Change Log  . . . . . . . . . . . . . . . . . . . . . 26
     A.1.  Changes between -00 and -01  . . . . . . . . . . . . . . . 26
     A.2.  Changes between -01 and -02  . . . . . . . . . . . . . . . 27
     A.3.  Changes between -02 and -03  . . . . . . . . . . . . . . . 28
     A.4.  Changes between -03 and -04  . . . . . . . . . . . . . . . 28
     A.5.  Changes between -04 and -05  . . . . . . . . . . . . . . . 29
     A.6.  Changes between -05 and -06  . . . . . . . . . . . . . . . 29
     A.7.  Changes between -06 and -07  . . . . . . . . . . . . . . . 29
     A.8.  Changes between -07 and -08 (after IETF Last Call) . . . . 29
     A.9.  Changes between -08 and -09  . . . . . . . . . . . . . . . 29
     A.10. Changes between -09 and -10  . . . . . . . . . . . . . . . 29
     A.11. Changes between -10 and -11  . . . . . . . . . . . . . . . 29

























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1.  Introduction

   Note in Draft and to RFC Editor: The keyword represented in this
   document by "UTF8SMTPbis" (and in the XML source by &EAISMTPkeyword;)
   is a placeholder.  The actual keyword will be assigned when the
   standards track SMTP extension in this series [RFC5336bis-SMTP] is
   approved for publication and should be substituted here.  This
   paragraph should be treated as normative reference to that SMTP
   extension draft, creating a reference hold until it is approved by
   the IESG.  The paragraph should be removed before RFC publication.

   In order to use internationalized email addresses, we need to
   internationalize both the domain part and the local part of email
   addresses.  The domain part of email addresses is already
   internationalized [RFC5890], while the local part is not.  Without
   the extensions specified in this document, the mailbox name is
   restricted to a subset of 7-bit ASCII [RFC5321].  Though MIME
   [RFC2045] enables the transport of non-ASCII data, it does not
   provide a mechanism for internationalized email addresses.  In RFC
   2047 [RFC2047], MIME defines an encoding mechanism for some specific
   message header fields to accommodate non-ASCII data.  However, it
   does not permit the use 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 any part of
   email addresses is to use RFC 2047 coding to embed them in what RFC
   5322 [RFC5322] calls the "display name" (known as a "name phrase" or
   by other terms elsewhere) of the relevant header fields.  Information
   coded into the display name is invisible in the message envelope and,
   for many purposes, is not part of the address at all.

   This document is a replacement for RFC 4952 [RFC4952]; it reflects
   additional issues, shared terminology, and some architectural changes
   identified since that document was published.  It obsoletes that
   document.  The experimental descriptions of in-transit downgrading
   [RFC5504][RFC5825], are now irrelevant and no longer needed due to
   the changes discussed in Section 12.  The RFC Editor is requested to
   move all three of those documents to Historic.

   The pronouns "he" and "she" are used interchangeably to indicate a
   human of indeterminate gender.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119
   [RFC2119].  Although this document is Informational, those
   requirements are consistent with requirements specified in the
   Standards Track documents in this set as described in Section 5.




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2.  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 header
   fields, but also associated transport and delivery models.  A prior
   version of this specification, RFC 4952 [RFC4952], also provided an
   introduction to a series of experimental protocols [RFC5335]
   [RFC5336] [RFC5337] [RFC5504] [RFC5721] [RFC5738] [RFC5825].  This
   revised form provides overview and conceptual information for the
   standards-track successors of a subset of those protocols.  Details
   of the documents and the relationships among them appear in Section 5
   and a discussion of what was learned from the Experimental protocols
   and their implementations appears in Section 6.

   Taken together, these specifications provide the details for a way to
   implement and support internationalized email.  The document itself
   describes how the various elements of email internationalization fit
   together and the relationships among the primary specifications
   associated with message transport, header formats, and handling.

   This document, and others that comprise the collection described
   above, assume a reasonable familiarity with the basic Internet
   electronic mail specifications and terminology [RFC5321][RFC5322] and
   the MIME [RFC2045] and 8BITMIME [RFC6152] ones as well.  While not
   strictly required to implement this specification, a general
   familiarity with the terminology and functions of IDNA
   [RFC5890][RFC5891] [RFC5892][RFC5893] [RFC5894] are also assumed.

3.  Problem Statement

   Internationalizing Domain Names in Applications (IDNA) [RFC5890]
   permits internationalized domain names, but deployment has not yet
   reached most 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
   internationalized.  In many contexts, until more of those identifiers
   are internationalized, internationalized domain names alone have
   little value.

   Email addresses are prime examples of why it is not good enough to
   just internationalize the domain name.  As most observers have
   learned from experience, users strongly prefer email addresses that
   resemble names or initials to those involving seemingly meaningless
   strings of letters or numbers.  Unless the entire email address can
   use familiar characters and formats, users will perceive email as
   being culturally unfriendly.  If the names and initials used in email
   addresses can be expressed in the native languages and writing



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   systems of the users, the Internet will be perceived as more natural,
   especially by 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
   header fields; or of permitting upgraded Mail User Agents (MUAs) to
   decode a special coding and respond by displaying local characters.
   To be perceived as usable, the addresses must be internationalized
   and handled consistently in all of the contexts in which they occur.
   This requirement has far-reaching implications: collections of
   patches and workarounds are not adequate.  Even if they were
   adequate, a workaround-based approach may result in an assortment of
   implementations with different sets of patches and workarounds having
   been applied with consequent user confusion about what is actually
   usable and supported.  Instead, we need to build a fully
   internationalized email environment, focusing on permitting efficient
   communication among those who share a language and writing system.
   That, in turn, implies changes to the mail header environment to
   permit those header fields that are appropriately internationalized
   to utilize the full range of Unicode characters, an SMTP Extension to
   permit UTF-8 [RFC3629] [RFC5198] mail addressing and delivery of
   those extended header fields, support for internationalization of
   delivery and service notifications [RFC3461] [RFC3464], and (finally)
   a requirement for support of the 8BITMIME SMTP Extension [RFC6152] so
   that all of these can be transported through the mail system without
   having to overcome the limitation that header fields do not have
   content-transfer-encodings.

4.  Terminology

   This document assumes a reasonable understanding of the protocols and
   terminology of the core email standards as documented in [RFC5321]
   and [RFC5322].

4.1.  Mail User and Mail Transfer Agents

   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 application of the "protocols on the wire"
   principle to it.  That email architecture, as it has evolved, and
   that "on 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).

   However, the term "final delivery MTA" is used in this document in a



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   fashion equivalent to the term "delivery system" or "final delivery
   system" of RFC 5321.  This is the SMTP server that controls the
   format of the local parts of addresses and is permitted to inspect
   and interpret them.  It receives messages from the network for
   delivery to mailboxes or for other local processing, including any
   forwarding or aliasing that changes envelope addresses, rather than
   relaying.  From the perspective of the network, any local delivery
   arrangements such as saving to a message store, handoff to specific
   message delivery programs or agents, and mechanisms for retrieving
   messages are all "behind" the final delivery MTA and hence are not
   part of the SMTP transport or delivery process.

4.2.  Address Character Sets

   In this document, an address is "all-ASCII", or just an "ASCII
   address", if every character in the address is in the ASCII character
   repertoire [ASCII]; an address is "non-ASCII", or an "i18n-address",
   if any character is not in the ASCII character repertoire.  Such
   addresses MAY be restricted in other ways, but those restrictions are
   not relevant to this definition.  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 umbrella term to describe the email address internationalization
   specified by this document and its companion documents is
   "UTF8SMTPbis".
   [[anchor3: Note in Draft: Keyword to be changed before publication.]]
   For example, an address permitted by this specification is referred
   to as a "UTF8SMTPbis (compliant) address".

   Please note that, according to the definitions given here, the set of
   all "all-ASCII" addresses and the set of all "non-ASCII" addresses
   are mutually exclusive.  The set of all addresses permitted when
   UTF8SMTPbis appears is the union of these two sets.

4.3.  User Types

   An "ASCII user" (i) exclusively uses email addresses that contain
   ASCII characters only, and (ii) cannot generate recipient addresses
   that contain non-ASCII characters.

   An "i18mail user" has one or more non-ASCII email addresses, or is
   able to generate recipient addresses that contain non-ASCII
   characters.  Such a user may have ASCII addresses too; if the user
   has more than one email account and a corresponding address, or more
   than one alias for the same address, he or she has some method to
   choose which address to use on outgoing email.  Note that under this
   definition, it is not possible to tell from an ASCII address if the



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   owner of that address is an i18mail user or not.  (A non-ASCII
   address implies a belief that the owner of that address is an i18mail
   user.)  There is no such thing as an "i18mail message"; the term
   applies only to users and their agents and capabilities.  In
   particular, the use of non-ASCII message content is an integral part
   of the MIME specifications [RFC2045] and does not require these
   extensions (although it is compatible with them).

4.4.  Messages

   A "message" is sent from one user (sender) using a particular email
   address to one or more other recipient email addresses (often
   referred to just as "users" or "recipient users").

4.5.  Mailing Lists

   A "mailing list" is a mechanism whereby a message may be distributed
   to multiple recipients by sending it to one recipient address.  An
   agent (typically not a human being) at that single address then
   causes the message to be redistributed to the target recipients.
   This agent sets the envelope return address of the redistributed
   message to a different address from that of the original single
   recipient message.  Using a different envelope return address
   (reverse-path) causes error (and other automatically generated)
   messages to go to an error handling address.

   Special provisions for managing mailing lists that might contain non-
   ASCII addresses are discussed in a document that is specific to that
   topic [RFC5983] [RFC5983bis-MailingList].

4.6.  Conventional Message and Internationalized Message

   o  A conventional message is one that does not use any extension
      defined in the SMTP extension document [RFC5336] or in the
      UTF8header specification [RFC5335], and is strictly conformant to
      RFC 5322 [RFC5322].

   o  An internationalized message is a message utilizing one or more of
      the extensions defined in this set of specifications, so that it
      is no longer conformant to the traditional specification of an
      email message or its transport.

4.7.  Undeliverable Messages, Notification, and Delivery Receipts

   As specified in RFC 5321, a message that is undeliverable for some
   reason is expected to result in notification to the sender.  This can
   occur in either of two ways.  One, typically called "Rejection",
   occurs when an SMTP server returns a reply code indicating a fatal



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   error (a "5yz" code) or persistently returns a temporary failure
   error (a "4yz" code).  The other involves accepting the message
   during SMTP processing and then generating a message to the sender,
   typically known as a "Non-delivery Notification" or "NDN".  Current
   practice often favors rejection over NDNs because of the reduced
   likelihood that the generation of NDNs will be used as a spamming
   technique.  The latter, NDN, case is unavoidable if an intermediate
   MTA accepts a message that is then rejected by the next-hop server.

   A sender MAY also explicitly request message receipts [RFC3461] that
   raise the same issues for these internationalization extensions as
   NDNs.

5.  Overview of the Approach and Document Plan

   This set of specifications changes both SMTP and the character
   encoding of email message 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 below, also contains informational documents whose
   purpose is to provide implementation suggestions and guidance for the
   protocols.

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

   o  SMTP extension.  The SMTP extension document [RFC5336bis-SMTP]
      provides an SMTP extension (as provided for in RFC 5321) for
      internationalized addresses.

   o  Email message headers in UTF-8.  The email message header document
      [RFC5335bis-Hdrs] essentially updates RFC 5322 to permit some
      information in email message headers to be expressed directly by
      Unicode characters encoded in UTF-8 when the SMTP extension
      described above is used.  This document, possibly with one or more
      supplemental ones, will also need to address the interactions with
      MIME, including relationships between UTF8SMTPbis and internal
      MIME headers and content types.

   o  Extensions to delivery status and notification handling to adapt
      to internationalized addresses [RFC5337bis-DSN].

   o  Forthcoming documents will specify extensions to the IMAP protocol
      [RFC3501] to support internationalized message headers
      [RFC5738bis-IMAP], Parallel extensions to the POP protocol
      [RFC5721] [RFC5721bis-POP3], and some common properties of the two
      [POPIMAP-downgrade].




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6.  Review of Experimental Results

   The key difference between this set of protocols and the experimental
   set that preceded them [RFC5335] [RFC5336] [RFC5337] [RFC5504]
   [RFC5721] [RFC5738] [RFC5825] is that the earlier group provided a
   mechanism for in-transit downgrading of messages (described in detail
   in RFC 5504).  That mechanism permitted, and essentially required,
   that each non-ASCII address be accompanied by an all-ASCII
   equivalent.  That, in turn, raised security concerns associated with
   pairing of addresses that could not be authenticated.  It also
   introduced the first incompatible change to Internet mail addressing
   in many years, raising concerns about interoperability issues if the
   new address forms "leaked" into legacy email implementations.  The WG
   concluded that the advantages of in-transit downgrading, were it
   feasible operationally, would be significant enough to overcome those
   concerns.

   That turned out not to be the case, with interoperability problems
   among initial implementations.  Prior to starting on the work that
   led to this set of specifications, the WG concluded that the
   combination of requirements and long-term implications of that
   earlier model were too complex to be satisfactory and that work
   should move ahead without it.

   The other significant change to the protocols themselves is that the
   UTF8SMTPbis keyword is now required as an SMTP client announcement if
   the extension is needed; in the experimental version, only the server
   announcement that an extended envelope and/or content were permitted
   was necessary.

7.  Overview of Protocol Extensions and Changes

7.1.  SMTP Extension for Internationalized Email Address

   An SMTP extension, "UTF8SMTPbis" is specified as follows:

   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 message
      headers (see Section 7.2).

   o  Requires that the server advertise the 8BITMIME extension
      [RFC6152] and that the client support 8-bit transmission so that
      header information can be transmitted without using a special
      content-transfer-encoding.

   Some general principles affect the development decisions underlying



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   this work.

   1.  Email addresses enter subsystems (such as a user interface) that
       may perform charset conversions or other encoding changes.  When
       the local part of the address includes characters outside the
       ASCII character repertoire, use of ASCII-compatible encoding
       (ACE) [RFC3492] [RFC5890] in the domain part is discouraged to
       promote consistent processing of characters throughout the
       address.

   2.  An SMTP relay MUST

       *  Either recognize the format explicitly, agreeing to do so via
          an ESMTP option, or

       *  Reject the message or, if necessary, return a non-delivery
          notification message, so that the sender can make another
          plan.

   3.  If the message cannot be forwarded because the next-hop system
       cannot accept the extension, it MUST be rejected or a non-
       delivery message MUST be generated and sent.

   4.  In the interest of interoperability, charsets other than UTF-8
       are prohibited in mail addresses and message headers being
       transmitted over the Internet.  There is no practical way to
       identify multiple charsets 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 and the UTF-8 Header specification.  If the system
   implements IMAP or POP, it MUST conform to the i18n IMAP
   [RFC5738bis-IMAP] or POP [RFC5721bis-POP3] specifications
   respectively.

7.2.  Transmission of Email Header Fields in UTF-8 Encoding

   There are many places in MUAs or in a user presentation in which
   email addresses or domain names appear.  Examples include the
   conventional From, To, or Cc header fields; Message-ID and
   In-Reply-To header fields that normally contain domain names (but
   that may be a special case); and in message bodies.  Each of these
   must be examined 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 ASCII-Compatible Encoding (ACE) variants, are used,
   the user will inevitably, if only occasionally, see them rather than



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   "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.  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 similar to the encodings used in
   message headers and message bodies as possible.

   When email local parts are internationalized, they SHOULD be
   accompanied by arrangements for the message headers to be in the
   fully internationalized form.  That form SHOULD use UTF-8 rather than
   ASCII as the base character set for the contents of header fields
   (protocol elements such as the header field names themselves are
   unchanged and remain entirely in ASCII).  For transition purposes and
   compatibility with legacy systems, this can done by extending the
   traditional MIME encoding models for non-ASCII characters in headers
   [RFC2045] [RFC2231], but even these should be based on UTF-8, rather
   than other encodings, if at all possible [RFC6055].  However, the
   target is fully internationalized message headers, as discussed in
   [RFC5335bis-Hdrs] and not an extended and painful transition.

7.3.  SMTP Service Extension for DSNs

   The existing Draft Standard Delivery status notifications (DSNs)
   specification [RFC3461] is limited to ASCII text in the machine
   readable portions of the protocol.  "International Delivery and
   Disposition Notifications" [RFC5337bis-DSN] adds a new address type
   for international email addresses so an original recipient address
   with non-ASCII characters can be correctly preserved even after
   downgrading.  If an SMTP server advertises both the UTF8SMTPbis and
   the DSN extension, that server MUST implement internationalized DSNs
   including support for the ORCPT parameter specified in RFC 3461
   [RFC3461].

8.  Downgrading before and after SMTP Transactions

   An important issue with these extensions is how to handle
   interactions between systems that support non-ASCII addresses and
   legacy systems that expect ASCII.  There is, of course, no problem
   with ASCII-only systems sending to those that can handle
   internationalized forms because the ASCII forms are just a proper
   subset.  But, when systems that support these extensions send mail,
   they MAY include non-ASCII addresses for senders, receivers, or both
   and might also provide non-ASCII header information other than
   addresses.  If the extension is not supported by the first-hop system
   (SMTP server accessed by the Submission server acting as an SMTP
   client), message originating systems SHOULD be prepared to either



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   send conventional envelopes and message headers or to return the
   message to the originating user so the message may be manually
   downgraded to the traditional form, possibly using encoded words
   [RFC2047] in the message headers.  Of course, such transformations
   imply that the originating user or system must have ASCII-only
   addresses available for all senders and recipients.  Mechanisms by
   which such addresses may be found or identified are outside the scope
   of these specifications as are decisions about the design of
   originating systems such as whether any required transformations are
   made by the user, the originating MUA, or the Submission server.

   A somewhat more complex situation arises when the first-hop system
   supports these extensions but some subsequent server in the SMTP
   transmission chain does not.  It is important to note that most cases
   of that situation with forward-pointing addresses will be the result
   of configuration errors: especially if it hosts non-ASCII addresses,
   a final delivery MTA that accepts these extensions SHOULD NOT be
   configured with lower-preference MX hosts that do not.  When the only
   non-ASCII address being transmitted is backward-pointing (e.g., in an
   SMTP MAIL command), recipient configuration can not help in general.
   On the other hand, alternate, all-ASCII, addresses for senders are
   those most likely to be authoritatively known by the submission
   environment or the sender herself.  Consequently, if an intermediate
   SMTP relay that requires these extensions then discovers that the
   next system in the chain does not support them, it will have little
   choice other than to reject or return the message.

   As discussed above, downgrading to an ASCII-only form may occur
   before or during the initial message submission.  It might also occur
   after the delivery to the final delivery MTA in order to accommodate
   messages stores or IMAP or POP servers or clients that have different
   capabilities than the delivery MTA.  These two cases are discussed in
   the subsections below.

8.1.  Downgrading before or during Message Submission

   The IETF has traditionally avoided specifying the precise behavior of
   MUAs to provide maximum flexibility in the associated user
   interfaces.  The SMTP standard [RFC5321], Section 6.4, gives wide
   latitude to MUAs and Submission servers as to what might be supplied
   by the user as long as the result conforms with "on the wire"
   standards once it is injected into the public Internet.  In that
   tradition, the discussion in the remainder of Section 8 is provided
   as general guidance rather than normative requirements.

   Messages that require these extensions will sometimes be transferred
   to a system that does not support these extensions; it is likely that
   the most common cases will involve the combination of ASCII-only



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   forward-pointing addresses with a non-ASCII backward-pointing one.
   Until the extensions described here have been universally implemented
   in the Internet email environment, senders who prefer to use non-
   ASCII addresses (or raw UTF-8 characters in header fields) even when
   their intended recipients use and expect all-ASCII ones will need to
   be especially careful about the error conditions that can arise,
   especially if they are working in an environment in which non-
   delivery messages (or other indications from submission servers) are
   routinely dropped or ignored.

   Perhaps obviously, the most convenient time to find an ASCII address
   corresponding to an internationalized address is at the originating
   MUA or closely-associated systems.  This can occur either before the
   message is sent or after the internationalized form of the message is
   rejected.  It is also the most convenient time to convert a message
   from the internationalized form into conventional ASCII form or to
   generate a non-delivery message to the sender if either is necessary.
   At that point, the user has a full range of choices available,
   including changing backward-pointing addresses, 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 operations or perhaps even upgrading ones.
   Such operations would permit 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 the submission server encounters.

8.2.  Downgrading or Other Processing After Final SMTP Delivery

   When an email message is received by a final delivery MTA, it is
   usually stored in some form.  Then it is retrieved either by software
   that reads the stored form directly or by client software via some
   email retrieval mechanisms such as POP or IMAP.

   The SMTP extension described in Section 7.1 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 message headers from accessing
   stored internationalized emails.




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   Since the final delivery MTA (or, to be more specific, its
   corresponding mail storage agent) cannot safely assume that agents
   accessing email storage will always be capable of handling the
   extensions proposed here, it MAY downgrade internationalized emails,
   specially identify messages that utilize these extensions, or both.
   If this is done, the final delivery MTA SHOULD include a mechanism to
   preserve or recover the original internationalized forms without
   information loss to support access by UTF8SMTPbis-aware agents.

9.  Downgrading in Transit

   The base SMTP specification (Section 2.3.11 of RFC 5321 [RFC5321])
   states that "due to a long history of problems when intermediate
   hosts have attempted to optimize transport by modifying them, the
   local-part MUST be interpreted and assigned semantics only by the
   host specified in the domain part of the address".  This is not a new
   requirement; equivalent statements appeared in specifications in 2001
   [RFC2821] and even in 1989 [RFC1123].

   Adherence to this rule means that a downgrade mechanism that
   transforms the local-part of an email address cannot be utilized in
   transit.  It can only be applied at the endpoints, specifically by
   the MUA or submission server or by the final delivery MTA.

   One of the reasons for this rule has to do with legacy email systems
   that embed mail routing information in the local-part of the address
   field.  Transforming the email address destroys such routing
   information.  There is no way a server other than the final delivery
   server can know, for example, whether the local-part of
   user%foo@example.com is a route ("user" is reached via "foo") or
   simply a local address.

10.  User Interface and Configuration Issues

   Internationalization of addresses and message headers, especially in
   combination with variations on character coding that are inherent to
   Unicode, may make careful choices of addresses and careful
   configuration of servers and DNS records even more important than
   they are for traditional Internet email.  It is likely that, as
   experience develops with the use of these protocols, it will be
   desirable to produce one or more additional documents that offer
   guidance for configuration and interfaces.  A document that discusses
   issues with mail user agents (MUAs), especially with regard to
   downgrading, is expected to be developed in the EAI Working Group.
   The subsections below address some other issues.






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10.1.  Choices of Mailbox Names and Unicode Normalization

   It has long been the case that the email syntax permits choices about
   mailbox names that are unwise in practice if one actually intends the
   mailboxes to be accessible to a broad range of senders.  The most-
   often-cited examples involve the use of case-sensitivity and tricky
   quoting of embedded characters in mailbox local parts.  These
   deliberately-unusual constructions are permitted by the protocols and
   servers are expected to support them.  Although they can provide
   value in special cases, taking advantage of them is almost always bad
   practice unless the intent is to create some form of security by
   obscurity.

   In the absence of these extensions, SMTP clients and servers are
   constrained to using only those addresses permitted by RFC 5321.  The
   local parts of those addresses MAY be made up of any ASCII characters
   except the control characters that 5321 prohibits, although some 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
   was permitted by RFC 821 [RFC0821] but is prohibited by RFC 5321
   because it requires a backspace character (a prohibited C0 control).
   Because RFC 5321 (and its predecessor, RFC 2821) prohibit the use of
   this character in ASCII mailbox names and it is even more problematic
   (for canonicalization and normalization reasons) in non-ASCII
   strings, backspace MUST NOT appear in UTF8SMTPbis mailbox names.

   For the particular case of mailbox names that contain non-ASCII
   characters in the local part, domain part, or both, special attention
   MUST be paid to Unicode normalization [Unicode-UAX15], in part
   because Unicode strings may be normalized by other processes
   independent of what a mail protocol specifies (this is exactly
   analogous to what may happen with quoting and dequoting in
   traditional addresses).  Consequently, the following principles are
   offered as advice to those who are selecting names for mailboxes:

   o  In general, it is wise to support addresses in Normalized form,
      using at least Normalization Form NFC.  Except in circumstances in
      which NFKC would map characters together that the parties
      responsible for the destination mail server would prefer to be
      kept distinguishable, supporting the NFKC-conformant form would
      yield even more predictable behavior for the typical user.

   o  It will usually be wise to support other forms of the same local-
      part string, either as aliases or by normalization of strings
      reaching the delivery server: the sender should not be depended



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      upon to send the strings in normalized form.

   o  Stated differently and in more specific terms, the rules of the
      protocol for local-part strings essentially provide that:

      *  Unnormalized strings are valid, but sufficiently bad practice
         that they may not work reliably on a global basis.  Servers
         should not depend on clients to send normalized forms but
         should be aware that procedures on client machines outside the
         control of the MUA may cause normalized strings to be sent
         regardless of user intent.

      *  C0 (and presumably C1) controls (see The Unicode Standard
         [Unicode]) are prohibited, the first in RFC 5321 and the second
         by an obvious extension from it [RFC5198].

      *  Other kinds of punctuation, spaces, etc., are risky practice.
         Perhaps they will work, and SMTP receiver code is required to
         handle them without severe errors (even if such strings are not
         accepted in addresses to be delivered on that server), but
         creating dependencies on them in mailbox names that are chosen
         is usually a bad practice and may lead to interoperability
         problems.

11.  Additional Issues

   This section identifies issues that are not covered, or not covered
   comprehensively, as part of this set of specifications, but that will
   require ongoing review as part of deployment of email address and
   header internationalization.

11.1.  Impact on URIs and IRIs

   The mailto: schema [RFC6068], and the discussion of it in the
   Internationalized Resource Identifier (IRI) specification [RFC3987]
   may need to be modified when this work is completed and standardized.

11.2.  Use of Email Addresses as Identifiers

   There are a number of places in contemporary Internet usage in which
   email addresses are used as identifiers for individuals, including as
   identifiers to Web servers supporting some electronic commerce sites
   and in some X.509 certificates [RFC5280].  These documents do not
   address those uses, but it is reasonable to expect that some
   difficulties will be encountered when internationalized addresses are
   first used in those contexts, many of which cannot even handle the
   full range of addresses permitted today.




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11.3.  Encoded Words, Signed Messages, and Downgrading

   One particular characteristic of the email format is its persistency:
   MUAs are expected to handle messages that were originally sent
   decades ago and not just those delivered seconds ago.  As such, MUAs
   and mail filtering software, such as that specified in Sieve
   [RFC5228], will need to continue to accept and decode header fields
   that use the "encoded word" mechanism [RFC2047] to accommodate non-
   ASCII characters in some header fields.  While extensions to both
   POP3 [RFC1939] and IMAP [RFC3501] have been defined that include
   automatic upgrading of messages that carry non-ASCII information in
   encoded form -- including RFC 2047 decoding -- of messages by the
   POP3 [RFC5721bis-POP3] or IMAP [RFC5738bis-IMAP] server, there are
   message structures and MIME content-types for which that cannot be
   done or where the change would have unacceptable side effects.

   For example, message parts that are cryptographically signed, using
   e.g., S/MIME [RFC5751] or Pretty Good Privacy (PGP) [RFC3156], cannot
   be upgraded from the RFC 2047 form to normal UTF-8 characters without
   breaking the signature.  Similarly, message parts that are encrypted
   may contain, when decrypted, header fields that use the RFC 2047
   encoding; such messages cannot be 'fully' upgraded without access to
   cryptographic keys.

   Similar issues may arise if messages are signed and then subsequently
   downgraded, e.g., as discussed in Section 8.1, and then an attempt is
   made to upgrade them to the original form and then verify the
   signatures.  Even the very subtle changes that may result from
   algorithms to downgrade and then upgrade again may be sufficient to
   invalidate the signatures if they impact either the primary or MIME
   bodypart headers.  When signatures are present, downgrading MUST be
   performed with extreme care if at all.

11.4.  Other Uses of Local Parts

   Local parts are sometimes used to construct domain labels, e.g., the
   local part "user" in the address user@domain.example could be
   converted into a vanity host user.domain.example with its Web space
   at <http://user.domain.example> and the catchall addresses
   any.thing.goes@user.domain.example.

   Such schemes are obviously limited by, among other things, the SMTP
   rules for domain names, and will not work without further
   restrictions for other local parts such as the <utf8-local-part>
   specified in [RFC5335bis-Hdrs].  Whether those limitations are
   relevant to these specifications is an open question.  It may be
   simply another case of the considerable flexibility accorded to
   delivery MTAs in determining the mailbox names they will accept and



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   how they are interpreted.

11.5.  Non-Standard Encapsulation Formats

   Some applications use formats similar to the application/mbox format
   defined in [RFC4155] instead of the message/digest form described in
   RFC 2046, Section 5.1.5 [RFC2046] to transfer multiple messages as
   single units.  Insofar as such applications assume that all stored
   messages use the message/rfc822 format described in RFC 2046, Section 
   5.2.1 [RFC2046] with ASCII message headers, they are not ready for
   the extensions specified in this series of documents and special
   measures may be needed to properly detect and process them.

12.  Key Changes From the Experimental Protocols and Framework

   The original framework for internationalized email addresses and
   headers was described in RFC 4952 and a subsequent set of
   experimental protocol documents.  Those relationships are described
   in Section 3.  The key architectural difference between the
   experimental specifications and this newer set is that the earlier
   specifications supported in-transit downgrading.  Those mechanisms
   included the definition of syntax and functions to support passing
   alternate, all-ASCII, addresses with the non-ASCII ones as well as
   special headers to indicate the downgraded status of messages.  Those
   features were eliminated after experimentation indicated that they
   were more complex and less necessary than had been assumed earlier.
   Those issues are described in more detail in Section 6 and Section 9.

13.  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
   requirements.

14.  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
   the proposed fix involving forcing all displayed elements into
   normalized lower-case works for domain names in URLs, but not for
   email local parts since those are case sensitive.

   Since email addresses are often transcribed from business cards and



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   notes on paper, they are subject to problems arising from confusable
   characters (see [RFC4690]).  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 message headers must
   not leave the Internet less secure than it is 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
   (see, e.g., RFC 4690 [RFC4690]) -- and, potentially, some issues with
   UTF-8 normalization, discussed in RFC 3629 [RFC3629], and other
   transformations.  Normalization and other issues associated with
   transformations and standard forms are also part of the subject of
   work described elsewhere [RFC5198] [RFC5893] [RFC6055].

   Some issues specifically related to internationalized addresses and
   message headers 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.  This potential problem can be
   mitigated somewhat by enforcing the expectation that most or all such
   transformations will be performed prior to final delivery by systems
   that are presumed to be under the administrative control of the
   sending user (as opposed to being performed in transit by entities
   that are not under the administrative control of the sending user).

   The new UTF-8 header and message formats might also raise, or
   aggravate, another known issue.  If the model creates new forms of an
   'invalid' or 'malformed' message, then a new email attack is created:
   in an effort to be robust, some or most agents will accept such
   message and interpret them as if they were well-formed.  If a filter
   interprets such a message differently than the MUA used by the
   recipient, then it may be possible to create a message that appears
   acceptable under the filter's interpretation but that should be
   rejected under the interpretation given to it by that MUA.  Such
   attacks already exist for existing messages and encoding layers,
   e.g., invalid MIME syntax, invalid HTML markup, and invalid coding of
   particular image types.

   In addition, email addresses are used in many contexts other than
   sending mail, such as for identifiers under various circumstances
   (see Section 11.2).  Each of those contexts will need to be



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   evaluated, in turn, to determine whether the use of non-ASCII forms
   is appropriate and what particular issues they raise.

   This work will clearly affect any systems or mechanisms that are
   dependent on digital signatures or similar integrity protection for
   email message headers (see also the discussion in Section 11.3).
   Many conventional uses of PGP and S/MIME are not affected since they
   are used to sign body parts but not message headers.  On the other
   hand, the developing work on domain keys identified mail (DKIM)
   [RFC5863] will eventually need to consider this work and vice versa:
   while this specification does not address or solve the issues raised
   by DKIM and other signed header mechanisms, the issues will have to
   be coordinated and resolved eventually if the two sets of protocols
   are to co-exist.  In addition, to the degree to which email addresses
   appear in PKI (Public Key Infrastructure) certificates [RFC5280],
   standards addressing such certificates will need to be upgraded to
   address these internationalized addresses.  Those upgrades will need
   to address questions of spoofing by look-alikes of the addresses
   themselves.

15.  Acknowledgments

   This document is an update to, and derived from, RFC 4952.  This
   document would have been impossible without the work and
   contributions acknowledged in it.  The present document benefited
   significantly from discussions in the EAI WG and elsewhere after RFC
   4952 was published, especially discussions about the experimental
   versions of other documents in the internationalized email
   collection, and from RFC errata on RFC 4952 itself.

   Special thanks are due to Ernie Dainow for careful reviews and
   suggested text in this version and to several IESG members for a
   careful review and specific suggestions.

16.  References

16.1.  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.



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

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

   [RFC5321]                 Klensin, J., "Simple Mail Transfer
                             Protocol", RFC 5321, October 2008.

   [RFC5322]                 Resnick, P., Ed., "Internet Message
                             Format", RFC 5322, October 2008.

   [RFC5335bis-Hdrs]         Yang, A., Steele, S., and N. Freed,
                             "Internationalized Email Headers",
                             September 2011, <https://
                             datatracker.ietf.org/doc/
                             draft-ietf-eai-rfc5335bis/>.

   [RFC5336bis-SMTP]         Yao, J. and W. Mao, "SMTP Extension for
                             Internationalized Email Address",
                             August 2011, <https://datatracker.ietf.org/
                             doc/draft-ietf-eai-rfc5336bis/>.

   [RFC5337bis-DSN]          Hansen, T., Newman, C., and A. Melnikov,
                             "Internationalized Delivery Status and
                             Disposition Notifications", October 2011, <
                             https://datatracker.ietf.org/doc/
                             draft-ietf-eai-rfc5337bis-dsn/>.

   [RFC5890]                 Klensin, J., "Internationalized Domain
                             Names for Applications (IDNA): Definitions
                             and Document Framework", RFC 5890,
                             August 2010.

   [RFC6152]                 Klensin, J., Freed, N., Rose, M., and D.
                             Crocker, "SMTP Service Extension for 8-bit
                             MIME Transport", STD 71, RFC 6152,
                             March 2011.

16.2.  Informative References

   [POPIMAP-downgrade]       Fujiwara, K., "Post-delivery Message
                             Downgrading for Internationalized Email
                             Messages", Work in Progress, July 2011, <ht
                             tps://datatracker.ietf.org/doc/
                             draft-ietf-eai-popimap-downgrade/>.



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   [RFC0821]                 Postel, J., "Simple Mail Transfer
                             Protocol", STD 10, RFC 821, August 1982.

   [RFC1123]                 Braden, R., "Requirements for Internet
                             Hosts - Application and Support", STD 3,
                             RFC 1123, October 1989.

   [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.

   [RFC2046]                 Freed, N. and N. Borenstein, "Multipurpose
                             Internet Mail Extensions (MIME) Part Two:
                             Media Types", RFC 2046, 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: Characte
                             r Sets, Languages, and Continuations",
                             RFC 2231, November 1997.

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

   [RFC3156]                 Elkins, M., Del Torto, D., Levien, R., and
                             T. Roessler, "MIME Security with OpenPGP",
                             RFC 3156, August 2001.

   [RFC3461]                 Moore, K., "Simple Mail Transfer Protocol
                             (SMTP) Service Extension for Delivery
                             Status Notifications (DSNs)", RFC 3461,
                             January 2003.

   [RFC3464]                 Moore, K. and G. Vaudreuil, "An Extensible
                             Message Format for Delivery Status
                             Notifications", RFC 3464, January 2003.

   [RFC3492]                 Costello, A., "Punycode: A Bootstring
                             encoding of Unicode for Internationalized



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                             Domain Names in Applications (IDNA)",
                             RFC 3492, March 2003.

   [RFC3501]                 Crispin, M., "INTERNET MESSAGE ACCESS
                             PROTOCOL - VERSION 4rev1", RFC 3501,
                             March 2003.

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

   [RFC4155]                 Hall, E., "The application/mbox Media
                             Type", RFC 4155, September 2005.

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

   [RFC4690]                 Klensin, J., Faltstrom, P., Karp, C., and
                             IAB, "Review and Recommendations for
                             Internationalized Domain Names (IDNs)",
                             RFC 4690, September 2006.

   [RFC4952]                 Klensin, J. and Y. Ko, "Overview and
                             Framework for Internationalized Email",
                             RFC 4952, July 2007.

   [RFC5198]                 Klensin, J. and M. Padlipsky, "Unicode
                             Format for Network Interchange", RFC 5198,
                             March 2008.

   [RFC5228]                 Guenther, P. and T. Showalter, "Sieve: An
                             Email Filtering Language", RFC 5228,
                             January 2008.

   [RFC5280]                 Cooper, D., Santesson, S., Farrell, S.,
                             Boeyen, S., Housley, R., and W. Polk,
                             "Internet X.509 Public Key Infrastructure
                             Certificate and Certificate Revocation List
                             (CRL) Profile", RFC 5280, May 2008.

   [RFC5335]                 Abel, Y., "Internationalized Email
                             Headers", RFC 5335, September 2008.

   [RFC5336]                 Yao, J. and W. Mao, "SMTP Extension for
                             Internationalized Email Addresses",
                             RFC 5336, September 2008.

   [RFC5337]                 Newman, C. and A. Melnikov,



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                             "Internationalized Delivery Status and
                             Disposition Notifications", RFC 5337,
                             September 2008.

   [RFC5504]                 Fujiwara, K. and Y. Yoneya, "Downgrading
                             Mechanism for Email Address
                             Internationalization", RFC 5504,
                             March 2009.

   [RFC5721]                 Gellens, R. and C. Newman, "POP3 Support
                             for UTF-8", RFC 5721, February 2010.

   [RFC5721bis-POP3]         Gellens, R., Yao, J., and K. Fujiwara,
                             "POP3 Support for UTF-8", Work in Progress,
                             July 2011, <https://datatracker.ietf.org/
                             doc/draft-ietf-eai-rfc5721bis>.

   [RFC5738]                 Resnick, P. and C. Newman, "IMAP Support
                             for UTF-8", RFC 5738, March 2010.

   [RFC5738bis-IMAP]         Resnick, P., Newman, C., and S. Shen, "IMAP
                             Support for UTF-8", Work in Progress,
                             July 2011, <https://datatracker.ietf.org/
                             doc/draft-ietf-eai-5738bis/>.

   [RFC5751]                 Ramsdell, B. and S. Turner, "Secure/
                             Multipurpose Internet Mail Extensions
                             (S/MIME) Version 3.2 Message
                             Specification", RFC 5751, January 2010.

   [RFC5825]                 Fujiwara, K. and B. Leiba, "Displaying
                             Downgraded Messages for Email Address
                             Internationalization", RFC 5825,
                             April 2010.

   [RFC5863]                 Hansen, T., Siegel, E., Hallam-Baker, P.,
                             and D. Crocker, "DomainKeys Identified Mail
                             (DKIM) Development, Deployment, and
                             Operations", RFC 5863, May 2010.

   [RFC5891]                 Klensin, J., "Internationalized Domain
                             Names in Applications (IDNA): Protocol",
                             RFC 5891, August 2010.

   [RFC5892]                 Faltstrom, P., "The Unicode Code Points and
                             Internationalized Domain Names for
                             Applications (IDNA)", RFC 5892,
                             August 2010.



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   [RFC5893]                 Alvestrand, H. and C. Karp, "Right-to-Left
                             Scripts for Internationalized Domain Names
                             for Applications (IDNA)", RFC 5893,
                             August 2010.

   [RFC5894]                 Klensin, J., "Internationalized Domain
                             Names for Applications (IDNA): Background,
                             Explanation, and Rationale", RFC 5894,
                             August 2010.

   [RFC5983]                 Gellens, R., "Mailing Lists and
                             Internationalized Email Addresses",
                             RFC 5983, October 2010.

   [RFC5983bis-MailingList]  "Mailing Lists and Internationalized Email
                             Addresses", Unwritten waiting for I-D,
                             2011.

   [RFC6055]                 Thaler, D., Klensin, J., and S. Cheshire,
                             "IAB Thoughts on Encodings for
                             Internationalized Domain Names", RFC 6055,
                             February 2011.

   [RFC6068]                 Duerst, M., Masinter, L., and J. Zawinski,
                             "The 'mailto' URI Scheme", RFC 6068,
                             October 2010.

   [Unicode]                 The Unicode Consortium.  The Unicode
                             Standard, Version 5.2.0, defined by:, "The
                             Unicode Standard, Version 5.2.0", (Mountain
                             View, CA: The Unicode Consortium,
                             2009. ISBN 978-1-936213-00-9)., <http://
                             www.unicode.org/versions/Unicode5.2.0/>.

   [Unicode-UAX15]           The Unicode Consortium, "Unicode Standard
                             Annex #15: Unicode Normalization Forms",
                             March 2008,
                             <http://www.unicode.org/reports/tr15/>.

Appendix A.  Change Log

   [[RFC Editor: Please remove this section prior to publication.]]

A.1.  Changes between -00 and -01

   o  Because there has been no feedback on the mailing list, updated
      the various questions to refer to this version as well.




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   o  Reflected RFC Editor erratum #1507 by correcting terminology for
      headers and header fields and distinguishing between "message
      headers" and different sorts of headers (e.g., the MIME ones).

A.2.  Changes between -01 and -02

   Note that section numbers in the list that follows may refer to -01
   and not -02.

   o  Discussion of RFC 5825 ("downgraded display") has been removed per
      the earlier note and on-list discussion.  Any needed discussion
      about reconstructed messages will need to appear in the IMAP and
      POP documents.  However, the introductory material has been
      reworded to permit keeping 5504 and 5825 on the list there,
      without which the back chain would not be complete.  For
      consistency with this change, 5504 and 5825 have been added to the
      "Obsoletes" list (as far as I know, an Informational spec can
      obsolete or update Experimental ones, so no downref problem here
      --JcK).

   o  Reference to alternate addresses dropped from (former) Section
      7.1.

   o  Reference to RFC 5504 added to (former) Section 8 for
      completeness.

   o  Ernie's draft comments added (with some minor edits) to replace
      the placeholder in (former) Section 9 ("Downgrading in Transit").
      It is intended to capture at least an introduction the earlier
      discussions of algorithmic downgrading generally and ACE/Punycode
      transformations in particular.  Anyone who is unhappy with it
      should say so and propose alternate text.  RSN.

   o  In the interest of clarity and consistency with the terminology in
      Section 4.1, all uses of "final delivery SMTP server" and "final
      delivery server" have been changed to "final delivery MTA".

   o  Placeholder at the end of Section 2 has been removed and the text
      revised to promise less.  The "Document Plan" (Section 5) has been
      revised accordingly.  We need to discuss this at IETF 78 if not
      sooner.

   o  Sections 5 and 6 have been collapsed into one -- there wasn't
      enough left in the former Section 5 to justify a separate section.

   o  Former Section 11.1 has been dropped and the DSN document moved up
      into the "Document Plan" as suggested earlier.




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   o  Section 12, "Experimental Targets", has been removed.

   o  Updated references for the new version EAI documents and added
      placeholders for all of the known remaining drafts that will
      become part of the core EAI series but that have not been written.

   o  Inserted an additional clarification about the relationship of
      these extensions to non-ASCII messages.

   o  Changed some normative/informative reference classifications based
      on review of the new text.

   o  Removed references to the pre-EAI documents that were cited for
      historical context in 4952.

   o  Got rid of a remaining pointer to address downgrading in the
      discussion of an updated MAILTO URI.

   o  Minor additional editorial cleanups and tuning.

A.3.  Changes between -02 and -03

   o  Inserted paragraph clarifying the status of the UTF8SMTPbis
      keyword as a result of discussion prior to and during IETF 79.

   o  Adjusted some references including adding an explicit citation of
      RFC 821.

   o  Removed the discussion of the experimental work from an inline
      aside to a separate section, Section 6.

   o  Rewrote the discussion of configuration errors in MX setups to
      make it clear that they are an issue with forward-pointing
      addresses only and improved the discussion of backward-pointing
      addresses.

   o  Removed some now-obsolete placeholder notes and resolved the
      remaining one to a dangling reference.

A.4.  Changes between -03 and -04

   o  Several minor editorial changes.

   o  Added a discussion of the relationship to the base email, MIME,
      and IDNA specifications.






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A.5.  Changes between -04 and -05

   o  Several more minor editorial changes.

A.6.  Changes between -05 and -06

   o  Corrections to more precisely reflect RFC 2119 language
      requirements and closely-related issues..

A.7.  Changes between -06 and -07

   o  Added a new section (now Section 12) to explicitly discuss the
      changes from the previous version.

   o  Removed the discussion of LMTP from Section 11; it is more
      appropriately placed in the SMTP Extension document (5336bis).

A.8.  Changes between -07 and -08 (after IETF Last Call)

   o  Modified Section 7.2 to make the last paragraph less tentative and
      more clear.

   o  Modified Section 8.1 to add an introductory paragraph that
      clarifies what the IETF does and does not specify about email
      protocols.

A.9.  Changes between -08 and -09

   This version incorporates responses to a last set of public comments
   and changes made in response to IESG discussion and comments as part
   of the balloting process.

   o  Many small editorial changes made at IESG request.

   o  Several other small editorial corrections, removal of uncited
      reference to LMTP, added a few citations for clarity.

A.10.  Changes between -09 and -10

   This version contains additional small editorial changes resulting
   from IESG comments and review of -09 changes.  Some more significant
   clarifications appear in Section 10.1

A.11.  Changes between -10 and -11

   While -10 was approved for publication by the IESG (after IETF Last
   Call) in September 2010, the document then went into a reference hold
   in the RFC Editor queue.  Issued identified during and after Last



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   Call for the other three core EAI documents (5335bis, 5336bis, and
   5337bis) required reopening this document and making some minor
   additional changes.

   o  Reworded the descriptions of the POP, IMAP, and mailing list
      documents and moved them to Informative.  Notes in the XML of
      earier versions of this draft indicate that they were listed as
      Normative merely as a temporary convenience.  Examination and
      reclassification of them apparently slipped through the cracks.

   o  Reclassified the document to standards track to eliminate
      normative reference problems from other EAI documents.

   o  References, other than the two Unicode ones, have been updated for
      the convenience of reviewers and the RFC Editor.  A note has been
      inserted into the XML requesting that the RFC Editor update the
      Unicode references to be current at the time of publication.

   o  Explicitly notes status of documents obsoleted by this one and
      moves them to Historic.

   o  Updated author contact information.

Authors' Addresses

   John C KLENSIN
   1770 Massachusetts Ave, #322
   Cambridge, MA  02140
   USA

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


   YangWoo KO
   112-202 Malgeunachim APT. Nae-dong
   Seo-gu, Daejeon  302-981
   Republic of Korea

   EMail: yangwooko@gmail.com











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