Email Address Internationalization                            J. Klensin
Internet-Draft                                                     Y. Ko
Obsoletes: RFC4952 RFCs 4952, 5504, 5825                                     ICU
(if approved)                                              July 3, 12, 2010
Intended status: Informational
Expires: January 4, 13, 2011

           Overview and Framework for Internationalized Email


   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 an update of RFC 4952 that
   reflects additional issues identified since that document was

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

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 January 4, 13, 2011.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Role of This Specification . . . . . . . . . . . . . . . . . .  4  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 . . . . . . . . . . . . . . . . . . . . . . . . .  7  8
     4.5.  Mailing Lists  . . . . . . . . . . . . . . . . . . . . . .  8
     4.6.  Conventional Message and Internationalized Message . . . .  8
     4.7.  Undeliverable Messages and Notification  . . . . . . . . .  8
   5.  Overview of the Approach . . . . . . . . . . . . . . . . . . .  8
   6. and Document Plan . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Overview of Protocol Extensions and Changes  . . . . . . . . .  9
     6.1.  SMTP Extension for Internationalized Email Address . . . .  9
     6.2.  Transmission of Email Header Fields in UTF-8 Encoding  . . 10
     6.3.  SMTP Service Extension for DSNs  . . . . . . . . . . . . . 11
   7.  Downgrading before and after SMTP Transactions . . . . . . . . 11
     7.1.  Downgrading before or during Message Submission  . . . . . 12
     7.2.  Downgrading or Other Processing After Final SMTP
           Delivery . . . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  Downgrading in Transit . . . . . . . . . . . . . . . . . . . . 13
   9.  User Interface and Configuration Issues  . . . . . . . . . . . 13
     10.1. 14
     9.1.  Choices of Mailbox Names and Unicode Normalization . . . . 14
   10. Additional Issues  . . . . . . . . . . . . . . . . . . . . . . 15
     10.1. Impact on URIs and IRIs  . . . . . . . . . . . . . . . . . 15
     11.2. Interaction with Delivery Notifications  . . . . . . . . . 15
     11.3. 16
     10.2. Use of Email Addresses as Identifiers  . . . . . . . . . . 16
     10.3. Encoded Words, Signed Messages, and Downgrading  . . . . . 16
     10.4. LMTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.6. 17
     10.5. Other Uses of Local Parts  . . . . . . . . . . . . . . . . 16
     11.7. 17
     10.6. Non-Standard Encapsulation Formats . . . . . . . . . . . . 17
   12. Experimental Targets . . . . . . . . . . . . . . . . . . . . . 17
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   15. Acknowledgements
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 19
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     14.2. Informative References . . . . . . . . . . . . . . . . . . 20 21
   Appendix A.  Change Log  . . . . . . . . . . . . . . . . . . . . . 22 24
     A.1.  Changes between -00 and -01  . . . . . . . . . . . . . . . 23 24
     A.2.  Changes between -01 and -02  . . . . . . . . . . . . . . . 24

1.  Introduction

   [[anchor1: Note to EAI WG: these two initial drafts are intended to
   initiate discussion All comments received on what should, and should not, be in the
   Framework mailing list
   about this document and how we want those topics covered. have been incorporated into the -02 draft.  As such,
   indicated in earlier notes, most placeholders have been removed from
   is more of an intermediate draft between RFC 4952 and to make the first draft
   of 4952bis that could be document a WG Last Call candidate.  If we are going  A note was posted
   on 9 July listing outstanding topics needing discussion in -01
   ( to
   keep the rather aggressive schedule we agreed
   which there was no response before -02 was submitted.  It may be
   useful to in the charter, we
   need refer to have enough discussion on critical-path points that a
   revision suitable (at least) for final review prior to Last Call can
   be posted before the 12 July I-D cutoff.  For that to happen, we
   should have enough discussion to start determining consensus within
   the next ten days.  So, focused comments note and soon, please.]] the change log below as part of
   review of this draft.]]

   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.
   [[anchor2: Note in Draft: RFC 5890, formerly draft-ietf-idnabis-defs,
   and the closely-related RFC 5891 - 5894, have been in AUTH48 since
   June 6.  None of the the author, WG leadership, or ADs are holding it
   up.  Using the I-D references here is just extra work.]]
   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 an update of RFC 4952 [RFC4952] that reflects
   additional issues, shared terminology, and some architectural changes
   identified since that document was published.

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

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

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].
   [[anchor2: Note in Draft: Is 5825 still relevant, or is a victim of
   the "no in-transit downgrade" decision.??]]  This
   revised document 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 6. 5.

   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 [[anchor3: ??? provides a
   roadmap for navigating the]] various documents are involved. primary specifications
   associated with message transport, header formats, and handling.

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 of us 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
   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 or other community.
   That, in turn, implies changes to the mail header environment to
   permit the full range of Unicode characters where that makes sense,
   an SMTP Extension to permit UTF-8 [RFC3629] [RFC5198] mail addressing
   and delivery of those extended header fields, support for
   internationalized delivery and service notifications [RFC3461]
   [RFC3464], and (finally) a requirement for support of the 8BITMIME
   SMTP Extension [RFC1652] 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. 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 "wire" "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
   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
   [[anchor5: 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.  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 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").

   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

   An internationalized message is a message utilizing one or more of
   the extensions defined in this specification or in the UTF8header
   specification [RFC5335], so that it is no longer conformant to the
   RFC 5322 specification of a message.

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 [EAI-Mailinglist]. [EAI-Mailinglist] [RFCNNNNbis-MailingList].

4.6.  Undeliverable Messages  Conventional Message and Notification

   As specified in RFC 5321, a Internationalized Message

   o  A conventional message that is undeliverable for some
   reason is expected to result one that does not use any extension
      defined in notification to the sender.  This can
   occur in either of two ways.  One, typically called "Rejection",
   occurs when an SMTP server 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 and Notification

   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
   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.

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 in the next section, below, also contains informational documents whose
   purpose is to provide implementation suggestions and guidance for the

6.  Document Plan

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

   [[anchor12: ...  Note to WG: if we actually include a list here, the
   result will be that this document can be approved, but not published,
   until those documents on the list are complete.  I'm inclined to list
   the SMTP extension and headers documents only and hand-wave about the
   rest, but we need to discuss.  Versions -00 and -01 simply refer to
   the current Experimental documents --Editor.]]

   o  SMTP extensions.  This document [RFC5336] [RFC5336bis-SMTP] provides an SMTP
      extension (as provided for in RFC 5321) for internationalized

   o  Email message headers in UTF-8.  This document [RFC5335] [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  Extensions to the IMAP protocol to support internationalized
      message headers [RFC5738]. [RFC5738bis-IMAP].

   o  Parallel extensions to the POP protocol [RFC5721].

   o  Description of internationalization changes for delivery
      notifications (DSNs) [RFC5337].

7. [RFC5721]

6.  Overview of Protocol Extensions and Changes


6.1.  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). 6.2).

   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

   Some general principles affect the development decisions underlying
   this work.

   1.  Email addresses enter subsystems (such as a user interface) that
       may perform charset conversions or other encoding changes.  When
       the left hand side of the address includes characters outside the
       US-ASCII character repertoire, use of punycode ASCII-compatible (ACE)
       encoding [RFC3492] [RFC5890] on the right hand side 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

   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 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, including recognition of the keywords associated with
   alternate addresses,
   specification and the UTF-8 Header specification.  If the system
   implements IMAP or POP, it MUST conform to the i18n IMAP or POP
   specifications respectively.


6.2.  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
   "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, it seems clear that
   they should be accompanied by arrangements for the message headers to
   be in the fully internationalized form.  That form should presumably
   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 will 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 of for non-ASCII
   characters in headers [RFC2045] and [RFC2231].  However, the target is
   fully internationalized message headers, as discussed in [RFC5335]
   [RFC5335bis-Hdrs] and not an extended and painful transition.

8.  Downgrading before and after

6.3.  SMTP Transactions

   An important issue with these extensions Service Extension for DSNs

   The existing Draft Standard Delivery status notifications (DSNs)
   specification [RFC3461] is how limited to handle
   interactions between systems that support non-ASCII addresses 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

7.  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
   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 will be the result of configuration errors:
   especially if it hosts non-ASCII addresses, a final delivery server MTA that
   accepts these extensions should not be configured with lower-
   preference MX hosts that do not.  While the experiments that preceded
   these specifications included a mechanism for passing backup ASCII
   addresses to intermediate relay systems and having those systems
   alter the relevant message header fields and substitute the
   addresses, addresses
   [RFC5504], the requirements and long-term implications of that system
   proved too complex to be satisfactory.  Consequently, if an
   intermediate SMTP relay that is transmitting a message that requires
   these extensions and 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.


7.1.  Downgrading before or during Message Submission

   Perhaps obviously, the most convenient time to find an ASCII address
   corresponding to an internationalized address is at the originating
   MUA.  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 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 [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 encounters.


7.2.  Downgrading or Other Processing After Final SMTP Delivery

   When an email message is received by a final delivery SMTP server, 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 6.1 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.

   Since the final delivery SMTP server 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 either downgrade internationalized
   emails or specially identify messages that utilize these extensions,
   or both.  If this is done, the final delivery SMTP server MTA SHOULD include a
   mechanism to preserve or recover the original internationalized forms
   without information loss to support access by UTF8SMTPbis-aware


8.  Downgrading in Transit

   [[anchor16: Note in Draft and Question for

   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 WG: We could discuss
   local-part MUST be interpreted and assigned semantics only by the various issues with in-transit downgrading including
   host specified in the
   complexities domain part of carrying backup addresses, the problems that
   motivated the "don't mess with addresses address".  This is not a new
   requirement; equivalent statements appeared in transit" (paraphrased,
   obviously) rule specifications in RFC 5321 and friends, 2001
   [RFC2821] and so on.  Or we could omit
   it (and this section).  Pragmatically, I think it would take us some
   time even in 1989 [RFC1123].

   Adherence to reach consensus on what, exactly, should be said and that
   might delay progress.  But input is clearly needed -- if it is not
   received before we prepared -02, this section will simply be

10.  User Interface and Configuration Issues

   Internationalization rule means that a downgrade mechanism that
   transforms the local-part of addresses and message headers, especially an email address cannot be done in
   transit.  It can only be done at the endpoints, namely by the MUA or
   submission server or by the final delivery MTA.

   One of the reasons for this rule has to do with variations on character legacy email systems
   that use source routing 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& is
   a route ("user" is reached via "foo") or simply a local address.

9.  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.
   [[anchor15: Note in Draft: What do we want to do about this?]]
   The subsections below address some other issues.


9.1.  Choices of Mailbox Names and Unicode Normalization

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

   In the absence of this extension, 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 but is prohibited by RFC 5321 because it
   requires a backspace character (a prohibited C0 control).  The
   practice SHOULD be phased out as this extension becomes widely
   deployed but backward-compatibility considerations may require that
   it continue to be recognized.

   For the particular case of EAI mailbox names, special attention must
   be paid to Unicode normalization, 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 for servers to provide support addresses only in Normalized form and to normalize strings on receipt, form,
      using either Normalization Form NFC and, except in unusual
      circumstances, NFKC.
      [[anchor19: Note in Draft: "Normalize on receipt" is consistent
      with the recommendations in draft-iab-i18n-encoding.  The issue
      with NFKC is that some of the characters mapped out may be
      significant, especially in personal names.  Anyone with objections
      should speak up.  Soon.]]

   o  It may be wise to support other forms of the same local-part
      string, either as aliases or by normalization of strings reaching
      the delivery server, in the event that the sender does not 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.

      *  C0 (and presumably C1) controls (see The Unicode Standard) Standard
         [Unicode52]) are prohibited, the first in RFC 5321 and the
         second by an obvious extension from it. it [RFC5198].

      *  Other kinds of punctuation, spaces, etc., are risky practice.
         Perhaps they will work, and SMTP receiver code is required to
         handle them, but creating dependencies on them in mailbox names
         that are chosen is usually a bad practice and may lead to
         interoperability problems.


10.  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.


10.1.  Impact on URIs and IRIs

   The mailto: schema defined in [RFC2368] and discussed in the Internationalized
   Resource Identifier (IRI) specification [RFC3987] may need to be
   modified when this work is completed and standardized.
   In particular, providing an alternate address as part of a mailto:
   URI may require some fairly careful work on the syntax of that URI.

11.2.  Interaction with Delivery Notifications

   The advent of UTF8SMTPbis will make necessary consideration of the
   interaction with delivery notification mechanisms, including the
   ASCII-only SMTP extension for requesting delivery notifications
   (DSNs) [RFC3461], and the format of delivery notifications [RFC3464].
   A new document, "International Delivery and Disposition
   Notifications" [RFC5337] 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.


10.2.  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. 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.


10.3.  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 proposed to enable defined that include
   automatic EAI-upgrade --
   including RFC 2047 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 [RFC3851] 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 7.1, 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.

10.4.  LMTP

   LMTP [RFC2033] may be used as part of the final delivery agent.  In
   such cases, LMTP may be arranged to deliver the mail to the mail
   store.  The mail store may not have UTF8SMTPbis capability.  LMTP may
   need to be updated to deal with these situations.


10.5.  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

   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 [RFC5335]. [RFC5335bis-Hdrs].  Whether this issue is 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
   how they are interpreted.


10.6.  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] form 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] format with US-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.  Experimental Targets

   [[anchor26: Note in draft: this section is left in this draft for
   convenience in review.  It will be removed with -02.]]

   In addition to the simple question of whether the model outlined here
   can be made to work in a satisfactory way for upgraded systems and
   provide adequate protection for un-upgraded ones, we expect that
   actually working with the systems will provide answers to two
   additional questions: what restrictions such as character lists or
   normalization should be placed, if any, on the characters that are
   permitted to be used in address local-parts and how useful, in
   practice, will downgrading turn out to be given whatever restrictions
   and constraints that must be placed upon it.


11.  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


12.  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 email
   local parts since those are case sensitive.

   Since email addresses are often transcribed from business cards and
   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 ongoing
   work discussed
   in [RFC5198], in described elsewhere [RFC5198] [RFC5893] and elsewhere. [IAB-idn-encoding].

   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.  Expecting and most or all
   such transformations prior to final delivery be done by systems that
   are presumed to be under the administrative control of the sending
   user ameliorates the potential problem somewhat as compared to what
   it would be if the relationships were changed in transit.

   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 final MUA, MUA used by the
   recipient, then it may be possible to create a message that appears
   acceptable under the filter's interpretation but should be rejected
   under the interpretation given to it by the final 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 11.3). 10.2).  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.

   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 11.4). 10.3).
   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]) (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, 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

15.  Acknowledgements

13.  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.

14.  References


14.1.  Normative References

   [ASCII]                   American National Standards Institute
                             (formerly United States of America
                             Standards Institute), "USA Code for
                             Information Interchange", ANSI X3.4-1968,

                             ANSI X3.4-1968 has been replaced by newer
                             versions with slight modifications, but the
                             1968 version remains definitive for the

   [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,
                             BCP 14, 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.

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

   [RFC5322]                 Resnick, P., Ed., "Internet Message
                             Format", RFC 5890, June 2010.

16.2.  Informative References

   [EAI-Mailinglist]    Gellens, R., "Mailing Lists 5322, October 2008.

   [RFC5335bis-Hdrs]         Yang, A. and
                        Internationalized S. Steele, "Internationalized
                             Email Addresses", June Headers", July 2010, <

   [Hoffman-IMAA]       Hoffman, P. and A. Costello, "Internationalizing
                        Mail Addresses in Applications (IMAA)", Work
                        in Progress, October 2003.

   [JET-IMA] <https://

   [RFC5336bis-SMTP]         Yao, J. and J. Yeh, "Internationalized eMail
                        Address (IMA)", Work in Progress, W. Mao, "SMTP Extension for
                             Internationalized Email Address",
                             June 2005.

   [Klensin-emailaddr] 2010, <

   [RFC5337bis-DSN]          Not yet posted?, "Internationalized
                             Delivery Status and Disposition
                             Notifications", Unwritten waiting for I-D,

   [RFC5721bis-POP3]         Not yet posted?, "POP3 Support for UTF-8",
                             Unwritten waiting for I-D, 2010.

   [RFC5738bis-IMAP]         Not yet posted?, "IMAP Support for UTF-8",
                             Unwritten waiting for I-D, 2010.

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

   [RFCNNNNbis-MailingList]  Not yet posted?, "Mailing Lists and
                             Internationalized Email Addresses", Work First
                             Version still not in Progress, July 2005. RFC Editor queue https
                             Unwritten waiting for I-D, 2010.

14.2.  Informative References

   [EAI-Mailinglist]         Gellens, R., "Mailing Lists and
                             Internationalized Email Addresses",
                             March 2010, <

   [IAB-idn-encoding]        Thaler, D., Klensin, J., and S. Cheshire,
                             "IAB Thoughts on Encodings for
                             Internationalized Domain Names", 2010, <htt

   [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.

   [RFC2033]                 Myers, J., "Local Mail Transfer Protocol",
                             RFC 2033, October 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:
                        Character Characte
                             r Sets, Languages, and Continuations",
                             RFC 2231, November 1997.

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

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

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

   [RFC3851]                 Ramsdell, B., "Secure/Multipurpose Internet
                             Mail Extensions (S/MIME) Version 3.1
                             Message Specification", RFC 3851,
                             July 2004.

   [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.

   [RFC5322]            Resnick, P., Ed.,

   [RFC5280]                 Cooper, D., Santesson, S., Farrell, S.,
                             Boeyen, S., Housley, R., and W. Polk,
                             "Internet Message Format", X.509 Public Key Infrastructure
                             Certificate and Certificate Revocation List
                             (CRL) Profile", RFC 5322, October 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,
                             "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.

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

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

   [Unicode-UAX15]           The Unicode Consortium, "Unicode Standard
                             Annex #15: Unicode Normalization Forms",
                             March 2008,

   [Unicode52]               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://

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.

   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  Merged  Discussion of RFC 5825 ("downgraded display") has been removed per
      the earlier sections 4.4 note and 4.6 into 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 expanded Informational spec can
      obsolete or update Experimental ones, so no downref problem here

   o  Reference to alternate addresses dropped from (former) Section 4.4.

   o  Merged earlier  Reference to RFC 5504 added to (former) Section 11.6 into 8 for

   o  Ernie's draft comments added (with some minor edits) to replace
      the placeholder in (former) Section 11.2 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 eliminated propose alternate text.  RSN.

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

   o  Eliminated former last paragraph  Placeholder at the end of Section 11.4 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

   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.

   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 artifact additional clarification about the relationship of
      in-transit downgrading.
      these extensions to non-ASCII messages.

   o  Updated  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 few references. remaining pointer to address downgrading in the
      discussion of an updated MAILTO URI.

   o  Minor additional editorial cleanups and tuning.

Authors' Addresses

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

   Phone: +1 617 491 5735

   YangWoo Ko KO
   119 Munjiro
   Yuseong-gu, Daejeon  305-732
   Republic of Korea