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Versions: 00 01 02 03 04 05 06 RFC 5337

Network Working Group                                          C. Newman
Internet-Draft                                          Sun Microsystems
Updates: 3461,3464,3798 (if                             January 25, 2007
approved)
Expires: July 29, 2007


          International Delivery and Disposition Notifications
                       draft-ietf-eai-dsn-00.txt

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   By submitting this Internet-Draft, each author represents that any
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   This Internet-Draft will expire on July 29, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2007).

Abstract

   Delivery status notifications (DSNs) are critical to the correct
   operation of an email system.  However, the existing draft standard
   is presently limited to US-ASCII text in the machine readable
   portions of the protocol.  This specification adds a new address type
   for international email addresses so an original recipient address
   with non-US-ASCII characters can be correctly preserved even after
   downgrading.  This also provides updated content return media types



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   for delivery status notifications and message disposition
   notifications to support use of the new address type.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in this Document  . . . . . . . . . . . . . .  3
   3.  UTF-8 Address Type . . . . . . . . . . . . . . . . . . . . . .  3
   4.  UTF-8 Encoded Address Type . . . . . . . . . . . . . . . . . .  4
   5.  UTF-8 Delivery Status Notifications  . . . . . . . . . . . . .  5
   6.  UTF-8 Message Disposition Notifications  . . . . . . . . . . .  6
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
     7.1.  UTF-8 Mail Address Type Registration . . . . . . . . . . .  7
     7.2.  UTF-8-ENC Mail Address Type Registration . . . . . . . . .  7
     7.3.  Update to 'smtp' Diagnostic Type Registration  . . . . . .  8
     7.4.  message/utf-8-headers  . . . . . . . . . . . . . . . . . .  8
     7.5.  message/utf-8  . . . . . . . . . . . . . . . . . . . . . .  9
     7.6.  message/utf-8-delivery-status  . . . . . . . . . . . . . . 11
     7.7.  message/utf-8-disposition-notification . . . . . . . . . . 12
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 15
   Appendix B.  Open Issues . . . . . . . . . . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
   Intellectual Property and Copyright Statements . . . . . . . . . . 17























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1.  Introduction

   When an email message is transmitted using the UTF8SMTP [I-D.ietf-
   eai-smtpext] extension and Internationalized Email Headers [I-D.ietf-
   eai-utf8headers], it is sometimes necessary to return that message or
   generate a Message Disposition Notification [RFC3798] (MDN).  As a
   message sent to multiple recipients can generate a status and
   disposition notification for each recipient, it is helpful if a
   client can correlate these returns based on the recipient address it
   provided, thus preservation of the original recipient is important.
   This specification describes how to preserve the original recipient
   and updates the MDN and DSN formats to support the new address types.


2.  Conventions Used in this Document

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
   in this document are to be interpreted as defined in "Key words for
   use in RFCs to Indicate Requirement Levels" [RFC2119].

   The formal syntax use the Augmented Backus-Naur Form (ABNF) [RFC4234]
   notation including the core rules defined in Appendix B of RFC 4234
   and the rules in section 4 of RFC 3629.


3.  UTF-8 Address Type

   An Extensible Message Format for Delivery Status Notifications
   [RFC3464] defines the concept of an address type.  The address format
   introduced in Internationalized Email Headers [I-D.ietf-eai-
   utf8headers] is a new address type.  The syntax for the new address
   type follows in the context of status notifications follows:

   utf-8-type-addr     = "utf-8;" utf-8-address

   utf-8-address       = "<" Mailbox [ *WSP "<" Mailbox ">" ] ">"
          ; The first  occurrence of 'Mailbox' is defined in [utf8smtp]
          ; The second occurrence of 'Mailbox' is defined in RFC 2821

   This address type definition requires 8-bit characters and provides
   no encoding mechanism.  As a result, it is only suitable for use in
   newly defined protocols capable of native representation of 8-bit
   characters.  This address type MUST NOT be used in the SMTP ORCPT
   parameter or a message/delivery-status body part field, but SHOULD be
   used in a message/utf-8-delivery-status body part Original-Recipient
   or Final-Recipient field.





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4.  UTF-8 Encoded Address Type

   An SMTP [RFC2821] server which advertises both the UTF8SMTP extension
   [I-D.ietf-eai-smtpext] and the DSN extension [RFC3461] MUST accept a
   utf-8-enc address type in the ORCPT parameter including 8-bit UTF-8
   characters.  This address type also includes a 7-bit encoding
   suitable for use in a message/delivery-status body part or an ORCPT
   parameter sent to an SMTP server which does not advertise UTF8SMTP.

   The utf-8-enc address type requires that US-ASCII CTLs, SP, %, + and
   = be encoded using '%' encoding as described in the ABNF below.  As a
   result, the xtext encoding defined in section 4 of the SMTP DSN
   extension [RFC3461] is not used with the utf-8-enc address type
   because it is never necessary.  In addition, plane 1 Unicode
   characters MAY be included in a utf-8-enc address type using a
   "%u####" syntax (QMIDCHAR, where # is a hexadecimal digit) and other
   Unicode characters MAY be encoded using "%U########" syntax
   (QHIGHCHAR).  When sending data to a UTF8SMTP capable server, native
   UTF-8 characters SHOULD be used instead of the QMIDCHAR and QHIGHCHAR
   encodings described below.  When sending data to an SMTP server which
   does not advertise UTF8SMTP, then the QMIDCHAR and QHIGHCHAR
   encodings MUST be used instead of UTF-8.

   When the ORCPT parameter is placed in a message/utf-8-delivery-status
   Original-Recipient field, the utf-8-enc address type SHOULD be
   converted to a utf-8 address type by removing all '%' encoding.
   However, if an address is labeled with the utf-8-enc address type but
   does not conform to utf-8-enc syntax, then it MUST be copied into the
   message/utf-8-delivery-status field without alteration.

   The ability to encode characters with the QMIDCHAR or QHIGHCHAR
   encodings should be viewed as a transitional mechanism.  It is hoped
   that as systems lacking support for UTF8SMTP become less common over
   time, these encodings can eventually be phased out.

   The formal syntax for this address type follows:















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   utf-8-enc-type-addr = "utf-8-enc;" utf-8-enc-addr

   utf-8-enc-addr      = 1*(QUCHAR / QLOWCHAR / QMIDCHAR / QHIGHCHAR)
                       ; MUST follow utf-8-address ABNF when dequoted

   QUCHAR              = %x21-24 / %x26-2a / %x2c-3c / %x3e-7e /
                         UTF8-2 / UTF8-3 / UTF8-4
                       ; Printable except CTLs SP, %, + and =
   QLOWCHAR            = ("%0" NZHEXDIG) / ("%1" HEXDIG) / "%20" / "%25"
                       / "%2B" / "%3D" / "%7F"
                       ; Only permitted for CTLs, SPACE, %, + and =
   QMIDCHAR            = "%" %x75 UCHAR-HEX-QUAD
                       ; %u#### excluding surrogates and US-ASCII
   QHIGHCHAR           = "%" %x55 (UCHAR-HEX-5 / UCHAR-HEX-6)
                       ; %U######## excluding plane 1
   UCHAR-HEX-QUAD      = UCHAR-HEX-2 / UCHAR-HEX-3
                       / UCHAR-HEX-4 / UCHAR-HEX-4D
   UCHAR-HEX-2         = "00" HEXDIG8 HEXDIG
   UCHAR-HEX-3         = "0" NZHEXDIG 2(HEXDIG)
   UCHAR-HEX-4         = NZDHEXDIG 3(HEXDIG)
   UCHAR-HEX-4D        = "D" %x30-37 2(HEXDIG)
   UCHAR-HEX-5         = "000" NZHEXDIG 4(HEXDIG)
   UCHAR-HEX-6         = "00" NZHEXDIG 5(HEXDIG)
   HEXDIG8             = %x38-39 / "A" / "B" / "C" / "D" / "E" / "F"
                       ; HEXDIG excluding 0-7
   NZHEXDIG            = %x31-39 / "A" / "B" / "C" / "D" / "E" / "F"
                       ; HEXDIG excluding "0"
   NZDHEXDIG           = %x31-39 / "A" / "B" / "C" / "E" / "F"
                       ; HEXDIG excluding "0" and "D"


5.  UTF-8 Delivery Status Notifications

   A traditional delivery status notification [RFC3464] comes in a
   three-part multipart/report [RFC3462] container, where the first part
   is human readable text describing the error, the second part is a
   7-bit-only message/delivery-status and the optional third part is
   used for content (message/rfc822) or header (text/rfc822-headers)
   return.  An SMTP server which advertises both UTF8SMTP and DSN SHOULD
   return an undeliverable UTF8SMTP message without downgrading it
   (assuming the return SMTP server supports UTF8SMTP).  As the present
   DSN format does not permit this, three new media types are needed.

   The first type, message/utf-8-delivery-status has the syntax of
   message/delivery-status with two modifications.  First, the charset
   for message/utf-8-delivery-status is UTF-8 and thus any field MAY
   contain UTF-8 characters when appropriate.  Second, systems
   generating a message/utf-8-delivery-status body part SHOULD use the



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   utf-8 address type for all addresses containing characters outside
   the US-ASCII repertoire.  These systems SHOULD up-convert a utf-8-enc
   address type in the ORCPT parameter to a utf-8 address type in the
   Original-Recipient field.

   The second type, used for content return, is message/utf-8 is similar
   to message/rfc822, except it contains a message with UTF-8 headers.
   This type has profound implications on the email infrastructure.
   First, Internet Message Access Protocol [RFC3501] servers MUST NOT
   descend a message/utf-8 when generating the message BODYSTRUCTURE, it
   is likely a new variant on BODYSTRUCTURE will be necessary that does
   descend message/utf-8 body parts.  Second, if this type is sent to a
   7-bit-only system, it could be encoded in base64 or quoted-printable
   [RFC2045].  As a result, SMTP servers and other systems which
   transfer a message/utf-8 body part MAY choose to down-convert it to a
   message/rfc822 body part using the rules described in Downgrading
   mechanism for Email Address Internationalization [I-D.ietf-eai-
   downgrade].

   The third type, used for header return, is message/utf-8-headers and
   contains only the UTF-8 headers of a message (all lines prior to the
   first blank line in a UTF8SMTP message).  Unlike message/utf-8, this
   body part provides no difficulties for present infrastructure.


6.  UTF-8 Message Disposition Notifications

   Message Disposition Notifications [RFC3798] have a similar design and
   structure to DSNs.  As a result, they use the same basic return
   format.  When generating a MDN for a UTF-8 header message, content or
   header return is the same as for DSNs.  The second part of the
   multipart/report uses a new media type, message/
   utf-8-disposition-notification, which has the syntax of message/
   disposition-notification with two modifications.  First, the charset
   for message/utf-8-disposition-notification is UTF-8 and thus any
   field MAY contain UTF-8 characters when appropriate.  Second, systems
   generating a message/utf-8-disposition-notification body part
   (typically a mail user agent) SHOULD use the utf-8 address type for
   all addresses containing characters outside the US-ASCII repertoire.

   The MDN specification also defines the Original-Recipient header
   which is added with a copy of the contents of ORCPT at delivery time.
   A delivery agent writing a UTF-8 header message in native format
   SHOULD convert a utf-8-enc address type in the ORCPT parameter to a
   utf-8 address type when generating an Original-Recipient header
   field.

   The MDN specification also defines the Disposition-Notification-To



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   header which is an address header and thus follows the same 8-bit
   rules as other address headers such as "From" and "To" when used in a
   UTF-8 header message.


7.  IANA Considerations

   This specification does not create any new IANA registries.  However
   the following items are registered as a result of this document:

7.1.  UTF-8 Mail Address Type Registration

   The mail address type registry was created by RFC 3464.  The
   registration template response follows:

   (a) The proposed address-type name.

   UTF-8

   (b) The syntax for mailbox addresses of this type, specified using
   BNF, regular expressions, ASN.1, or other non-ambiguous language.

   See Section 3.

   (c) If addresses of this type are not composed entirely of graphic
   characters from the US-ASCII repertoire, a specification for how they
   are to be encoded as graphic US-ASCII characters in a DSN Original-
   Recipient or Final-Recipient DSN field.

   This address type MUST NOT be used in the ORCPT parameter or in a
   7-bit transport environment including a message/delivery-status
   Original-Recipient or Final-Recipient field.  The UTF-8-ENC address
   type is used for that purpose.  This address type MAY be used in a
   message/utf-8-delivery-status Original-Recipient or Final-Recipient
   DSN field or an Original-Recipient header [RFC3798] if the message is
   a UTF-8 header message.

7.2.  UTF-8-ENC Mail Address Type Registration

   (a) The proposed address-type name.

   UTF-8-ENC

   (b) The syntax for mailbox addresses of this type, specified using
   BNF, regular expressions, ASN.1, or other non-ambiguous language.

   See Section 4.




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   (c) If addresses of this type are not composed entirely of graphic
   characters from the US-ASCII repertoire, a specification for how they
   are to be encoded as graphic US-ASCII characters in a DSN Original-
   Recipient or Final-Recipient DSN field.

   When it is necessary to transport a UTF-8 address type in a 7-bit
   context or in a context where not all legal US-ASCII characters are
   permitted (e.g. the ORCPT parameter forbids SP and =), this encoding
   MUST be used.

7.3.  Update to 'smtp' Diagnostic Type Registration

   The mail diagnostic type registry was created by RFC 3464.  The
   registration for the 'smtp' diagnostic type should be updated to
   reference RFC XXXX in addition to RFC 3464.

   When the 'smtp' diagnostic type is used in the context of a message/
   delivery-status body part, it remains as presently defined.  When the
   'smtp' diagnostic type is used in the context of a message/
   utf-8-delivery-status body part, the codes remain the same, but the
   text portion MAY contain UTF-8 characters.

7.4.  message/utf-8-headers

   Type name: message

   Subtype name: utf-8-headers

   Required parameters: none

   Optional parameters: none

   Encoding considerations: This media type contains Internationalized
      Email Headers [I-D.ietf-eai-utf8headers] with no message body.
      Whenever possible, the 8-bit content transfer encoding SHOULD be
      used.  When this media type passes through a 7-bit-only SMTP
      infrastructure it MAY be encoded with the base64 or quoted-
      printable content transfer encoding.

   Security considerations: See Section 8

   Interoperability considerations: It is important this media type is
      not converted to a charset other than UTF-8.  As a result,
      implementations MUST NOT include a charset parameter with this
      media type.  Although it might be possible to downconvert this
      media type to the text/rfc822-header media type, such conversion
      is discouraged as it loses information.




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   Published specification: RFC XXXX

   Applications that use this media type: UTF8SMTP servers and email
      clients that support multipart/report generation or parsing.

   Additional information:

   Magic number(s): none

   File extension(s): In the event this is saved to a file, the
      extension ".u8hdr" is suggested.

   Macintosh file type code(s): The 'TEXT' type code is suggested as
      files of this type are typically used for diagnostic purposes and
      suitable for analysis in a UTF-8 aware text editor.  A uniform
      type identifier (UTI) of "public.utf8-email-message-header" is
      suggested.  This type conforms to "public.utf8-plain-text" and
      "public.plain-text".

   Person & email address to contact for further information: See the
      Author's address section of this document.

   Intended usage: COMMON

   Restrictions on usage: This media type contains textual data in the
      UTF-8 charset.  It typically contains octets with the 8th bit set.
      As a result a transfer encoding is required when a 7-bit transport
      is used.

   Author: See Author's Address section of this document.

   Change controller: IETF Standards Process

7.5.  message/utf-8

   Type name: message

   Subtype name: utf-8

   Required parameters: none

   Optional parameters: none

   Encoding considerations: This media type contains Internationalized
      Email Headers [I-D.ietf-eai-utf8headers] and MIME message body
      content.  The 8-bit or binary content-transfer-encoding MUST be
      used unless this media type is sent over a 7-bit only transport.




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   Security considerations: See Section 8

   Interoperability considerations: The media type provides
      functionality similar to the message/rfc822 content type for email
      messages with international email headers.  When there is a need
      to embed or return such content in another message, there is
      generally an option to use this media type and leave the content
      unchanged or downconvert the content to message/rfc822.  Both of
      these choices will interoperate with the installed base, but with
      different properties.  Systems unaware of international headers
      will typically treat a message/utf-8 body part as an unknown
      attachment, while they will understand the structure of a message/
      rfc822.  However, systems which understand message/utf-8 will
      provide functionality superior to the result of a down-conversion
      to message/rfc822.  The most interoperable choice depends on the
      deployed software.

   Published specification: RFC XXXX

   Applications that use this media type: SMTP servers and email clients
      that support multipart/report generation or parsing.  Email
      clients which forward messages with international headers as
      attachments.

   Additional information:

   Magic number(s): none

   File extension(s): The extension ".u8msg" is suggested.

   Macintosh file type code(s): A uniform type identifier (UTI) of
      "public.utf8-email-message" is suggested.  This conforms to
      "public.message" and "public.composite-content" but does not
      necessarily conform to "public.utf8-plain-text".

   Person & email address to contact for further information: See the
      Author's address section of this document.

   Intended usage: COMMON

   Restrictions on usage: This is a structured media type which embeds
      other MIME media types.  The 8-bit or binary content-transfer-
      encoding MUST be used unless this media type is sent over a 7-bit
      only transport.







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   Author: See Author's Address section of this document.

   Change controller: IETF Standards Process

7.6.  message/utf-8-delivery-status

   Type name: message

   Subtype name: utf-8-delivery-status

   Required parameters: none

   Optional parameters: none

   Encoding considerations: This media type contains delivery status
      notification attributes in the UTF-8 charset.  The 8-bit content
      transfer encoding MUST be used with this content-type, unless it
      is sent over a 7-bit transport environment in which case quoted-
      printable or base 64 may be necessary.

   Security considerations: See Section 8

   Interoperability considerations: This media type provides
      functionality similar to the message/delivery-status content type
      for email message return information.  Clients of the previous
      format will need to be upgraded to interpret the new format,
      however the new media type makes it simple to identify the
      difference.

   Published specification: RFC XXXX

   Applications that use this media type: SMTP servers and email clients
      that support delivery status notification generation or parsing.

   Additional information:

   Magic number(s): none

   File extension(s): The extension ".u8dsn" is suggested.

   Macintosh file type code(s): A uniform type identifier (UTI) of
      "public.utf8-email-message-delivery-status" is suggested.  This
      type conforms to "public.utf8-plain-text".

   Person & email address to contact for further information: See the
      Author's address section of this document.





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   Intended usage: COMMON

   Restrictions on usage: This is expected to be the second part of a
      multipart/report.

   Author: See Author's Address section of this document.

   Change controller: IETF Standards Process

7.7.  message/utf-8-disposition-notification

   Type name: message

   Subtype name: utf-8-disposition-notification

   Required parameters: none

   Optional parameters: none

   Encoding considerations: This media type contains disposition
      notification attributes in the UTF-8 charset.  The 8-bit content
      transfer encoding MUST be used with this content-type, unless it
      is sent over a 7-bit transport environment in which case quoted-
      printable or base 64 may be necessary.

   Security considerations: See Section 8

   Interoperability considerations: This media type provides
      functionality similar to the message/disposition-notification
      content type for email message disposition information.  Clients
      of the previous format will need to be upgraded to interpret the
      new format, however the new media type makes it simple to identify
      the difference.

   Published specification: RFC XXXX

   Applications that use this media type: Email clients or servers that
      support message disposition notification generation or parsing.

   Additional information:

   Magic number(s): none

   File extension(s): The extension ".u8mdn" is suggested.







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   Macintosh file type code(s): A uniform type identifier (UTI) of
      "public.utf8-email-message-disposition-notification" is suggested.
      This type conforms to "public.utf8-plain-text".

   Person & email address to contact for further information: See the
      Author's address section of this document.

   Intended usage: COMMON

   Restrictions on usage: This is expected to be the second part of a
      multipart/report.

   Author: See Author's Address section of this document.

   Change controller: IETF Standards Process


8.  Security Considerations

   Automated use of report types without authentication presents several
   security issues.  Forging negative reports presents the opportunity
   for denial-of-service attacks when the reports are used for automated
   maintenance of directories or mailing lists.  Forging positive
   reports may cause the sender to incorrectly believe a message was
   delivered when it was not.

   Malicious users can generate report structures designed to trigger
   coding flaws in report parsers.  Report parsers need to use secure
   coding techniques to avoid the risk of buffer overflow or denial-of-
   service attacks against parser coding mistakes.  Code reviews of such
   parsers are also recommended.

   Malicious users of the email system regularly send messages with
   forged envelope return paths and these messages trigger delivery
   status reports that result in a large amount of unwanted traffic on
   the Internet.  Many users choose to ignore delivery status
   notifications because they are usually the result of "blowback" from
   forged messages and thus never notice when messages they sent go
   undelivered.  As a result, support for correlation of delivery status
   and message disposition notification messages with sent-messages has
   become a critical feature of mail clients and possibly mail stores if
   the email infrastructure is to remain reliable.  In the short term,
   simply correlating message-IDs may be sufficient to distinguish true
   status notifications from those resulting from forged originator
   addresses.  But in the longer term, including cryptographic signature
   material that can securely associate the status notification with the
   original message is advisable.




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   As this specification permits UTF-8 in additional fields, the
   security considerations of UTF-8 [RFC3629] apply.


9.  References

9.1.  Normative References

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

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

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

   [RFC3462]  Vaudreuil, G., "The Multipart/Report Content Type for the
              Reporting of Mail System Administrative Messages",
              RFC 3462, January 2003.

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

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

   [RFC3798]  Hansen, T. and G. Vaudreuil, "Message Disposition
              Notification", RFC 3798, May 2004.

   [RFC4234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

   [I-D.ietf-eai-utf8headers]
              Yeh, J., "Internationalized Email Headers",
              draft-ietf-eai-utf8headers-02 (work in progress),
              October 2006.

   [I-D.ietf-eai-smtpext]
              Yao, J. and W. Mao, "SMTP extension for internationalized
              email address", draft-ietf-eai-smtpext-02 (work in
              progress), October 2006.







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9.2.  Informative References

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

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

   [I-D.ietf-eai-downgrade]
              Yoneya, Y. and K. Fujiwara, "Downgrading mechanism for
              Email Address Internationalization (EAI)",
              draft-ietf-eai-downgrade-02 (work in progress), Aug 2006.


Appendix A.  Acknowledgements

   Many thanks for input provided by Alexey Melnikov, Pete Resnick,
   James Galvin, Ned Freed, John Klensin and members of the EAI WG to
   help solidify this proposal.


Appendix B.  Open Issues

   Suggestion to change the utf-8-enc-addr format from %-encoded Unicode
   to %-encoded UTF-8 as used in URIs.

























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Author's Address

   Chris Newman
   Sun Microsystems
   3401 Centrelake Dr., Suite 410
   Ontario, CA  91761
   US

   Email: chris.newman@sun.com










































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