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Internationalized Email Headers

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6532.
Authors Abel Yang , Shawn Steele , Ned Freed
Last updated 2020-01-21 (Latest revision 2011-10-21)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state WG Document
Document shepherd (None)
IESG IESG state Became RFC 6532 (Proposed Standard)
Action Holders
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Pete Resnick
Send notices to
Email Address Internationalization                               A. Yang
(EAI)                                                              TWNIC
Internet-Draft                                                 S. Steele
Obsoletes: 5335 (if approved)                                  Microsoft
Updates: 2045 (if approved)                                     N. Freed
Intended status: Standards Track                                  Oracle
Expires: April 23, 2012                                 October 21, 2011

                    Internationalized Email Headers


   Internet mail was originally limited to 7-bit ASCII.  MIME added
   support for the use of 8-bit character sets in body parts, and also
   defined an encoded-word construct so other character sets could be
   used in certain header field values.  But full internationalization
   of electronic mail requires additional enhancements to allow the use
   of Unicode, including characters outside the ASCII repertoire, in
   mail addresses as well as direct use of Unicode in header fields like
   From:, To:, and Subject:, without requiring the use of complex
   encoded-word constructs.  This document specifies an enhancement to
   the Internet Message Format and to MIME that allows use of Unicode in
   mail addresses and most header field content.

   This specification replaces RFC 5335.  This specification also
   updates Section 6.4 of RFC 2045 to eliminate the restriction
   prohibiting the use of non-identity content-transfer-encodings on
   subtypes of "message/".

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
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 23, 2012.

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Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology Used In This Specification . . . . . . . . . . . .  3
   3.  Changes to Message Header Fields . . . . . . . . . . . . . . .  4
     3.1.  UTF-8 Syntax and Normalization . . . . . . . . . . . . . .  4
     3.2.  Syntax Extensions to RFC 5322  . . . . . . . . . . . . . .  5
     3.3.  Use of 8-bit UTF-8 in Message-Ids  . . . . . . . . . . . .  5
     3.4.  Effects on Line Length Limits  . . . . . . . . . . . . . .  6
     3.5.  Changes to MIME Message Type Encoding Restrictions . . . .  6
     3.6.  Use of MIME Encoded-Words  . . . . . . . . . . . . . . . .  6
     3.7.  The Message/global Media Type  . . . . . . . . . . . . . .  7
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Edit history . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  draft-ietf-eai-rfc5335bis-00 . . . . . . . . . . . . . . .  9
     7.2.  draft-ietf-eai-rfc5335bis-01 . . . . . . . . . . . . . . . 10
     7.3.  draft-ietf-eai-rfc5335bis-02 . . . . . . . . . . . . . . . 10
     7.4.  draft-ietf-eai-rfc5335bis-03 . . . . . . . . . . . . . . . 10
     7.5.  draft-ietf-eai-rfc5335bis-04 . . . . . . . . . . . . . . . 10
     7.6.  draft-ietf-eai-rfc5335bis-05 . . . . . . . . . . . . . . . 10
     7.7.  draft-ietf-eai-rfc5335bis-06 . . . . . . . . . . . . . . . 10
     7.8.  draft-ietf-eai-rfc5335bis-07 . . . . . . . . . . . . . . . 10
     7.9.  draft-ietf-eai-rfc5335bis-09 . . . . . . . . . . . . . . . 11
     7.10. draft-ietf-eai-rfc5335bis-10 . . . . . . . . . . . . . . . 11
     7.11. draft-ietf-eai-rfc5335bis-11 . . . . . . . . . . . . . . . 11
     7.12. draft-ietf-eai-rfc5335bis-12 . . . . . . . . . . . . . . . 11
     7.13. draft-ietf-eai-rfc5335bis-13 . . . . . . . . . . . . . . . 11
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 13

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1.  Introduction

   Internet mail distinguishes a message from its transport and further
   divides a message between a header and a body [RFC5322].  Internet
   mail header field values contain a variety of strings that are
   intended to be user-visible.  The range of supported characters for
   these strings was originally limited to 7-bit [ASCII].  MIME
   [RFC2045] [RFC2046] [RFC2047] provides the ability to use additional
   character sets, but this support is limited to body part data and to
   special encoded-word constructs that were only allowed in a limited
   number of places in header field values.

   Globalization of the Internet requires support of the much larger set
   of characters provided by Unicode [RFC5198] in both mail addresses
   and most header field values.  Additionally, complex encoding schemes
   like encoded-words introduce inefficiencies as well as significant
   opportunities for processing errors.  And finally, native support for
   the UTF-8 charset is now available on most systems.  Hence it is
   strongly desirable for Internet mail to support UTF-8 [RFC3629]

   This document specifies an enhancement to the Internet Message Format
   [RFC5322] and to MIME that permits the direct use of UTF-8, rather
   than only ASCII, in header field values, including mail addresses.  A
   new media type, message/global, is defined for messages that use this
   extended format.  This specification also lifts the MIME restriction
   on having non-identity content-transfer-encodings on any subtype of
   the message top-level type so that message/global parts can be safely
   transmitted across existing mail infrastructure.

   This specification is based on a model of native, end-to-end support
   for UTF-8, which depends on having an "8-bit clean" environment
   assured by the transport system.  Support for carriage across legacy,
   7-bit infrastructure and for processing by 7-bit receivers requires
   additional mechanisms that are not provided by these specifications.

   This specification is a revision of and replacement for [RFC5335].
   Section 6 of [I-D.ietf-eai-frmwrk-4952bis] describes the change in
   approach between this specification and the previous version.

2.  Terminology Used In This Specification

   A plain ASCII string is fully compatible with [RFC5321] and
   [RFC5322].  In this document, non-ASCII strings are UTF-8 strings if
   they are in header field values which contain at least one <UTF8-non-
   ascii> (see Section 3.1).

   Unless otherwise noted, all terms used here are defined in [RFC5321],

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   [RFC5322], [I-D.ietf-eai-frmwrk-4952bis], or

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The term "8-bit" means octets are present in the data with values
   above 0x7F.

3.  Changes to Message Header Fields

   To permit Unicode characters in field values, the header definition
   in [RFC5322] is extended to support the new format.  The following
   sections specify the necessary changes to RFC 5322's ABNF.

   The syntax rules not mentioned below remain defined as in [RFC5322].

   Note that this protocol does not change RFC 5322 rules for defining
   header field names.  The bodies of header fields are allowed to
   contain Unicode characters, but the header field names themselves
   must contain only ASCII characters.

   Also note that messages in this format require the use of the
   UTF8SMTPbis extension [I-D.ietf-eai-rfc5336bis] to be transferred via

3.1.  UTF-8 Syntax and Normalization

   UTF-8 characters can be defined in terms of octets using the
   following ABNF [RFC5234], taken from [RFC3629]:

   UTF8-non-ascii  =   UTF8-2 / UTF8-3 / UTF8-4

   UTF8-2          =   <Defined in Section 4 of RFC3629>

   UTF8-3          =   <Defined in Section 4 of RFC3629>

   UTF8-4          =   <Defined in Section 4 of RFC3629>

   See [RFC5198] for a discussion of Unicode normalization;
   normalization form NFC [UNF] SHOULD be used.  Actually, if one is
   going to do internationalization properly, one of the most often-
   cited goals is to permit people to spell their names correctly.
   Since many mailbox local parts reflect personal names, that principle
   applies to mailboxes as well.  The NFKC normalization form [UNF]

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   SHOULD NOT be used because it may lose information that is needed to
   correctly spell some names in some unusual circumstances.

3.2.  Syntax Extensions to RFC 5322

   The following rules extend the ABNF syntax defined in [RFC5322] and
   [RFC5234] in order to allow UTF-8 content.

   VCHAR   =/  UTF8-non-ascii

   ctext   =/  UTF8-non-ascii

   atext   =/  UTF8-non-ascii

   qtext   =/  UTF8-non-ascii

   text    =/  UTF8-non-ascii
                  ; note that this upgrades the body to UTF-8

   dtext   =/  UTF8-non-ascii

   The preceding changes mean that the following constructs now allow

   1.  Unstructured text, used in header fields like Subject: or

   2.  Any construct that uses atoms, including but not limited to the
       local parts of addresses and message-ids.  This includes
       addresses in the "for" clauses of Received: header fields.

   3.  Quoted strings.

   4.  Domains.

   Note that header field names are not on this list; these are still
   restricted to ASCII.

3.3.  Use of 8-bit UTF-8 in Message-Ids

   Implementers of message-id generation algorithms MAY prefer to
   restrain their output to ASCII since that has some advantages, such
   as when constructing In-reply-to: and References: header fields in
   mailing-list threads where some senders use EAI and others not.

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3.4.  Effects on Line Length Limits

   Section 2.1.1 of [RFC5322] limits lines to 998 characters and
   recommends that the lines be restricted to only 78 characters.  This
   specification changes the former limit to 998 octets.  (Note that in
   ASCII octets and characters are effectively the same but this is not
   true in UTF-8.)  The 78 character limit remains defined in terms of
   characters, not octets, since it is intended to address display width
   issues, not line length issues.

3.5.  Changes to MIME Message Type Encoding Restrictions

   This specification updates Section 6.4 of [RFC2045].  [RFC2045]
   prohibits applying a content-transfer-encoding to any subtypes of
   "message/".  This specification relaxes that rule -- it allows newly
   defined MIME types to permit content-transfer-encoding, and it allows
   content-transfer-encoding for message/global (see Section 3.7).

   Background: Normally, transfer of message/global will be done in
   8-bit-clean channels, and body parts will have "identity" encodings,
   that is, no decoding is necessary.

   But in the case where a message containing a message/global is
   downgraded from 8-bit to 7-bit as described in [RFC6152], an encoding
   might have to be applied to the message; if the message travels
   multiple times between a 7-bit environment and an environment
   implementing these extensions, multiple levels of encoding may occur.
   This is expected to be rarely seen in practice, and the potential
   complexity of other ways of dealing with the issue is thought to be
   larger than the complexity of allowing nested encodings where

3.6.  Use of MIME Encoded-Words

   The MIME encoded-words facility [RFC2047] provides the ability to
   place non-ASCII text, but only in a subset of the places allowed by
   this extension.  Additionally, encoded-words are substantially more
   complex since they allow the use of arbitrary charsets.  Accordingly,
   encoded-words SHOULD NOT be used when generating header fields for
   messages employing this extension.  Agents MAY, when incorporating
   material from another message, convert encoded-word use to direct use
   of UTF-8.

   Note that care must be taken when decoding encoded-words because the
   results after replacing an encoded-word with its decoded equivalent
   in UTF-8 may be syntactically invalid.  Processors that elect to
   decode encoded-words MUST NOT generate syntactically invalid fields.

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3.7.  The Message/global Media Type

   Internationalized messages in this format MUST only be transmitted as
   authorized by [I-D.ietf-eai-rfc5336bis] or within a non-SMTP
   environment that supports these messages.  A message is a "message/
   global message" if:

   o  it contains 8-bit UTF-8 header values as specified in this
      document, or

   o  it contains 8-bit UTF-8 values in the header fields of body parts.

   The content of a message/global part is otherwise identical to that
   of a message/rfc822 part.

   If an object of this type is sent to a 7-bit-only system, it MUST
   have an appropriate content-transfer-encoding applied.  (Note that a
   system compliant with MIME that doesn't recognize message/global is
   supposed to treat it as "application/octet-stream" as described in
   Section 5.2.4 of [RFC2046].)

   Type name:  message

   Subtype name:  global

   Required parameters:  none

   Optional parameters:  none

   Encoding considerations:  Any content-transfer-encoding is permitted.
      The 8-bit or binary content-transfer-encodings are recommended
      where permitted.

   Security considerations:  See Section 4.

   Interoperability considerations:  This media type provides
      functionality similar to the message/rfc822 content type for email
      messages with internationalized 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 down-convert the content to message/rfc822.  Both of
      these choices will interoperate with the installed base, but with
      different properties.  Systems unaware of internationalized
      headers will typically treat a message/global body part as an
      unknown attachment, while they will understand the structure of a
      message/rfc822.  However, systems that understand message/global
      will provide functionality superior to the result of a down-
      conversion to message/rfc822.  The most interoperable choice

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      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 that forward messages with internationalized 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 that embeds
      other MIME media types.  An 8-bit or binary content-transfer-
      encoding SHOULD be used unless this media type is sent over a
      7-bit-only transport.

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

   Change controller:  IETF Standards Process

4.  Security Considerations

   Because UTF-8 often requires several octets to encode a single
   character, internationalization may cause header field values in
   general and mail addresses in particular to become longer.  As
   specified in [RFC5322], each line of characters MUST be no more than
   998 octets, excluding the CRLF.  On the other hand, MDA (Mail
   Delivery Agent) processes that parse, store, or handle email
   addresses or local parts must take extra care not to overflow
   buffers, truncate addresses, or exceed storage allotments.  Also,
   they must take care, when comparing, to use the entire lengths of the

   There are lots of ways to use UTF-8 to represent something equivalent

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   or similar to a particular displayed character or group of
   characters; see the security considerations in [RFC3629] for details
   on the problems this can cause.  The normalization process described
   in Section 3.1 is recommended to minimize these issues.

   The security impact of UTF-8 headers on email signature systems such
   as Domain Keys Identified Mail (DKIM), S/MIME, and OpenPGP is
   discussed in [I-D.ietf-eai-frmwrk-4952bis], Section 14.

   If a user has a non-ASCII mailbox address and an ASCII mailbox
   address, a digital certificate that identifies that user might have
   both addresses in the identity.  Having multiple email addresses as
   identities in a single certificate is already supported in PKIX
   (Public Key Infrastructure for X.509 Certificates) [RFC5280] and
   OpenPGP [RFC3156], but there may be user interface issues associated
   with the introduction of UTF-8 into addresses in this context.

5.  IANA Considerations

   IANA is requested to update the registration of the message/global
   MIME type using the registration form contained in Section 3.7.

6.  Acknowledgements

   This document incorporates many ideas first described in Internet-
   Draft form by Paul Hoffman, although many details have changed from
   that earlier work.

   The author especially thanks Jeff Yeh for his efforts and
   contributions on editing previous versions.

   Most of the content of this document was provided by John C Klensin
   and Dave Crocker.  Significant comments and suggestions were received
   from Martin Duerst, Julien Elie, Arnt Gulbrandsen, Kristin Hubner,
   Kari Hurtta, Yangwoo Ko, Charles H. Lindsey, Alexey Melnikov, Chris
   Newman, Pete Resnick, Yoshiro Yoneya, and additional members of the
   JET team (Joint Engineering Team) and were incorporated into the
   document.  The editors wish to sincerely thank them all for their

7.  Edit history

   [[RFC Editor: Please remove this section before publishing.]]

7.1.  draft-ietf-eai-rfc5335bis-00

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   1.  Applied Errata suggested by Alfred Hoenes.

   2.  Adjust [RFC2821] and [RFC2822] to [RFC5321] and [RFC5322].

   3.  Abrogate <alt-address> in ABNF of <angle-addr>.

   4.  Revoke [RFC5504] from this document.

   5.  Upgrade some references from I-Ds to RFC.

7.2.  draft-ietf-eai-rfc5335bis-01

   1.  Author name revised.

7.3.  draft-ietf-eai-rfc5335bis-02

   1.  ABNF revised.

7.4.  draft-ietf-eai-rfc5335bis-03

   1.  Fix typos

   2.  ABNF revised

   3.  Improve sentence

7.5.  draft-ietf-eai-rfc5335bis-04

   1.  improve sentences and ABNF revised based on AD and Co-chairs

7.6.  draft-ietf-eai-rfc5335bis-05

   1.  ABNF revised based on AD comments

7.7.  draft-ietf-eai-rfc5335bis-06

   1.  ABNF revised

   2.  improve Section 5

7.8.  draft-ietf-eai-rfc5335bis-07

   1.  Minor ABNF revised in Section 3.2

   2.  improve Section 5

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7.9.  draft-ietf-eai-rfc5335bis-09

   Version -08 was posted in error and withdrawn.  Version 09 is is
   identical to version 07 except for a date change, addition of this
   note, and some vertical spacing compression on this page.

7.10.  draft-ietf-eai-rfc5335bis-10

   1.  Add appendix and overview of changes

   2.  Replace polls result in Abstract and Section 1

   3.  Minor Sentence modification

7.11.  draft-ietf-eai-rfc5335bis-11

   1.  Major rewrite of entire document to incorporate Dave Crocker's
       simplified ABNF.

   2.  The document has intentionally been refocused on implementors
       wishing to adapt their software to support EAI, so much of the
       explanatory and historical text has been removed.  (Some of it
       may be reintroduced later as an appendix.

7.12.  draft-ietf-eai-rfc5335bis-12

   1.  Added a section on the handling of MIME encoded-words.

   2.  Updated the security considerations to refer to the more complete
       discussion in RFC 3629.

   3.  Added a section on the effects on line length limits.

   4.  Removed the syntax restriction on the use of 8-bit UTF-8 in

   5.  Added text recommending that 8-bit UTF-8 be avoided in message-

7.13.  draft-ietf-eai-rfc5335bis-13

   1.  Updated and alphabetized the contributor list.

   2.  Corrected various typos, reworded some sections to make them

   3.  Replaced the reference to RFC 5598 with a reference to RFC 5322.

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   4.  Removed the Updates: RFC 5322.  RFC 5322 is extended by this
       document, not updated.

   5.  Added some text to the Introduction referring to the framework
       document for information about changes between this specification
       and RFC 5335.

   6.  Added text to the Abstract to say that this document replaces RFC
       5335 and that RFC 2045 is updated.

8.  References

8.1.  Normative References

   [ASCII]                        "Coded Character Set -- 7-bit American
                                  Standard Code for Information
                                  Interchange", ANSI X3.4, 1986.

   [I-D.ietf-eai-frmwrk-4952bis]  Klensin, J. and Y. Ko, "Overview and
                                  Framework for Internationalized
                                  draft-ietf-eai-frmwrk-4952bis-10 (work
                                  in progress), September 2010.

   [I-D.ietf-eai-rfc5336bis]      Yao, J. and W. MAO, "SMTP Extension
                                  for Internationalized Email Address",
                                  draft-ietf-eai-rfc5336bis-07 (work in
                                  progress), December 2010.

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

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

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

   [RFC5234]                      Crocker, D. and P. Overell, "Augmented
                                  BNF for Syntax Specifications: ABNF",
                                  STD 68, RFC 5234, January 2008.

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

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

   [UNF]                          Davis, M. and K. Whistler, "Unicode
                                  Standard Annex #15: Unicode
                                  Normalization Forms", September 2010,

8.2.  Informative References

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

   [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.

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

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

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

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

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Authors' Addresses

   Abel Yang
   4F-2, No. 9, Sec 2, Roosevelt Rd.
   Taipei,   100

   Phone: +886 2 23411313 ext 505

   Shawn Steele


   Ned Freed
   800 Royal Oaks
   Monrovia, CA  91016-6347


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