Displaying Downgraded Messages for Email Address Internationalization
RFC 5825

Internet Engineering Task Force (IETF)                       K. Fujiwara
Request for Comments: 5825                                          JPRS
Category: Experimental                                          B. Leiba
ISSN: 2070-1721                                      Huawei Technologies
                                                              April 2010

 Displaying Downgraded Messages for Email Address Internationalization

Abstract

   This document describes a method for displaying downgraded messages
   that originally contained internationalized email addresses or
   internationalized header fields.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5825.

Copyright Notice

   Copyright (c) 2010 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
   (http://trustee.ietf.org/license-info) 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.

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Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................2
   3. Converting Downgraded Message Headers for Display ...............3
      3.1. Considerations .............................................3
      3.2. The Process ................................................3
           3.2.1. No Reconstruction of the Envelope
                  Information Preservation ............................4
           3.2.2. Reconstructing the Address Header Fields'
                  Preservation Header .................................4
           3.2.3. The Unknown Header Fields' Preservation
                  Header Fields .......................................5
   4. Security Considerations .........................................6
   5. Acknowledgements ................................................6
   6. References ......................................................6
      6.1. Normative References .......................................6
      6.2. Informative References .....................................7
   Appendix A.  Examples ..............................................8
     A.1.  Displaying Example ........................................11

1.  Introduction

   The Email Address Internationalization (UTF8SMTP) extension document
   set [RFC4952] [RFC5336] [RFC5335] [RFC5337] expands Email address
   structure, syntax, and email header format.  To avoid rejection of
   internationalized email messages, the downgrading mechanism [RFC5504]
   converts an internationalized message to a traditional email message
   when a server in the delivery path does not support the UTF8SMTP
   extension.  The downgraded message is a traditional email message,
   except the message has "Downgraded-" header fields.

   A perfect reverse-function of the downgrading does not exist because
   the encoding defined in [RFC2047] is not exactly reversible and
   "Received" header field downgrading may remove FOR clause
   information.  The restoration of the downgrading should be done once
   at the final destination of the downgraded message such as Mail User
   Agents (MUAs) or IMAP servers.  This document describes the
   restoration methods for displaying downgraded messages in MUAs.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

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   Specialized terms used in this specification are defined in the EAI
   overview [RFC4952] or in [RFC5321], [RFC5322], or the MIME documents
   [RFC2045], [RFC2047], [RFC2183], and [RFC2231].

   This document depends on [RFC5335] and [RFC5504].  Key words used in
   those documents are used in this document, too.

   The term "MIME decode" is used for both "encoded-word" decoding
   defined by [RFC2047] and MIME parameter value decoding defined by
   [RFC2231].

3.  Converting Downgraded Message Headers for Display

3.1.  Considerations

   The order of some header fields (such as "Resent-*" fields) is
   significant.  The process of regenerating the original fields from
   the downgraded ones MUST NOT reorder the fields.

   In order to regenerate a field from a specific downgraded header
   field, it's necessary to find the corresponding replacement in the
   current message.  If the corresponding field cannot be found, the
   downgraded header field in question cannot be regenerated and used.

   In any case where reconstruction of a particular downgraded header
   field fails, both header fields (the "downgraded-YYY" header field
   and the "YYY" header field) SHOULD be left in the message as they
   are.  The MUA MAY choose to communicate the situation to the user
   (see the "Security Considerations" section).

3.2.  The Process

   A MUA MAY decode and regenerate the original header fields of the
   message (Mail Transport Agents (MTAs) and Mail Delivery Agents (MDAs)
   SHOULD NOT attempt to do this; it SHOULD be left to the MUA).  This
   procedure can be used to approximately reverse the downgrade process,
   but it will not always construct the original header fields exactly.

   Three types of downgraded header fields are described in Section 3 of
   [RFC5504]:

   1.  "Envelope Information Preservation Header Fields", described in
       RFC5504 Section 3.1 and in Section 3.2.1, below.

   2.  "Address Header Fields' Preservation Header Fields", described in
       RFC5504 Section 3.2 and in Section 3.2.2, below.

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   3.  "Unknown Header Fields' Preservation Header Fields", described in
       RFC5504 Section 3.3 and in Section 3.2.3, below.

   After processing downgraded header fields, decode all header fields,
   as described in [RFC2047] and [RFC2231].

3.2.1.  No Reconstruction of the Envelope Information Preservation
        Header Fields

   Envelope information preservation header fields are new fields that
   might have been added by the downgrade process.  Because they do not
   represent fields that appeared in the original message, this process
   is not applicable to them.

3.2.2.  Reconstructing the Address Header Fields' Preservation Header
        Fields

   Reconstructing address header fields' preservation header fields is
   OPTIONAL, and a decision MAY be made on each field, individually.  In
   particular, it might be less important to process the "Resent-*"
   header fields, so an implementation MAY choose to skip those.

   To construct a displayable copy of a header field from one of these
   downgraded header fields, follow this procedure:

   1.  In an edit buffer, create a new header field:

       (a)  For the field name, remove the "Downgraded-" prefix from the
            downgraded field name.  For example, "Downgraded-From"
            becomes "From", and "Downgraded-Resent-To" becomes
            "Resent-To".

       (b)  For the field value, decode the MIME-encoded value of the
            downgraded field according to [RFC2047].

   2.  Apply "Email Header Fields Downgrading", defined in Section 5 of
       [RFC5504], to the field in the edit buffer.  The process
       generates two header fields, one is ASCII header field and the
       other is the Address Header Fields' Preservation Header Field.
       Put the generated ASCII header field into comparison buffer 1.

   3.  Canonicalize the header field in the comparison buffer 1:

       1.  Unfold all header field continuation lines as described in
           [RFC5322].

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       2.  Ensure that there is one space character before and one after
           the <mailbox-list> separator ",".  If a space character is
           missing, insert one.

       3.  Ensure that there is one space character before and one after
           each <comment>.  If a space character is missing, insert one.

       4.  Decode each <encoded-word> whose charset is "UTF-8".

       5.  Convert all sequences of one or more WSP characters to a
           single space character.  WSP characters here include those
           before and after a line-folding boundary.

       6.  Delete all WSP characters at the end of each unfolded header
           field value.

       7.  Delete any WSP characters remaining before and after the
           colon separating the header field name from the header field
           value, retaining the colon separator.

   4.  Locate the first instance of the corresponding field in the
       message's headers.

   5.  Canonicalize the located field as in step 3, and put the result
       into comparison buffer 2.

   6.  Compare the header field in comparison buffer 1 with the header
       field in comparison buffer 2.  If they match, go to step 8.

   7.  Locate the next instance of the corresponding field in the
       message's headers.  If one is found, go to step 5.  If none is
       found, stop: you cannot use this downgraded field because you
       can't find its replacement in the message.

   8.  Replace the located header field with the one in the edit buffer.
       You MUST NOT reorder the header fields when you do this; it's
       important to replace the field in the same place.  Remove the
       target downgraded header field in the message header.

3.2.3.  The Unknown Header Fields' Preservation Header Fields

   The unknown header fields' preservation header fields SHOULD be left
   as they are unless the MUA has special knowledge of a particular
   field.  An MUA with such knowledge MAY use the procedure similar to
   the procedure in Section 3.2.2, above, for those fields about which
   it knows.  (Note that the whitespace canonicalization rule might not
   be applicable to some header fields.)

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4.  Security Considerations

   While information in any email header should usually be treated with
   some suspicion, current email systems commonly employ various
   mechanisms and protocols to make the information more trustworthy.
   For example, an organization's boundary MTA can modify "From" lines
   so that messages arriving from outside the organization are easily
   distinguishable from internal emails.  As a result of that rewriting,
   the "From" header field might not match the "Downgraded-From" header
   field.

   A MUA MAY emphasize bogus or broken address header fields'
   preservation header fields found in step 7 of Section 3.2.2.

   Hiding the information from the actual header fields when using the
   "Downgraded-" header fields does not cause loss of information if
   generating MIME-decoded header fields in step 1 of Section 3.2.2 and
   the comparison done in step 7 are successful.  To ensure that no
   information is lost, a MUA SHOULD have a function that uses the
   actual message that was received (with/without MIME decoding) to
   render the message.

   We have focused, here, on issues with displaying downgraded messages.
   For more discussion of downgraded and internationalized messages in
   general, see the "Security Considerations" section in [RFC5504] and
   [RFC4952].

5.  Acknowledgements

   This document was separated from [RFC5504].  Both documents were
   developed in the EAI WG.  Significant comments and suggestions were
   received from John Klensin, Harald Alvestrand, Chris Newman, Randall
   Gellens, Charles Lindsey, Marcos Sanz, Alexey Melnikov, Pasi Eronen,
   Frank Ellermann, Edward Lewis, S. Moonesamy, and JET members.

6.  References

6.1.  Normative References

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

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

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

   [RFC2183]  Troost, R., Dorner, S., and K. Moore, "Communicating
              Presentation Information in Internet Messages: The
              Content-Disposition Header Field", RFC 2183, August 1997.

   [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
              Word Extensions:
              Character Sets, Languages, and Continuations", RFC 2231,
              November 1997.

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

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

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

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

6.2.  Informative References

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

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Appendix A.  Examples

   This section shows an example of displaying a downgraded message.
   First, an example of the original UTF8SMTP message and its downgraded
   message are shown.  The example comes from "Example 1" of [RFC5504]
   and three header fields, "Unknown-Field", "Resent-From", and
   "Resent-To", are added.  The example UTF8SMTP message is shown in
   Figure 1.

   Message-Id: MESSAGE_ID
   Mime-Version: 1.0
   Content-Type: text/plain; charset="UTF-8"
   Content-Transfer-Encoding: 8bit
   Subject: NON-ASCII-SUBJECT
   Unknown-Field: NON-ASCII-Unknown
   From: DISPLAY-local <NON-ASCII-local@example.com
    <ASCII-local@example.com>>
   To: DISPLAY-remote1 <NON-ASCII-remote1@example.net
    <ASCII-remote1@example.net>>
   Cc: DISPLAY-remote2 <NON-ASCII-remote2@example.org>
   Resent-From: DISPLAY-remote1 <NON-ASCII-remote1@example.net
    <ASCII-remote1@example.net>>
   Resent-To: DISPLAY-reto <NON-ASCII-reto@example.net
    <ASCII-reto@example.net>>
   Date: DATE

   MAIL_BODY

                        Figure 1: Original message

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   A delivered downgraded message is shown in Figure 2.  A Return-Path
   header will be added by the final destination MTA.  Some "Received"
   header fields may be added.

Return-Path: <ASCII-local@example.com>
Received: ...
Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com_?=
 =?UTF-8?Q?<ASCII-local@example.com>>?=
Downgraded-Rcpt-To: =?UTF-8?Q?<NON-ASCII-remote1@example.net_?=
 =?UTF-8?Q?<ASCII-remote1@example.net>>?=
Message-Id: MESSAGE_ID
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
Subject: =?UTF-8?Q?NON-ASCII-SUBJECT?=
Downgraded-Unknown-Field: =?UTF-8?Q?NON-ASCII-Unknown?=
From: =?UTF-8?Q?DISPLAY-local?= <ASCII-local@example.com>
Downgraded-From: =?UTF-8?Q?DISPLAY-local_<NON-ASCII-local@example.com_?=
 =?UTF-8?Q?<ASCII-local@example.com>>?=
To: =?UTF-8?Q?DISPLAY-remote1?= <ASCII-remote1@example.net>
Downgraded-To: =?UTF-8?Q?DISPLAY-remote1_?=
 =?UTF-8?Q?<NON-ASCII-remote1@example.net_<ASCII-remote1@example.net>>?=
Cc: =?UTF-8?Q?DISPLAY-remote2?= Internationalized address
 =?UTF-8?Q?NON-ASCII-remote2@example.org?= removed:;
Downgraded-Cc: =?UTF-8?Q?DISPLAY-remote2_?=
 =?UTF-8?Q?<NON-ASCII-remote2@example.org>?=
Resent-From: =?UTF-8?Q?DISPLAY-remote1?= <ASCII-remote1@example.net>
Downgraded-Resent-From: =?UTF-8?Q?DISPLAY-remote1_?=
 =?UTF-8?Q?<NON-ASCII-remote1@example.net_<ASCII-remote1@example.net>>?=
Resent-To: =?UTF-8?Q?DISPLAY-reto?= <ASCII-reto@example.net>
Downgraded-Resent-To: =?UTF-8?Q?DISPLAY-reto_?=
 =?UTF-8?Q?<NON-ASCII-reto@example.net_<ASCII-reto@example.net>>?=
Date: DATE

MAIL_BODY

                       Figure 2: Downgraded message

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   Figure 3 shows the MIME-decoded message of Figure 2.  The recipient
   can read the original "From", "To", "Cc", "Resent-From", "Resent-To"
   and "Unknown-Field" header fields as "Downgraded-From",
   "Downgraded-To", "Downgraded-Cc", "Downgraded-Resent-From",
   "Downgraded-Resent-To", and "Downgraded-Unknown-Field" header fields.

   Return-Path: <ASCII-local@example.com>
   Received: ...
   Downgraded-Mail-From: <NON-ASCII-local@example.com
    <ASCII-local@example.com>>
   Downgraded-Rcpt-To: <NON-ASCII-remote1@example.net
    <ASCII-remote1@example.net>>
   Message-Id: MESSAGE_ID
   Mime-Version: 1.0
   Content-Type: text/plain; charset="UTF-8"
   Content-Transfer-Encoding: 8bit
   Subject: NON-ASCII-SUBJECT
   Downgraded-Unknown-Field: NON-ASCII-Unknown
   From: DISPLAY-local <ASCII-local@example.com>
   Downgraded-From: DISPLAY-local <NON-ASCII-local@example.com
    <ASCII-local@example.com>>
   To: DISPLAY-remote1 <ASCII-remote1@example.net>
   Downgraded-To: DISPLAY-remote1 <NON-ASCII-remote1@example.net
    <ASCII-remote1@example.net>>
   Cc: DISPLAY-remote2 Internationalized address
    NON-ASCII-remote2@example.org removed:;
   Downgraded-Cc: DISPLAY-remote2 <NON-ASCII-remote2@example.org>
   Resent-From: DISPLAY-remote1 <ASCII-remote1@example.net>
   Downgraded-Resent-From: DISPLAY-remote1
    <NON-ASCII-remote1@example.net <ASCII-remote1@example.net>>
   Resent-To: DISPLAY-reto <ASCII-reto@example.net>
   Downgraded-Resent-To: DISPLAY-reto
    <NON-ASCII-reto@example.net <ASCII-reto@example.net>>
   Date: DATE

   MAIL_BODY

                      Figure 3: MIME-decoded message

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A.1.  Displaying Example

   This example shows how to display the message in Figure 2, above,
   using the process defined in Section 3.  For simplicity, we will show
   the reconstruction of all the applicable fields at once.

   Selecting all Downgraded-* fields gives this:

Downgraded-Mail-From: =?UTF-8?Q?<NON-ASCII-local@example.com_?=
 =?UTF-8?Q?<ASCII-local@example.com>>?=
Downgraded-Rcpt-To: =?UTF-8?Q?<NON-ASCII-remote1@example.net_?=
 =?UTF-8?Q?<ASCII-remote1@example.net>>?=
Downgraded-Unknown-Field: =?UTF-8?Q?NON-ASCII-Unknown?=
Downgraded-From: =?UTF-8?Q?DISPLAY-local_<NON-ASCII-local@example.com_?=
 =?UTF-8?Q?<ASCII-local@example.com>>?=
Downgraded-To: =?UTF-8?Q?DISPLAY-remote1_?=
 =?UTF-8?Q?<NON-ASCII-remote1@example.net_<ASCII-remote1@example.net>>?=
Downgraded-Cc: =?UTF-8?Q?DISPLAY-remote2_?=
 =?UTF-8?Q?<NON-ASCII-remote2@example.org>?=
Downgraded-Resent-From: =?UTF-8?Q?DISPLAY-remote1_?=
 =?UTF-8?Q?<NON-ASCII-remote1@example.net_<ASCII-remote1@example.net>>?=
Downgraded-Resent-To: =?UTF-8?Q?DISPLAY-reto_?=
 =?UTF-8?Q?<NON-ASCII-reto@example.net_<ASCII-reto@example.net>>?=

                    Figure 4: Downgraded header fields

   Two of the fields, "Downgraded-Mail-From" and "Downgraded-Rcpt-To",
   are envelope information preservation header fields, and will not be
   reconstructed.  One field, "Downgraded-Unknown-Field", is an unknown
   header fields' preservation header field and will also not be
   reconstructed.  That leaves the address header fields' preservation
   header fields to be reconstructed.

Downgraded-From: =?UTF-8?Q?DISPLAY-local_<NON-ASCII-local@example.com_?=
 =?UTF-8?Q?<ASCII-local@example.com>>?=
Downgraded-To: =?UTF-8?Q?DISPLAY-remote1_?=
 =?UTF-8?Q?<NON-ASCII-remote1@example.net_<ASCII-remote1@example.net>>?=
Downgraded-Cc: =?UTF-8?Q?DISPLAY-remote2_?=
 =?UTF-8?Q?<NON-ASCII-remote2@example.org>?=
Downgraded-Resent-From: =?UTF-8?Q?DISPLAY-remote1_?=
 =?UTF-8?Q?<NON-ASCII-remote1@example.net_<ASCII-remote1@example.net>>?=
Downgraded-Resent-To: =?UTF-8?Q?DISPLAY-reto_?=
 =?UTF-8?Q?<NON-ASCII-reto@example.net_<ASCII-reto@example.net>>?=

              Figure 5: Header fields for the reconstruction

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   Now, perform step 1 to the downgraded header fields shown in Figure 5
   and create an edit buffer.

   From: DISPLAY-local <NON-ASCII-local@example.com
    <ASCII-local@example.com>>
   To: DISPLAY-remote1 <NON-ASCII-remote1@example.net
    <ASCII-remote1@example.net>>
   Cc: DISPLAY-remote2 <NON-ASCII-remote2@example.org>
   Resent-From: DISPLAY-remote1
    <NON-ASCII-remote1@example.net <ASCII-remote1@example.net>>
   Resent-To: DISPLAY-reto
    <NON-ASCII-reto@example.net <ASCII-reto@example.net>>

                  Figure 6: edit buffer: Output of step 1

   Apply "Email Header Fields Downgrading" to the "edit buffer".  It
   generates downgraded ASCII header fields and the address header
   fields' preservation header fields.  The latter fields are the same
   as the downgraded header fields.  Put the former fields into
   "comparison buffer 1".

   From:DISPLAY-local <ASCII-local@example.com>
   To:DISPLAY-remote1 <ASCII-remote1@example.net>
   Cc:DISPLAY-remote2 Internationalized address
    NON-ASCII-remote2@example.org removed:;
   Resent-From:DISPLAY-remote1 <ASCII-remote1@example.net>
   Resent-To:DISPLAY-reto <ASCII-reto@example.net>

              Figure 7: comparison buffer 1: Output of step 3

   Perform steps 4 to 6, comparison, for each header field.  Five header
   fields, "From", "To", "Cc", "Resent-From" and "Resent-To" fields will
   match, and we will proceed to step 8.  (Step 7, iteration, does not
   apply in this example.

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   Perform step 8, replacing all applicable fields, without changing the
   order.  Then, do MIME decoding on everything, for display.

   Return-Path: <ASCII-local@example.com>
   Received: ...
   Downgraded-Mail-From: <NON-ASCII-local@example.com
    <ASCII-local@example.com>>
   Downgraded-Rcpt-To: <NON-ASCII-remote1@example.net>
    <ASCII-remote1@example.net>
   Message-Id: MESSAGE_ID
   Mime-Version: 1.0
   Content-Type: text/plain; charset="UTF-8"
   Content-Transfer-Encoding: 8bit
   Subject: NON-ASCII-SUBJECT
   Downgraded-Unknown-Field: NON-ASCII-Unknown
   From: DISPLAY-local <NON-ASCII-local@example.com
    <ASCII-local@example.com>>
   To: DISPLAY-remote1 <NON-ASCII-remote1@example.net
    <ASCII-remote1@example.net>>
   Cc: DISPLAY-remote2 <NON-ASCII-remote2@example.org>
   Resent-From: DISPLAY-remote1 <NON-ASCII-remote1@example.net
    <ASCII-remote1@example.net>>
   Resent-To: DISPLAY-reto <NON-ASCII-reto@example.net
    <ASCII-reto@example.net>>
   Date: DATE

                        Figure 8: The final result

   As a result, in this simple example, some original header fields are
   now displayed in their original form.  Differences between Figure 1
   and Figure 8 are Return-Path, Downgraded-Mail-From,
   Downgraded-Rcpt-To, and Downgraded-Unknown-Field.

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Authors' Addresses

   Kazunori Fujiwara
   Japan Registry Services Co., Ltd.
   Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
   Chiyoda-ku, Tokyo  101-0065
   Japan

   Phone: +81-3-5215-8451
   EMail: fujiwara@jprs.co.jp

   Barry Leiba
   Huawei Technologies

   Phone: +1 646 827 0648
   EMail: barryleiba@computer.org
   URI:   http://internetmessagingtechnology.org/

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