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Versions: 00 01                                                         
Network Working Group                                        J. Yeh, Ed.
Internet-Draft                                                     TWNIC
Expires: August 31, 2006                               February 27, 2006

                    Internationalized Email Headers

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   Full internationalization of electronic mail requires not only the
   capability to transmit non-ASCII content, to encode selected
   information in specific header fields, and to use international
   characters in envelope addresses.  It also requires being able to
   express those addresses and information based on them in mail header
   fields.  This document specifies the use of Unicode encoded in UTF-8,
   rather than ASCII, as the base form for Internet email header fields.
   This form is permitted in transmission only if authorized by an SMTP
   extension, as specified in an associated specification.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Role of this specification . . . . . . . . . . . . . . . .  3
   2.  Background and History . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Pre-requirement  . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Identification of internationalized email  . . . . . . . . . .  5
   6.  Impact on Message Header Fields  . . . . . . . . . . . . . . .  6
   7.  Additional issue . . . . . . . . . . . . . . . . . . . . . . .  7
     7.1.  POP3/IMAP  . . . . . . . . . . . . . . . . . . . . . . . .  7
     7.2.  Mailing list header fields . . . . . . . . . . . . . . . .  7
     7.3.  URI/IRI  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   9.  IANA considerations  . . . . . . . . . . . . . . . . . . . . .  8
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     11.1. Normative References . . . . . . . . . . . . . . . . . . .  8
     11.2. Informative References . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11

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1.  Introduction

1.1.  Role of this specification

   Full internationalization of electronic mail requires several

   o  The capability to transmit non-ASCII content, provided for as part
      of the basic MIME specification [RFC2045], [RFC2046].
   o  The capability to encode selected information in specific header
      fields, provided for as another part of the MIME specification
   o  The capability to use international characters in envelope
      addresses, discussed in [IMA-overview] and specified in [IMA-SMTP-
      extension].  And, finally,
   o  The capability to express those addresses, and information related
      to and based on them, in mail header fields, defined in this

   This document specifies the use of Unicode encoded in UTF-8
   [RFC3629], rather than ASCII, as the base form for Internet email
   header fields.  This form is permitted in transmission, if authorized
   by the SMTP extension specified in [IMA-SMTP-extension].

2.  Background and History

   Mailbox names often represent the names of human users.  Many of
   these users throughout the world have names that are not normally
   represented with just the ASCII repertoire of characters, and would
   more the less like to use their real names in their mailbox names.
   These users are also likely to use non-ASCII text in their common
   names and subjects of email messages, both in what they send and what
   they receive.  This protocol specifies UTF-8 as the encoding to
   represent email header messages.

   The traditional format of email messages [RFC2822] only allows ASCII
   characters in the header fields of messages.  This prevents users
   from having email addresses that contain non-ASCII characters.  It
   further forces non-ASCII text in common names, comments, and in free
   text (such as in the Subject: field) to be in MIME format [RFC2047].
   This specification describes a change to the email message format
   that is connected to the SMTP message transport change described in
   the associated specifications [IMA-overview] and [IMA-SMTP-
   extension], and that allows non-ASCII characters throughout email
   header fields.  These changes affect SMTP clients, SMTP servers, and
   mail user agents (MUAs).

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   As specified in [IMA-SMTP-extension], an SMTP protocol extension
   [RFC2821] is used to prevent the transmission of messages with UTF-8
   header fields to systems that cannot handle such messages.

   Use this SMTP extension helps prevent against the introduction of
   such messages into message stores that might misrepresent or mangle
   such messages.  It should be noted that using an ESMTP extension does
   not prevent against transferring email messages with UTF-8 header
   fields to other systems that use the email format for messages and
   that may not be upgraded, such as the POP and IMAP protocols.  Those
   protocols will need to be changed in order to handle stored messages
   that have UTF-8 header fields.

   The objective for this protocol is to allow UTF-8 in email header
   fields.  Issues about how to handle messages that contain UTF-8
   header fields but are proposed to be delivered to systems that have
   not been upgraded to support this capability are discussed elsewhere,
   particularly in [IMA-downgrading].

   This protocol is workable even if IMA mailbox names are not
   presented.  For example, the protocol might still be used if just the
   subject header has non-ASCII characters, but the protocol MUST be
   used if other header fields (particularly trace header fields such as
   "Received:") contain non-ASCII characters.

3.  Terminology

   In this document, header fields are "UTF-8 header" if the bodies of
   headers contain UTF-8 characters.

   Unless otherwise noted, all terms used here are defined in [RFC2821]
   or [RFC2822] or in [IMA-overview].

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

   This document is being discussed on the ima mailing list.  See
   https://www1.ietf.org/mailman/listinfo/ima for information about
   subscribing.  The list's archive is at

4.  Pre-requirement

   The use of UTF-8 header fields is dependent on the use of an SMTP
   extension named "IMA".

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   That protocol is defined in [IMA-SMTP-extension].  If that extension
   is not supported, UTF-8 header fields MUST NOT be transmitted.

   Sending MUAs that follow this protocol MUST create all header fields
   encoded in UTF-8.  No other direct encodings are allowed.  MUAs MAY
   continue to use MIME to specify some text in other encodings; however
   this is not recommended because it is likely that this will not
   interoperate well with MUAs that follow this specification.

5.  Identification of internationalized email

   When a SMTP client tries to send a mail to a SMTP server that does
   not support IMA, the client should know whether the message requires
   the support for IMA or not.  In addition to this, identifiction of
   internationalized email is also required when a message is stored and
   presented.  Checking the presence of UTF-8 characters in the header
   whenever such an identification is required may also achieve the its
   goal.  However, this type of repeated processing wastes time and
   processing power of involved systems.  It is nice to have a mechanism
   (such as self-label) or some indicator to identify whether the
   message is new format(i.e.  IMA compliant) or old one (i.e.  RFC 2822

   To be able to do so, sending MUA should insert a new header field to
   identify the presence of i18n information (particularly UTF-8
   headers) in the message.  The new header specified as "i18n-email",
   and elements of the header is the version number of i18n email.  The
   i18n header field syntax specified like:

   i18n-email: 1.0

   [Note in draft: There should be more useful information can be place
   in the new header field. ]

   While we can't require ordering of headers, it would be good to have
   it appear as near the top of the headers as possible.  It would also
   be good to be able to guarantee that it will be there when the
   message is dropped into a mail store.  Thus, when a i18n email is

   o  The "i18n-email" header field MUST be inserted by the originating
   o  The "i18n-email" header field MUST be inserted, along with Return-
      path, by the final delivery MTA if not presented.
   o  The "i18n-email" header field, if present, MUST be removed as part
      of any downgrading process that eliminates the UTF-8 header

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   o  MTAs MAY check for duplicates of the "i18n-email" header field and
      eliminate all but one of them.  However, if a receiving MUA
      encounters more than one of these headers, it SHOULD simply ignore
      any excess ones.

   This combination guarantees that the header will be present on
   delivery even if it is deleted in transit.

6.  Impact on Message Header Fields

   This protocol does NOT change the definition of header field names.
   That is, only the bodies of header fields are allowed to have UTF-8
   characters; the rules in RFC 2822 for header names are not changed.

   SMTP client can send header fields in UTF-8 format, if the IEmail
   extensionextension advertised by SMTP server.  However, the
   Message-ID is the unique identifier of a single email.  [Note in
   draft: Extension name depends on the SMTP extension defined in [IMA-
   SMTP-extension]] In order to maintain the identity, message
   identifiers of the Message-ID fields MUST be created in all ASCII.

   To be specific, when IEE smtp extension is advertised.
   o  <unstructured>, <comment> and <phrase> are allowed to use UTF-8.
   o  <date-time>, <msg-id> remains the same definition as in RFC2822.

   In this specification, internationalized email address will be
   presented in UTF-8.  Thus, all header fields involving <mailbox>es
   may be different from traditional ones.  There might be IMA
   unawareMTAs in the mail routing path.  In that case, MTA may bounce
   the message with reply code 558, or downgrade the non-ASCII contents
   of all header bodies before continuing to send the message, as
   described in [IMA-downgrading].  However, MTAs never know if there
   are any data or instructions embedded in the email address.  Or there
   also email addresses do not contain embedded operations.  The only
   one way is to let the mail address owner to tell if the address is ok
   for downgrade process or not.  Hence, the ATOMIC and ALT-ADDRESS
   options are introduced.  The detail of ATOMIC and ALT-ADDRESS options
   can be found in [IMA-SMTP-extension].  With these two different
   cases, there are two possible representation of <mailbox>.
   o  ATOMIC:
      ATOMIC, it means that the email address can be downgraded safely
      without damage to the mail delivery.  In this case, the <mailbox>
      syntax remains the same to RFC2822.  The only difference is that
      the <local-part> and <domain> of <addr-spec> allows UTF-8

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      If user provides an alternative address for the internationalized
      email address for the mail delivery.  The <mailbox> syntax will be

           mailbox =       new-name-addr / new-addr-spec
           new-name-addr   =       [display-name] new-angle-addr
           new-angle-addr  =       [CFWS] "<" new-addr-spec ">" [CFWS]
           new-angle-addr  =/      obs-angle-addr
           new-addr-spec   =       [addr-spec] non-ASCII-addr-spec
           new-addr-spec   =/      addr-spec

   In any time, SMTP server can reject with a reply code of 558 whenever
   ALT-ADDRESS is not provided and downgrade is not feasible.

   [Note in draft: The detail ABNF will need to be prepared in this
   document when proper WG establish.]

7.  Additional issue

   This section identifies issues that are not covered as part of this
   set of specifications, but that will need to be considered as part of
   IEE deployment.

7.1.  POP3/IMAP

   Receiving MUAs that follow this protocol MUST able to handle email
   header fields encoded in UTF-8.  Which means that the email fetching
   protocol such as POP3 or IMAP MAY need to be updated.

7.2.  Mailing list header fields

   All mailing list and mail redistribution related header fields may
   need further investigation.

7.3.  URI/IRI

   The mailto schema in URI/IRI may need further investigation.

8.  Security Considerations

   If a user has a non-ASCII mailbox address and a all-ASCII mailbox
   address, a digital certificate that identifies that user SHOULD have
   both addresses in the identity.  Having multiple email addresses as
   identities in a single certificate is already supported in PKIX and

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   Because UTF-8 often requires several octets to encode a single
   character, internationalized local parts may cause mail addresses to
   become longer.  Then may possibly make it harder to keep lines in a
   header under 78 octets.  Lines that are longer than 78 octets (which
   is a SHOULD specification, not a MUST specification, in RFC 2822)
   could possibly cause mail user agents to fail in ways that affect

9.  IANA considerations

   The ESMTP extension needed to support this specification is specified
   in [IMA-SMTP-extension].  This specification does not require any
   additional IANA actions in that regard.

10.  Acknowledgements

   This document was created by incorporating a good deal of material
   from an old Internet Draft by Paul Hoffman [Hoffman-utf8-headers].
   While many of the concepts and details have changed, the
   contributions from that draft are greatly appreciated.

   Most of the content of this document is provided by John C Klensin.
   Also some significant comments and suggestions were received from
   Charles H. Lindsey, Yangwoo KO, Yoshiro YONEYA, and other members of
   the JET team and were incorporated into the document.  The editor is
   much great thanks to their contribution sincerely.

11.  References

11.1.  Normative References

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

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

              Yao, J., Ed. and X. LEE, "SMTP extension for
              internationalized email address",
              draft-yao-ima-smtpext-00.txt (work in progress),
              January 2006.

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              Klensin, J. and Y. Ko, "Overview and Framework of
              Internationalized Email Address Delivery",
              draft-klensin-ima-framework-00.txt (work in progress),
              September 2005.

   [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.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

   [RFC3066]  Alvestrand, H., "Tags for the Identification of
              Languages", BCP 47, RFC 3066, January 2001.

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

11.2.  Informative References

              Hoffman, P., "SMTP Service Extensions or Transmission of
              Headers in UTF-8 Encoding",
              draft-hoffman-utf8headers-00.txt (work in progress),
              December 2003.

              "whatever we call the downgrading document", 2005.

   [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.

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Author's Address

   Jeff Yeh (editor)
   4F-2, No. 9, Sec 2, Roosvelt Rd.
   Taipei,   100

   Phone: +886 2 23411313 ext 506
   Email: jeff@twnic.net.tw

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