Network Working Group                                        J. Yao, Ed.
Internet-Draft                                               W. Mao, Ed.
Intended status: Experimental                                      CNNIC
Expires: September 4, October 11, 2007                                         CNNIC
                                                           March 3,                                  April 9, 2007

           SMTP extension for internationalized email address
                     draft-ietf-eai-smtpext-04.txt
                     draft-ietf-eai-smtpext-05.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Internationalized email address includes two parts, the local part
   and the domain part.  The ways email addresses are used by protocols
   are different from the ways domain names are used.  The most critical
   difference is that emails are delivered through a chain of peering
   clients and servers while domain names are resolved by name servers
   by looking up their own tables.  In addition to this, email transport
   protocols SMTP and ESMTP provide a negotiation mechanism through
   which clients can make decisions for further processing.

   This document specifies the use of SMTP extension for
   internationalized email address delivery.  It also mentions the backward compatible
   mechanism for downgrade procedure, as  Communication with systems
   that do not implement this specification is specified in an associated
   specification. another
   document.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Role of this specification . . . . . . . . . . . . . . . .  3
     1.2.  Proposal Context . . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4  3
   2.  Mail Transport-level Protocol  . . . . . . . . . . . . . . . .  4
     2.1.  Framework for the Internationalization Extension . . . . .  4
     2.2.  The Address Internationalization Service UTF8SMTP Extension . . . . . . . . . . . . . . . . . .  5
     2.3.  Extended Mailbox Address Syntax  . . . . . . . . . . . . .  5  6
     2.4.  The ALT-ADDRESS parameter  . . . . . . . . . . . . . . . .  8
     2.5.  The Suggestion of the Value of  Using the ALT-ADDRESS parameter  . . . . . . . . . . . . . . . . . . . . . . . .  9  8
     2.6.  Body Parts and SMTP Extensions . . . . . . . . . . . . . .  9
     2.7.  Additional ESMTP Changes and Clarifications  . . . . . . . 10  9
       2.7.1.  The Initial SMTP Exchange  . . . . . . . . . . . . . . 10
       2.7.2.  Message Retry  . . . . . . . . . . . . . . . . . . . . 10
       2.7.3.  Trace Information  . . . . . . . . . . . . . . . . . . 11
       2.7.4.  Mailing List Question  .
   3.  Issues with Other Parts of the Email System  . . . . . . . . . 12
     3.1.  LMTP . . . . . . 12
       2.7.5.  POP and IMAP . . . . . . . . . . . . . . . . . . . . . 12
       2.7.6.
     3.2.  SMTP Service Extension for DSNs  . . . . . . . . . . . . . 12
   3.
     3.3.  POP and IMAP . . . . . . . . . . . . . . . . . . . . . . . 12
   4.  Potential problems . . . . . . . . . . . . . . . . . . . . . . 12
     3.1.
     4.1.  Impact to RFC 4409 and many email related RFC  . . . . . . 13
   4. . . . . . . . . 12
   5.  Implementation Advice  . . . . . . . . . . . . . . . . . . . . 13
   5.
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   6.
   7.  Security considerations  . . . . . . . . . . . . . . . . . . . 13
   7.
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   8. 13
   9.  Change History . . . . . . . . . . . . . . . . . . . . . . . . 14
     8.1.
     9.1.  draft-ietf-eai-smtpext: Version 00 . . . . . . . . . . . . 14
     8.2.
     9.2.  draft-ietf-eai-smtpext: Version 01 . . . . . . . . . . . . 14
     8.3.
     9.3.  draft-ietf-eai-smtpext: Version 02 . . . . . . . . . . . . 14
     8.4.
     9.4.  draft-ietf-eai-smtpext: Version 03 . . . . . . . . . . . . 14
     8.5.
     9.5.  draft-ietf-eai-smtpext: Version 04 . . . . . . . . . . . . 15
   9.
     9.6.  draft-ietf-eai-smtpext: Version 05 . . . . . . . . . . . . 15
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     9.1.
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     9.2.
     10.2. Informative References . . . . . . . . . . . . . . . . . . 17 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 18 19

1.  Introduction

   Internationalized email address is different from includes two parts, the
   internationalized domain name (IDN).  It can be solved local part
   and the domain part.  The ways email addresses are used by exploiting protocols
   are different from the ways domain names are used.  The most critical
   difference is that emails are delivered through a chain of peering
   clients and servers while domain names are resolved by name servers
   by looking up their own tables.  In addition to this, email transport
   protocol ESMTP[RFC1869] provide a negotiation mechanism while IDN through which
   clients can not use the negotiation
   mechanism.  So internationalized email address SHOULD be solved make decisions for further processing; please see more in
   [EAI-framework].  Email addresses can exploit the mail transport-level using the SMTP extension
   negotiation mechanism, which mechanism while Internationalized Domain Name(IDN) does
   not have such a facility.  This is an
   architecturally also more desirable approach.
   architecturally.  This document specifies a
   protocol an SMTP extension to solve the problem of permit
   internationalized email address
   based on ESMTP. addresses in envelopes, and UTF-8 in headers.
   The protocol proposed described here is MTA-level an MTA solution which is feasible,
   architecturally elegant, and not as difficult to
   be deployed in relevant communities. deploy.

1.1.  Role of this specification

   An overview framework document [EAI-overview] [EAI-framework] specifies the requirements for,
   and components of, full internationalization of electronic mail.  To
   understand and implement this specification, understanding the
   context presented in [EAI-framework] is necessary.

   This document specifies an element of that work, specifically the
   definition of an SMTP extension [RFC1869] for the internationalized
   email address transport delivery.

1.2.  Proposal Context

   In order

   This specification describes a change to use internationalized the email addresses, we need to
   internationalize transport
   mechanism that permits non-ASCII characters in both the domain part envelope and the local part of the email
   address.  The domain part
   header fields of messages while the email address has been
   internationalized through IDNA RFC 3490 [RFC3490].  But specification in [EAI-utf8header]
   specifies the local
   part details of how and where non-ASCII characters are
   permitted in the email address still remains as non-internationalized.

   The syntax of Internet email addresses is restricted to a subset header fields of
   7-bit ASCII for the domain-part, with a less-restricted subset messages.  The context for the local-part.  These restrictions
   change is described in [EAI-framework].

1.3.  Terminology

   The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED",
   and "MAY" in this document are specified in RFC 2821
   [RFC2821].  To be able to deliver internationalized email through
   SMTP servers, we need to upgrade SMTP server to be able to carry the
   internationalized email address.  Since the older SMTP servers, the
   mail-reading clients and other systems that are downstream from them
   might not be prepared to handle these extended addresses, an SMTP
   extension is specified to identify and protect the addressing
   mechanism.

   This specification describes a change to the email transport
   mechanism that permits non-ASCII address in both the envelope and
   header fields of messages.  The context for the change is described
   in [EAI-overview] and the details of the header changes are described
   in [EAI-utf8header].

1.3.  Terminology

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

   All specialized terms used in this specification are defined in the
   EAI overview [EAI-overview] framework [EAI-framework] or in [RFC2821] and [RFC2822].  The
   terms "ASCII address", "internationalized email address", "non-ASCII
   address", "i18mail address", "UTF8SMTP", "message" and "mailing list"
   are used with the definitions from the EAI overview [EAI-framework] document.

   This document defines only those ABNF [RFC4234] syntax rules that are
   different from those of the base email specifications
   [RFC2821][RFC2822] and, where the earlier rules are upgraded or
   extended, gives them new names.  When the new rule is a small upgrade
   to the older one, it is typically given a name starting with "u".
   Rules that are undefined here may be found in the base email
   documents under the same names.

   [[anchor4: RFC EDITOR'S NOTE: The following text should be deleted
   before publication.]]  This document is being discussed on the EAI
   mailing list.  See https://www1.ietf.org/mailman/listinfo/ima for
   information about subscribing.  The list's archive is at
   http://www1.ietf.org/mail-archive/web/ima/index.html.

2.  Mail Transport-level Protocol

2.1.  Framework for the Internationalization Extension

   The following service extension is defined:

   1.  The name of the SMTP service extension is "Email Address
       Internationalization";
   2.  The EHLO keyword value associated with this extension is
       "UTF8SMTP";
   3.  No parameter values are defined for this EHLO keyword value.  In
       order to permit future (although unanticipated) extensions, the
       EHLO response MUST NOT contain any parameters for that keyword.
       If a parameter appears, the SMTP client
       Clients MUST ignore any parameters, that is conformant to
       this version of this specification is, clients MUST treat the ESMTP response behave
       as if the "UTF8SMTP" keyword did parameters do not appear.  If a server includes
       UTF8SMTP in its EHLO response, it MUST be fully compliant with
       this version of this specification.
   4.  An  One optional parameter parameter, ALT-ADDRESS, is added to the SMTP MAIL
       and RCPT commands.  The parameter is named as ALT-ADDRESS.  The "ALT-
       ADDRESS" requires  ALT-ADDRESS specifies an all-ASCII address
       which can be used as a substitute for the i18mail addresses that
       we call the primary address; you can learn more in [EAI-overview]
       [EAI-framework] or [EAI-downgrading].  The value of "ALT-
       ADDRESS" is set by the sender when MUA and the Submission server
       have a communication.
   5.  No additional SMTP verbs are defined by this extension.
   6.  Servers offering this extension MUST provide support for, and
       announce, the 8BITMIME extension [RFC1652].
   7.  The reverse-path and forward-path of SMTP MAIL and RCPT commands
       are extended to allow UTF-8 characters in the specified mailbox
       address.
   8.  The mail data is extended on compliance with [EAI-utf8header]
   9.  The maximum length of a MAIL FROM and RCPT TO command lines is
       increased by 396 characters by the possible addition of the ALT-
       ADDRESS keyword and value.

2.2.  The Address Internationalization Service UTF8SMTP Extension

   An SMTP Server that announces this extension MUST be prepared to
   accept a UTF-8 string [RFC3629] in any position in which RFC 2821
   specifies that a "mailbox" MAY appear.  That string MUST be parsed
   only as specified in RFC 2821, i.e., by separating the mailbox into
   source route, local part and domain part, using only the characters
   colon (U+003A), comma (U+002C), and at-sign (U+0040) as specified
   there.  Once isolated by this parsing process, the local part MUST be
   treated as opaque unless the SMTP Server is the final delivery MTA.
   Any domain names that are to be looked up in the DNS MUST first be
   processed into the form as specified in IDNA [RFC3490] by means of the
   ToASCII() operation unless they are already in that form.  Any domain
   names that are to be compared to local strings SHOULD be checked for
   validity and then MUST be compared as specified in section 3.4 of
   IDNA.

   An

   The UTF8SMTP extension is valid on the submission port [RFC4409].

   An SMTP Client that receives the UTF8SMTP extension keyword in
   response to the "EHLO" command MAY transmit a mailbox name as an
   internationalized string in UTF-8 form and MAY send an
   internationalized mail UTF-8 header
   [EAI-utf8header].  It MAY transmit the domain part of that string in
   either punycode (derived from the IDNA
   process) form of ACE labels specified in [RFC3490] or UTF-8 form.
   If it sends the domain in UTF-8 form, the
   original Submission SMTP client that
   first injects the message into the Internet SHOULD first verify that
   the string is valid for a domain name according to IDNA rules.  As required by  The
   presence of the UTF8SMTP extension does not change the requirement of
   RFC 2821, it 2821 that servers MUST not attempt to parse, evaluate, or
   transform the local part in any way way.  We say that an ASCII address is
   "available" for a forwarding path or return path if the UTF8SMTP SMTP extension address is offered by
   all-ASCII or an ALT-ADDRESS parameter is specified for the server. path.  If
   the UTF8SMTP SMTP extension is not offered by the Server, the SMTP
   Client
   client MUST NOT transmit an internationalized address and MUST NOT
   transmit a mail message which contains internationalized mail headers
   [EAI-utf8header].  Instead,
   [EAI-utf8header] at any level within it MIME structure.  Instead, an
   SMTP client other than the Submission MTA MUST either make one of the
   following three choices:
   1.  Reject or return the message as undeliverable.
   2.  Find an alternate route to the
   user as undeliverable destination that permits UTF8SMTP.
   3.  If and only if an ASCII address is available for the return path
       and one or replace it with more of the alternate ASCII specific forwarding paths being attempted,
       downgrade the message to an all-ASCII form as specified in
       [EAI-downgrading].

   To be able to deliver internationalized email through SMTP servers,
   we need to upgrade SMTP server to be able to carry the
   internationalized email address.
   If it is replaced,  Submission servers [RFC4409] are
   permitted to perform a broader range of changes to allow the replacement MUST
   internationalized email address.  The older SMTP servers, the mail-
   reading clients and other systems that are downstream from them might
   not be prepared to handle these extended addresses.  If a SMTP server
   does not support the ASCII-only address
   specified with UTF8SMTP extension, then the ALT-ADDRESS parameter.[EAI-downgrading]. SMTP client MUST
   NOT, under any circumstances, attempt to send UTF8SMTP message to
   this server or attempt to use UTF-8 characters of the MAIL FROM or
   RCPT TO commands.

2.3.  Extended Mailbox Address Syntax

   RFC 2821, section 4.1.2, defines the syntax of a mailbox entirely in
   terms of ASCII characters, using the production for "Mailbox" and
   those on which it depends.

   The key changes made by this specification are, informally, to

   o  Change the definition of "sub-domain" to permit either the
      definition above or a UTF-8 string representing a DNS label that
      is conformant with IDNA [RFC3490].  That label MUST NOT contain
      the characters "@" or ".", even though those characters can
      normally be inserted into a DNS label.
   o  Change
   o  Change the definition of "Atom" to permit either the definition
      above or a UTF-8 string.  That string MUST NOT contain any of the
      ASCII characters (either graphics or controls) that are not
      permitted in "atext"; it is otherwise unrestricted.

   According to the description above, define the syntax of an
   internationalized email mailbox with ABNF [RFC4234] as
         uMailbox = uLocal-part "@" uDomain
                   ; Replace Mailbox in RFC 2821, section 4.1.2

         uLocal-part = uDot-string / uQuoted-string
           ; MAY be case-sensitive
                   ; Replace Local-part in RFC 2821, section 4.1.2

         uDot-string = uAtom *("." uAtom)
                   ; Replace Dot-string in RFC 2821, section 4.1.2

         uAtom = 1*ucharacter
               ; Replace Atom in RFC 2821, section 4.1.2

         ucharacter = atext / UTF8-non-ASCII UTF8-xtra-char
                   ; Replace character in RFC 2821, section 4.1.2
                   ; atext is defined in RFC 2822

         uQuoted-string = DQUOTE *uqcontent DQUOTE
                   ; Replace Quoted-string in RFC 2821, section 4.1.2
                   ; DQUOTE is Double Quote defined in RFC 4234

         uqcontent = qcontent / UTF8-non-ASCII UTF8-xtra-char
                   ; qcontent is defined in RFC 2822, section 3.2.5

         uDomain = (sub-udomain 1*("." sub-udomain)) / address-literal
                   ; Replace Domain in RFC 2821, section 4.1.2
                   ; address-literal is defined in RFC2821 section 4.1.2

         sub-udomain = uLet-dig [uLdh-str]
                   ; Replace sub-domain in RFC 2821, section 4.1.2

         uLet-dig = Let-dig / UTF8-non-ASCII UTF8-xtra-char
                   ; Let-dig is defined in RFC 2821, section 4.1.3

         uLdh-str = *( ALPHA / DIGIT / "-" / UTF8-non-ASCII) UTF8-xtra-char) uLet-dig
                   ; Replace Ldh-str in RFC 2821, section 4.1.3

         UTF8-non-ASCII

         UTF8-xtra-char = UTF8-2 / UTF8-3 / UTF8-4
               ; UTF8-2, UTF8-3 and UTF8-4 are defined in RFC 3629

   The value of "udomain" SHOULD be verified with IDNA [RFC3490]; If
   failed, the email address with that udomain can not be regarded as
   the valid email address.

2.4.  The ALT-ADDRESS parameter

   If the UTF8SMTP extension is offered, the syntax of the SMTP MAIL and
   RCPT commands is extended to support the optional esmtp-keyword "ALT-
   ADDRESS", to specify the conditions under which the SMTP server specifies an alternate all-ASCII address which may
   use ALT-ADDRESS for the possible be
   used when downgrading.  If the ALT-ADDRESS esmtp-keyword is used, it
   MUST have an associated esmtp-value (ALT-
   ADDRESS-esmtp-value (ALT-ADDRESS-esmtp-value which is
   defined below) which requires an all-
   ASCII email address. below).

   Based on the definition of mail-parameters in [RFC2821], the ALT-
   ADDRESS parameter usage in the commands of "mail from" and "rcpt to"
   is defined below.

        "MAIL FROM:" SP <uReverse-path> [ SP <mail-parameters> ]<CRLF> <ALT-ADDRESS-parameter> ]
                   ; Update mail command in RFC 2821, section 3.3
                   ; The syntax for "esmtp-value" in RFC2821
                   ; does not allow "=", SP and control characters.
                   ; Therefore ALT-ADDRESS-paramater is extended.

            "RCPT TO:" SP <uForward-path> [ SP <rcpt-parameters> ]<CRLF> ]
               ; Update rcpt command in RFC 2821, section 3.3

        uReverse-path = uPath
               ; Replace Reverse-path in RFC 2821, section 4.1.2

        uForward-path = uPath
               ; Replace Forward-path in RFC 2821, section 4.1.2

        uPath = "<" [ A-d-l ":" ] uMailbox ">"
               ; Replace Path in RFC 2821, section 4.1.2
                   ; A-d-l is defined in RFC 2821, section 4.1.2
                   ; uMailbox is defined in section 2.3 of this document

        ALT-ADDRESS-esmtp-value=Mailbox

            ALT-ADDRESS-parameter="ALT-ADDRESS=" ALT-ADDRESS-esmtp-value

        ALT-ADDRESS-esmtp-value=xtext
           ; Mailbox xtext is defined in RFC 2821, 3461, section 4.1.2 4.2

   The use of the ALT-ADDRESS is specified below: If some involved SMTP
   servers can not support UTF8SMTP capability and if the sender has
   already set the ALT-ADDRESS value, the client SMTP server will use
   this address as the email address when the SMTP server does the
   subsequent operations.  If the ALT-ADDRESS value is not set by the
   sender, the email must be rejected to the original sender.  If the
   email is rejected due to the incapability of supporting UTF8SMTP, the
   relative server should issue the response error code "5.3.3" defined ALT-ADDRESS-parameter MUST NOT appear more than once in [RFC3463] which means that System is not capable of selected
   features, permanent failure.

2.5.  The Suggestion of the Value of the ALT-ADDRESS parameter

   The "ALT-ADDRESS" requires any MAIL
   or RCPT command.  ALT-ADDRESS-esmtp-value MUST be an all-ASCII address.  There are two
   alternative ways to set ALT-ADDRESS value: one is set by the sender
   using the all-ASCII address, the other is set using the transformed email address.

   Some may prefer transforming the non-ASCII address to the ASCII
   Compatible Encoding(ACE)
   address as the value of before xtext encoding.

2.5.  Using the ALT-ADDRESS. ALT-ADDRESS parameter

   A
   significant obstacle with applying an ACE to all local-parts is that
   the sending or converting system doesn't know if there are some
   specific data or instructions embedded in the address that the ACE
   process would hide.  Some SMTP servers may depend on these specific
   data or instructions to do some operations while the local parts
   applied with ACE will lose or hide these data or instructions.  SMTP
   [RFC2821] prohibits SMTP relays from converting local parts because
   the level of SMTP relays' knowledge on the structure of local parts
   is assumed to be zero.  However, we can raise the knowledge level by
   supplying additional information.  Many human users' email addresses
   do not have any embedded structure processed by the final delivery
   MTA.  In that case, the sender can specify that these email addresses
   are safe to be converted in the predefined way.  The final delivery
   SMTP server can revert the message containing non-ASCII envelope addresses even though they are as in all
   ASCII form.  Unless the MUA or the submission server clearly knows
   that the non-ASCII address can header fields
   MUST NOT be safely transformed into the all-
   ASCII address, the non-ASCII address should sent to an SMTP server which does not support UTF8SMTP.
   Such a message MAY be transformed
   because transformed email address may cause some potential problems.

   This document suggests that rejected due to lack of the ALT-ADDRESS as
   discussed in section 2.2 of this document.

   When messages are rejected because they require UTF8SMTP, response
   code "550" is set directly by the
   sender; In default, used, defined in [RFC2821], meaning "mailbox
   unavailable".  If enhanced mail system status codes [RFC3463] is
   used, the all-ASCII address response code should not be gotten from
   the transformation of "5.6.x" [SMTP-codes], meaning that
   "alt-address is required but not specified".

   [[anchor8: REMOVE THIS: IANA please assign the non-ASCII address. proper error codes for
   "5.6.x".]]

2.6.  Body Parts and SMTP Extensions

   Since there is no ESMTP parameter which tells whether the message is
   UTF8SMTP message, SMTP server needs to parse all message header
   fields and MIME header fields in the message body to discover which
   messages are UTF8SMTP.  While this specification requires that
   servers support the 8BITMIME extension [RFC1652] to ensure that
   servers have adequate handling capability for 8-bit data and to avoid
   a number of complex encoding problems, the use of internationalized
   addresses obviously does not require non-ASCII body parts in the MIME
   message.  The UTF8SMTP extension MAY be used with the BODY=8BITMIME
   parameter if that is appropriate given the body content or, if the
   server advertises it and it is appropriate, with the BODY=BINARYMIME
   parameter specified in [RFC3030].  This document does not modify the
   intent of BODY=BINARYMIME that text body parts and headers must still
   be handled in a line-oriented way.

   Assuming that the server advertises UTF8SMTP and 8BITMIME, and at
   least one non-ASCII address, with or without ALT-ADDRESS, the precise
   interpretation of these parameters on of "No 'Body' parameter", "BODY=
   8BITMIME", and "BODY= BINARYMIME" of the MAIL command is:
   1.  Headers  For No "Body" parameter, headers are in UTF-8, body parts are in
       ASCII.
   2.  Headers  For BODY=8BITMIME parameter, headers are in UTF-8, some or all
       body parts contain 8-bit line-
       oriented line-oriented data.
   3.  Headers  For BODY=BINARYMIME parameter, headers are in UTF-8, some or all
       body parts contain binary data without restriction as to line
       lengths or delimiters.

2.7.  Additional ESMTP Changes and Clarifications

   The mail transport process involves addresses ("mailboxes") and
   domain names in contexts in addition to the MAIL and RCPT commands
   and extended alternatives to them.  In general, the rule is that,
   when RFC 2821 specifies a mailbox, this document expects UTF-8 to be
   used for the entire string; when RFC 2821 specifies a domain name,
   the name SHOULD be in punycode the form of ACE labels if its raw form is non-ASCII. non-
   ASCII.

   The following subsections list and discuss all of the relevant cases.

   Support and use of this extension requires support for 8BITMIME.  It
   means that 8BITMIME MUST be advertised by the UTF8SMTP capability
   SMTP server.

2.7.1.  The Initial SMTP Exchange

   When an SMTP or ESMTP connection is opened, the server normally sends
   a
   "banner" "greeting" response consisting of the 220 '220' reply code and some
   information.  The client then sends the EHLO command.  Since the
   client cannot know whether the server supports UTF8SMTP until after
   it receives the response from EHLO, any domain names that appear in
   this dialogue, or in responses to EHLO, MUST be in the hostname form,
   i.e., internationalized ones MUST be in punycode form. the form of ACE labels.

2.7.2.  Message Retry

   When an

   An MSA or MTA encounters a server that doesn't support UTF8SMTP
   while relaying a message that requires such support, it is
   RECOMMENDED that an alternate MX be tried, and/or the message is
   requeued for a later attempt, rather than immediately downgrading or
   bouncing.  If the message is requeued, the total elapsed time before
   bouncing or downgrading SHOULD be smaller than the value used for
   other SMTP error conditions such as host unreachable or persistent
   4xx response codes.

   This alternate-MX-or-retry-later technique SHOULD NOT be used when
   the message's return path is a non-ASCII address and the specific
   forward path being attempted is an ASCII address (because the
   implication MTA may encounter a server that the delivery path normally supports doesn't support UTF8SMTP does
   not hold in this case).
   while relaying a message that requires such support.  The selection
   of submission servers is presumably under the control of the sender's
   client, while the selection of potential intermediate relays is under
   the control of the administration of the final delivery server.
   Hence, there is a presumption, at least when the recipient address is
   non-ASCII, that the delivery path servers normally support UTF8SMTP
   (if the sender's client or MSA didn't support UTF8SMTP, the message
   would not have been accepted for delivery in the first place).  Thus,
   a lack of UTF8SMTP support is likely to be a temporary situation, situation.  It
   is suggested that an alternate MX be tried, and/or the message is
   requeued for a later attempt, rather than immediately downgrading or
   rejecting.  If the message is requeued, the total elapsed time before
   rejecting or downgrading SHOULD be smaller than the value used for
   other SMTP error conditions such as host unreachable or persistent
   '4xx' response codes.

   An example of such an algorithm:

   If a normal inbound message requires UTF8SMTP but the contacted server
   being down and a cooperating site acting doesn't
   support it, treat this as a backup MX.  If temporary failure if the lack
   of UTF8SMTP in message has been
   queued for less than 24 hours, unless the delivery return-path is non-ASCII
   and the forward path is all-ASCII.  Normal temporary failure action
   is taken, such as, additional address records of a the current MX
   record are attempted, then additional MX records are attempted, then
   the message is a temporary
   situation, requeued with increasing back-off timers.

   If message has been queued for less than 24 hours and the message
   requires UTF8SMTP support but the contact server doesn't offer this,
   the following diagram describes some situations:

          return-path   forward-path   action
          -----------   ------------   ------
          ASCII         ASCII          reject or downgrade
          non-ASCII     non-ASCII      temp fail
          ASCII         non-ASCII      temp fail
          non-ASCII     ASCII          reject or downgrade

   This alternate-MX-or-retry-later technique SHOULD NOT be used when
   the message's return path is sent successfully after retrying, then
   it was a good thing to do.  Of course, if there non-ASCII address and the specific
   forward path being attempted is always an ASCII-
   only SMTP server in ASCII address (because the path, then retrying only adds delay to
   implication that the
   failure (reject or downgrade). delivery path normally supports UTF8SMTP does
   not hold in this case).

2.7.3.  Trace Information

   When an SMTP server receives a message for delivery or further
   processing, it MUST insert trace ("time stamp" or "Received")
   information at the beginning of the message content.  Time stamp
   appears in the form of "Received: lines".  The most important use of Received: lines
   Received: lines is for debugging mail faults.  When the delivery SMTP
   server makes the "final delivery" of a message, it inserts a return-
   path line at the beginning of the mail data.  The primary purpose of
   the Return-path is for debugging to designate the address to which messages
   indicating non-delivery or other mail faults. system failures are to be sent.
   For the trace information, we update the time stamp line and the
   return path line [RFC2821] formally defined as follows:

   uReturn-path-line = "Return-Path:" FWS uReverse-path <CRLF>
           ; Replaces Return-path-line in the section 4.4 of [RFC2821]
       ; uReverse-path is defined in Section 2.3

   uTime-stamp-line = "Received:" FWS uStamp <CRLF>
           ; Replaces Time-stamp-line in the section 4.4 of [RFC2821]

   uStamp = From-domain By-domain uOpt-info ";"  FWS date-time
           ; Replaces Stamp in the section 4.4 of [RFC2821]

   uOpt-info = [Via] [With] [ID] [uFor]
           ; Replaces Opt-info in the section 4.4 of [RFC2821]
           ; [With]'s protocl value will allow UTF8SMTP value

   uFor = "FOR" FWS 1*( uPath / uMailbox ) CFWS
           ; Replaces For in the section 4.4 of [RFC2821]
           ; uReverse-path uPath is defined in Section section 2.4 of this document
       ; uMailbox is defined in section 2.3 of this document

   Except in the 'uFor' and 'uReverse-path' line where non-ASCII domain
   name may be used, internationalized domain names in Received fields
   MUST be transmitted in the punycode form. form of ACE labels.  The [With]'s protocl value
   will have the protocol value of 'UTF8SMTP' for UTF8SMTP extension.  We will
   give more informaiton about this in "IANA consideration" section of
   this document.  If a "for"
   the WITH clause containing non-ASCII is encountered UTF8SMTP when downgrading a message, it is better to just drop the "for"
   clause rather than figure out some creative way to encode it.  When
   only the domain portion of a "for" clause address contains non-ASCII, this document suggests using the punycode form of the domain portion.
   For more detailed information, you may see it in [EAI-utf8header].

2.7.4.  Mailing List Question

   How a mixture of traditional and internationalized addresses on a
   mailing list will impact message flows, error reports, and delivery
   notifications in all plausible combinations of UTF8SMTP capability
   and un-capability servers extension is used.  More
   information is discussed and specified in the
   [EAI-mailing list].

2.7.5.  POP and IMAP

   While SMTP mainly takes care "IANA Considerations" section of the transportation this document,
   below.

3.  Issues with Other Parts of messages and
   the header fields on wire, POP essentially handles the retrieval of
   mail objects from Email System

3.1.  LMTP

   LMTP [RFC2033] may be used as the server by a client. final delivery agent.  In order such
   cases, LMTP may be arranged to use
   internationalized user names based on i18mail for deliver the retrieval of
   messages from a mail server using the POP protocol, a new capability
   SHOULD be introduced following the POP3 extension mechanism
   [RFC2449].  This is discussed and specified in the [EAI-pop].

   IMAP [RFC3501] uses the traditional user name which is based on
   ASCII.  IMAP SHOULD be updated to support the internationalized user
   names based on i18mail for the retrieval of messages from a mail
   server.  This is discussed and specified in the [EAI-imap].

2.7.6. store.
   The mail store may not have UTF8SMTP capability.  LMTP need to be
   updated to deal with these situations.

3.2.  SMTP Service Extension for DSNs

   The existing draft standard Delivery status notifications
   (DSNs)[RFC3461] is presently limited to US-ASCII text in the machine
   readable portions of the protocol.  "International Delivery and
   Disposition Notifications" [EAI-dsn] 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.  If an SMTP server advertises both the UTF8SMTP and the
   DSN extension, that server MUST implement EAI-dsn [EAI-dsn] including
   support for the ORCPT parameter.

3.  Potential problems

3.1.  Impact to RFC 4409

3.3.  POP and many email related RFC IMAP

   The [EAI-framework] has introduced two documents [EAI-pop] and
   [EAI-imap] to how to use internationalized email address protocols will impact user names based on UTF-8
   characters for the retrieval of messages from a mail server.

4.  Potential problems

4.1.  Impact many email related RFC documents such as Message Submission [RFC4409].
   Those protocols SHOULD be considered when implementing the
   internationalized

   Internationalized email has implications for all processes and
   protocols which examine, handle, generate, or otherwise deal with
   mail.  In particular, address protocols.

4. parsing or validity checks, message
   parsing or handling, etc.

5.  Implementation Advice

   In the absence of this extension, SMTP clients and servers are
   constrained to using only those addresses permitted by RFC 2821.  The
   local parts of those addresses MAY be made up of any ASCII
   characters, although some of them MUST be quoted as specified there.
   It is notable in an internationalization context that there is a long
   history on some systems of using overstruck ASCII characters (a
   character, a backspace, and another character) within a quoted string
   to approximate non-ASCII characters.  This form of
   internationalization SHOULD be phased out as this extension becomes
   widely deployed but backward-compatibility considerations require
   that it continue to be supported.

5.

6.  IANA Considerations

   IANA is requested to add "UTF8SMTP" to the SMTP extensions registry
   with the entry pointing to this specification for its definition.

   IANA is requested to assign the proper error codes "5.6.x" for this
   specification based on [SMTP-codes].

   The "Mail Transmission Types" registry is requested to be updated to
   include the following new entries:

  WITH protocol types  Description                             Reference
  -------------------  ----------------------------            ---------
  UTF8SMTP             UTF8SMTP with Service Extensions        [RFCxxxx]
  UTF8SMTPA            UTF8SMTP with SMTP AUTH                 [RFCxxxx]
  UTF8SMTPS            UTF8SMTP

   [[anchor20: REMOVE THIS: where RFCxxxx represents the future RFC N0.
   of this document.  When this document is published as RFC and
   assigned with STARTTLS                  [RFCxxxx]
  UTF8SMTPSA           UTF8SMTP a RFC No., "xxxx" should be replaced with both STARTTLS and         [RFCxxxx]
                       SMTP AUTH

6. 4-digits
   No..]]

7.  Security considerations

   See the extended security considerations discussion in [EAI-overview]

7.
   [EAI-framework]

8.  Acknowledgements

   Much of the text in the initial version of this document was derived
   or copied from [Klensin-emailaddr] with the permission of the author.

   Significant comments and suggestions were received from Xiaodong LEE,
   Nai-Wen Hsu, Yangwoo KO, Yoshiro YONEYA, and other members of the JET
   team and were incorporated into the document.  Special thanks to
   those contributors for this version of document, those includes (but
   not limited to) John C Klensin, Charles Lindsey, Dave Crocker, Harald
   Tveit Alvestrand, Marcos Sanz, Chris Newman, Martin Duerst, Edmon
   Chung, Tony Finch, Kari Hurtta, Randall Gellens, Frank Ellermann.

8.  Of
   course, none of the individuals are necessarily responsible for the
   combination of ideas represented here.

9.  Change History

   [[anchor19:

   [[anchor23: REMOVE THIS: This section is used for tracking the update
   of this document.  It may be useful to retain parts of it to
   facilitate establishing dates and documents for the history of this
   work.]]

8.1.

9.1.  draft-ietf-eai-smtpext: Version 00

   This version supercedes draft-yao-ima-smtpext-03.txt.  It refines the
   ABNF definiton definition of the internationalized email address.  It
   represents as the EAI working group document.

8.2.

9.2.  draft-ietf-eai-smtpext: Version 01

   o  Upgraded to reflect discussions during IETF 66.
   o  Remove the atomic parameter.
   o  Add the new section of "the Suggestion of the value of the ALT-
      ADDRESS parameter".

8.3.

9.3.  draft-ietf-eai-smtpext: Version 02

   o  Upgraded to reflect the recent discussion of the ima@ietf.org
      mailing list.
   o  Add the section of "Body Parts and SMTP Extensions".
   o  Add the new section of "Change History".
   o  Add the subsection about SMTP extensions for DSN.

8.4.

9.4.  draft-ietf-eai-smtpext: Version 03

   o  Update the syntax related to mailbox.
   o  Update the trace field section.
   o  Add the new section about message retry.
   o  Update the subsection about SMTP extensions for DSN.

8.5.

9.5.  draft-ietf-eai-smtpext: Version 04

   o  Refine some syntax.
   o  Delete "Message Header Label" section.
   o  Change "bounce" to "reject".

9.  References

9.1.  Normative References

   [ASCII]    Cerf, V., "ASCII format for network interchange", RFC 20,
              October 1969.

   [EAI-downgrading]
              YONEYA, Y., Ed. and K. Fujiwara, Ed., "Downgrading
              mechanism for Internationalized eMail Address (IMA)",
              draft-ietf-eai-downgrade-02 (work in progress),
              August 2006.

   [EAI-dsn]  Newman, C., "SMTP extensions for DSNs",
              draft-ietf-eai-dsn-00.txt (work in progress), 1 2007.

   [EAI-imap]
              Resnick, P.

9.6.  draft-ietf-eai-smtpext: Version 05

   o  Refine the abstract.
   o  Delete "The Suggestion of the Value of the ALT-ADDRESS parameter"
      section.
   o  Move original section 2.7.4 and C. Newman, "Considerations for IMAP in
              Conjunction 2.7.5 to section 3 with Email Address Internationalization",
              draft-ietf-eai-imap-utf8-00 (work in progress), May 2006.

   [EAI-mailing list]
              Gellens, R. and E. Chung, the name
      "Issues with other parts of the email system".
   o  Add the new section "LMTP".
   o  Refine some text according to suggestions from the EAI mailing
      list discussion
   o  Remove the section "Mailing Lists and
              Internationalized Email Addresses",
              draft-ietf-eai-mailinglist-01.txt (work in progress),
              January 2007.

   [EAI-overview] List Question"

10.  References

10.1.  Normative References

   [ASCII]    Cerf, V., "ASCII format for network interchange", RFC 20,
              October 1969.

   [EAI-framework]
              Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", draft-ietf-eai-framework-04.txt
              (work in progress), 12 2006.

   [EAI-pop]  Newman, C., "POP3 Support for UTF-8",
              draft-ietf-eai-pop-01.txt draft-ietf-eai-framework-05.txt
              (work in progress),
              January 2 2007.

   [EAI-utf8header]
              Yeh, J., "Transmission of Email Headers in UTF-8
              Encoding", draft-ietf-eai-utf8headers-01.txt draft-ietf-eai-utf8headers-05.txt (work in
              progress), August 2006. April 2007.

   [RFC1652]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
              Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
              RFC 1652, July 1994.

   [RFC1869]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
              Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
              November 1995.

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

   [RFC2449]  Gellens, R., Newman, C., and L. Lundblade, "POP3 Extension
              Mechanism", RFC 2449, November 1998.

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

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

   [RFC3030]  Vaudreuil, G., "SMTP Service Extensions for Transmission
              of Large and Binary MIME Messages", RFC 3030,
              December 2000.

   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

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

   [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes",
              RFC 3463, January 2003.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.

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

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

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

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

   [RFC4409]

10.2.  Informative References

   [EAI-downgrading]
              YONEYA, Y., Ed. and K. Fujiwara, Ed., "Downgrading
              mechanism for Internationalized eMail Address (IMA)",
              draft-ietf-eai-downgrade-02 (work in progress),
              August 2006.

   [EAI-dsn]  Newman, C., "SMTP extensions for DSNs",
              draft-ietf-eai-dsn-00.txt (work in progress), 1 2007.

   [EAI-imap]
              Resnick, P. and C. Newman, "Considerations for IMAP in
              Conjunction with Email Address Internationalization",
              draft-ietf-eai-imap-utf8-01 (work in progress),
              March 2007.

   [EAI-mailing list]
              Gellens, R. and J. Klensin, "Message Submission E. Chung, "Mailing Lists and
              Internationalized Email Addresses",
              draft-ietf-eai-mailinglist-01.txt (work in progress),
              January 2007.

   [EAI-pop]  Newman, C., "POP3 Support for Mail",
              RFC 4409, April 2006.

9.2.  Informative References UTF-8",
              draft-ietf-eai-pop-01.txt (work in progress),
              January 2007.

   [Klensin-emailaddr]
              Klensin, J., "Internationalization of Email Addresses",
              draft-klensin-emailaddr-i18n-03 (work in progress),
              July 2005.

   [RFC2033]  Myers, J., "Local Mail Transfer Protocol", RFC 2033,
              October 1996.

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

   [RFC3030]  Vaudreuil, G., "SMTP Service Extensions for Transmission
              of Large and Binary MIME Messages", RFC 3030,
              December 2000.

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

   [RFC4409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              RFC 4409, April 2006.

   [SMTP-codes]
              KLensin, J., "An IANA Registry for Extended SMTP Status
              Codes", draft-klensin-smtp-code-registry-00 (work in
              progress), April 2007.

Authors' Addresses

   Jiankang YAO (editor)
   CNNIC
   No.4 South 4th Street, Zhongguancun
   Beijing

   Phone: +86 10 58813007
   Email: yaojk@cnnic.cn

   Wei MAO (editor)
   CNNIC
   No.4 South 4th Street, Zhongguancun
   Beijing

   Phone: +86 10 58813055
   Email: mao@cnnic.cn maowei_ietf@cnnic.cn

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