Network Working Group                                             J. Yao
Internet-Draft                                                    W. Mao
Obsoletes: RFC5336 (if approved)                                   CNNIC
Updates: RFC5321 and 5322                                 April 11,                                   June 2, 2011
(if approved)
Intended status: Standards Track
Expires: October 13, December 4, 2011

           SMTP Extension for Internationalized Email Address
                    draft-ietf-eai-rfc5336bis-09.txt
                    draft-ietf-eai-rfc5336bis-10.txt

Abstract

   This document specifies an SMTP extension for transport and delivery
   of email messages with internationalized email addresses or header
   information.

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
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   This Internet-Draft will expire on October 13, December 4, 2011.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Role of This Specification . . . . . . . . . . . . . . . .  4
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Updates to Other Specifications  . . . . . . . . . . . . .  5
   2.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  5
   3.  Mail Transport-Level Protocol  . . . . . . . . . . . . . . . .  5
     3.1.  Framework for the Internationalization Extension . . . . .  5
     3.2.  The UTF8SMTPbis Extension  . . . . . . . . . . . . . . . .  6
     3.3.  Extended Mailbox Address Syntax  . . . . . . . . . . . . .  7
     3.4.  MAIL Command Parameter Usage . . . . . . . . . . . . . . . 10  9
     3.5.  Non-ASCII addresses and Reply-codes  . . . . . . . . . . . 10  9
     3.6.  Body Parts and SMTP Extensions . . . . . . . . . . . . . . 11 10
     3.7.  Additional ESMTP Changes and Clarifications  . . . . . . . 11 10
       3.7.1.  The Initial SMTP Exchange  . . . . . . . . . . . . . . 11 10
       3.7.2.  Mail eXchangers  . . . . . . . . . . . . . . . . . . . 12 11
       3.7.3.  Trace Information  . . . . . . . . . . . . . . . . . . 12 11
       3.7.4.  UTF-8 Strings in Replies . . . . . . . . . . . . . . . 14 11
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17 15
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18 16
   7.  Change History . . . . . . . . . . . . . . . . . . . . . . . . 18 16
     7.1.  draft-yao-eai-rfc5336bis: Version 00 . . . . . . . . . . . 18 16
     7.2.  draft-ietf-eai-rfc5336bis: Version 00  . . . . . . . . . . 19 16
     7.3.  draft-ietf-eai-rfc5336bis: Version 01  . . . . . . . . . . 19 17
     7.4.  draft-ietf-eai-rfc5336bis: Version 02  . . . . . . . . . . 19 17
     7.5.  draft-ietf-eai-rfc5336bis: Version 03  . . . . . . . . . . 19 17
     7.6.  draft-ietf-eai-rfc5336bis: Version 04  . . . . . . . . . . 19 17
     7.7.  draft-ietf-eai-rfc5336bis: Version 05  . . . . . . . . . . 19 17
     7.8.  draft-ietf-eai-rfc5336bis: Version 06  . . . . . . . . . . 19 17
     7.9.  draft-ietf-eai-rfc5336bis: Version 07  . . . . . . . . . . 19 17
     7.10. draft-ietf-eai-rfc5336bis: Version 08  . . . . . . . . . . 19 17
     7.11. draft-ietf-eai-rfc5336bis: Version 09  . . . . . . . . . . 20 17
     7.12. draft-ietf-eai-rfc5336bis: Version 10  . . . . . . . . . . 18
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 18
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 20 18
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 21 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22 20

1.  Introduction

   The Simple Mail Transfer Protocol [RFC5321] provides a negotiation
   mechanism about service extension by which SMTP clients can discover
   SMTP server capabilities and make decisions for further processing.
   This document uses this mechanism and specifies an SMTP extension to
   permit internationalized email addresses (see Section section 1.2) in the
   SMTP envelope, and Unicode characters encoded in UTF-8 [RFC3629] in
   the headers.  An extended overview of the extension model for
   internationalized email addresses and the email header appears in
   [RFC4952bis], referred to as "the framework document" or just as
   "framework" elsewhere in this specification.

   [[anchor1: Note in Draft and to RFC Editor: The keyword represented
   in this document by "UTF8SMTPbis" (and in the XML source
   byUTF8SMTPbis) is a placeholder.  The actual keyword will be assigned
   when the standards track SMTP extension in this series [RFC5336bis-
   SMTP] is approved for publication and should be substituted here.
   This paragraph should be treated as normative reference to that SMTP
   extension draft, creating a reference hold until it is approved by
   the IESG.  This paragraph should be removed before RFC publication.]]

1.1.  Role of This Specification

   The framework document [RFC4952bis] specifies the requirements for,
   and describes components of, full internationalization of electronic
   mail.  A thorough understanding of the information in that document
   and in the base Internet email specifications [RFC5321] [RFC5322] is
   necessary to understand and implement this specification.

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

1.2.  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].

   The terms "UTF-8 string" or "UTF-8 character" are used to refer to
   Unicode characters encoded in UTF-8.  All other specialized terms
   used in this specification are defined in the framework document or
   in the base Internet email specifications.  In particular, the terms
   "ASCII address", "internationalized email address", "non-ASCII
   address", "UTF8SMTPbis", "internationalized message", and "message"
   are used in this document according to the definitions in the
   framework document.

   Non-ASCII characters or strings referred in this document MUST be
   expressed in UTF-8, a standard Unicode Encoding Form.

   This specification uses Augmented BNF (ABNF) rules [RFC5234], with
   some modifications.  The modified [RFC5234].  Some
   basic rules are defined in this
   specification.  When a new rule has a name starting with "u", it is a
   small modification to an older rule.  Rules that are undefined here document can be found from [RFC5234] or [RFC5321]
   or [RFC5322] under the same names.

1.3.  Updates to Other Specifications

   This specification modifies RFC 5321 by permitting internationalized
   email address in the envelope.  It also updates some syntax rules
   defined in RFC 5321.  It modifies RFC 5322 by permitting data formats
   defined in [RFC5335bis].  It does not modify the 8BITMIME
   specification [RFC6152] in any way, but it does require that the
   8BITMIME extension be announced by the EAI-aware SMTP server and used
   with "BODY=8BMITMIME" by the EAI-aware SMTP client.

2.  Overview of Operation

   This specification describes an optional extension to the email
   transport mechanism that permits non-ASCII characters in both the
   envelope and header fields of messages, which are encoded in UTF-8.
   The extension is identified with the token "UTF8SMTPbis".

   The EAI UTF-8 header specification [RFC5335bis] provides the details
   of email header features enabled by this extension

3.  Mail Transport-Level Protocol

3.1.  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
        "UTF8SMTPbis".
   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 this keyword.
        The EAI-aware SMTP client MUST ignore any parameters if they
        appear for this keyword; that is, the EAI-aware SMTP client MUST
        behave as if the parameters do not appear.  If an SMTP server
        includes UTF8SMTPbis in its EHLO response, it MUST be fully
        compliant with this version of this specification.

   4.   One OPTIONAL parameter "UTF8SMTPbis" is added to the MAIL
        command.  The parameter has no value.  If this parameter is set
        in the MAIL command, it indicates that the SMTP client is EAI-
        aware and asserts that the envelope includes the non-ASCII
        address or the message being sent is internationalized message
        or the message being sent needs the UTF8SMTPbis support.
   5.   The maximum length of a MAIL command line is increased by 12
        characters by the possible addition of the UTF8SMTPbis
        parameter. [[anchor6: RFC Editor: the number '12' will be
        replaced by the new number (1 space + length of the new keyword
        supposed to replace "UTF8SMTPbis").]]
   6.   One OPTIONAL parameter "UTF8SMTPbis" is added to the VRFY and
        EXPN commands.  The parameter UTF8SMTPbis has no value.  The
        parameter indicates that the SMTP client can accept Unicode
        characters in UTF-8 encoding in replies from the VRFY and EXPN
        commands.
   7.   No additional SMTP verbs are defined by this extension.
   8.   Servers offering this extension MUST provide support for, and
        announce, the 8BITMIME extension [RFC6152].
   9.   The reverse-path and forward-path of the SMTP MAIL and RCPT
        commands are extended to allow Unicode characters encoded in
        UTF-8 in mailbox names (addresses).
   10.  The mail message body is extended as specified in [RFC5335bis].
   11.  The UTF8SMTPbis extension is valid on the submission port
        [RFC4409], and can be used with LMTP [RFC2033].

3.2.  The UTF8SMTPbis Extension

   An SMTP server that announces this UTF8SMTPbis extension MUST be
   prepared to accept a UTF-8 string [RFC3629] in any position in which
   RFC 5321 specifies that a <mailbox> can appear.  Although the
   characters in the <local-part> are permitted to contain non-ASCII
   characters, actual parsing of the <local-part>, and the delimiters
   used, are unchanged from the base email specification [RFC5321].  Any
   domain names to be looked up in the DNS MUST allow for [RFC5890]
   behavior.  When doing lookups, the EAI-aware SMTP client or server
   MUST either use a Unicode aware DNS library, or transform it to
   A-label defined in [RFC5890].

   An SMTP client that receives the UTF8SMTPbis extension keyword in
   response to the EHLO command MAY transmit mailbox names within SMTP
   commands as internationalized strings in UTF-8 form.  It MAY send a
   UTF-8 header [RFC5335bis] (which may also include mailbox names in
   UTF-8).  It MAY transmit the domain parts of mailbox names within
   SMTP commands or the message header as A-labels or U-labels
   [RFC5890].  The presence of the UTF8SMTPbis extension does not change
   RFC 5321 server relaying behaviors.

   If the UTF8SMTPbis SMTP extension is not offered by the SMTP server,
   the EAI-aware SMTP client MUST NOT transmit an internationalized
   email address and MUST NOT transmit a mail message containing
   internationalized mail headers as described in [RFC5335bis] at any
   level within its MIME structure [RFC2045] and [RFC2047].  (For this
   paragraph, the internationalized domain name in the form of A-labels
   as specified in IDNA definitions [RFC5890] is not considered to be
   "internationalized".)  Instead, if an EAI-aware SMTP client (EAI-
   aware SMTP sender) attempts to transfer an internationalized message
   and encounters an SMTP server that does not support the extension, it
   MUST make one of the following three choices and the priority order
   is 1, 2 and 3.

   1.  It MAY either reject the message during the SMTP transaction or
       accept the message and then generate and transmit a notification
       of non-deliverability.  Such notification MUST be done as
       specified in RFC 5321 [RFC5321], RFC 3464 [RFC3464], and the EAI
       delivery status notification (DSN) specification [RFC5337bis].
   2.  If and only if the EAI-aware SMTP client (sender) is a Message
       Submission Agent ("MSA") [RFC4409] [RFC5598], it MAY rewrite the
       envelope, headers, or message material MSA may choose its
       own way to make them entirely
       ASCII [ASCII] and consistent deal with this scenario according to the provisions of RFC 5321
       [RFC5321]
       [RFC4409] or its future versions.  But the detailed specification
       of this process and RFC 5322 [RFC5322]. its results is outside the scope of this
       document.
   3.  It MAY find an alternate route to the destination that permits
       UTF8SMTPbis.  That route MAY be discovered by trying alternate
       Mail eXchanger (MX) hosts (using preference rules as specified in
       RFC 5321) or using other means available to the EAI-aware SMTP
       client.

   This document applies only when an EAI-aware SMTP client is trying to
   send an internationalized message to an EAI-aware SMTP server.  For
   all other cases, and for addresses and messages that do not require
   an UTF8SMTPbis extension, EAI-aware SMTP clients and servers do not
   change the behavior specified in [RFC5321].

   An EAI-aware MUA/MSA sending to a legacy SMTP server [RFC5321] and
   [RFC5322] MAY convert an ASCII@U-label [RFC5890] address into the
   format of ASCII@A-label [RFC5890] if the email address is in the
   format of ASCII@U-label.

3.3.  Extended Mailbox Address Syntax

   RFC 5321, Section section 4.1.2, defines the syntax of a <mailbox> <Mailbox> entirely
   in terms of ASCII characters.  This document will make <Mailbox> to
   support non-ASCII characters.

   The key changes made by this specification include:

   o  Change  In order to update the <Mailbox> to support the internationalized
      email address, the <Mailbox> ABNF rule will be importted from RFC
      5321 directly, and other related rules are importted from RFC
      5321, RFC 5234, RFC 5890 or RFC 5335bis, or are extended in this
      document.
   o  Extend the definition of <Domain> <sub-domain> to permit both the RFC 5321
      definition and a UTF-8 string representing in a DNS label that is conforming
      with IDNA definitions [RFC5890].
   o  Change  Extend the definition of <Local-part> <atext> to permit both the RFC 5321
      definition and 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, the syntax of an
   internationalized email mailbox name (address) is defined in controls characters.

   The following ABNF
   [RFC5234] as follows.

   uMailbox = uLocal-part "@" ( uDomain / address-literal )
    ; Replace Mailbox in rules will be importted from RFC 5321, Section 4.1.2

   address-literal = <Defined in Section section
   4.1.2 of RFC 5321>

   uLocal-part = uDot-string / uQuoted-string
    ; MAY directly:
   o  <Mailbox>
   o  <Local-part>
   o  <Dot-string>
   o  <Quoted-string>
   o  <QcontentSMTP>
   o  <Domain>
   o  <Atom>

   The following ABNF rule will be case-sensitive
    ; Replace Local-part in importted from RFC 5321, Section 4.1.2

   uDot-string = uAtom *("." uAtom)
    ; Replace Dot-string in 5335bis, section
   4.1 directly:
   o  <UTF8-non-ascii>

   The following ABNF rule will be importted from RFC 5321, Section 4.1.2

   uAtom = 1*ucharacter 5234, appendix B.1
   directly:
   o  <DQUOTE>

   The following ABNF rule will be importted from RFC 5890, section
   2.3.2.1 directly:
   o  <U-label>

   The following rules are extended in ABNF [RFC5234] as follows.

   sub-domain   =/  U-label
    ; Replace Atom extend the defintion of sub-domain in RFC 5321, Section RFC5321, section 4.1.2

   ucharacter =

   atext /   =/  UTF8-non-ascii

   atext = <Defined in Section 3.2.3 of RFC 5322>
    ; Same definition with extend the defintion of atext in RFC 5321, Section 4.1.2

   uQuoted-string = DQUOTE *uQcontentSMTP DQUOTE
    ; Replace Quoted-string in RFC 5321, Section RFC5321, section 4.1.2

   DQUOTE = <Defined in appendix B.1 of RFC 5234>

   uQcontentSMTP = qtextSMTP /

   quoted-pairSMTP /  =/ %d92 UTF8-non-ascii

   qtextSMTP = <Defined in Section 4.1.2
    ; extend the defintion of RFC 5321> quoted-pairSMTP = <Defined in Section 4.1.2 of RFC 5321>

   uDomain = sub-udomain *("." sub-udomain)
    ; Replace Domain in RFC 5321, Section 4.1.2

   sub-udomain = uLet-dig [uLdh-str]
    ; Replace sub-domain in RFC 5321, Section RFC5321, section 4.1.2
    ; Note that uDomain may not be resolvable if sub-udomain is not a
    ; valid U-Label or LDH Label as defined in RFC 5890

   uLet-dig = Let-dig /

   qtextSMTP  =/ UTF8-non-ascii

   Let-dig = <Defined in Section 4.1.2 of RFC 5321>

   uLdh-str = *( ALPHA / DIGIT / "-" / UTF8-non-ascii) uLet-dig
    ; Replace Ldh-str extend the defintion of qtextSMTP in RFC 5321, Section RFC5321, section 4.1.2

   UTF8-non-ascii = <Defined in Section 4.1 of RFC5335bis>

3.4.  MAIL Command Parameter Usage

   If the envelope or message being sent requires the capabilities of
   the UTF8SMTPbis extension, the EAI-aware SMTP client MUST supply the
   UTF8SMTPbis parameter with the MAIL command.  If this parameter is
   provided, it MUST have no value.  If the EAI-aware SMTP client is
   aware that neither the envelope nor the message being sent requires
   any of the UTF8SMTPbis extension capabilities, it SHOULD NOT supply
   the UTF8SMTPbis parameter with the MAIL command.

   Because there is no guarantee that a next-hop SMTP server will
   support the UTF8SMTPbis extension, use of the UTF8SMTPbis extension
   always carries a risk of transmission failure.  In fact, during the
   early stages of deployment for the UTF8SMTPbis extension, the risk
   will be quite high.  Hence there is a distinct near-term advantage
   for ASCII-only messages to be sent without using this extension.  The
   long-term advantage of casting ASCII [ASCII] characters(0x7f and
   below) as UTF-8 form is that it permits pure-Unicode environments.

   This specification does not require that the EAI-aware SMTP client
   inspect the message or otherwise go to extraordinary lengths to
   assure itself whether the UTF8SMTPbis extension is REQUIRED for the
   particular message.

3.5.  Non-ASCII addresses and Reply-codes

   An EAI-aware SMTP client MUST only send an internationalized message
   to an SMTP server that supports UTF8SMTPbis.  If the SMTP server does
   not support this option, then the EAI-aware SMTP client has three
   choices according to section 3.2 of this specification.

   The three-digit Reply-codes used in this section are based on their
   meanings as defined in RFC 5321.

   When messages are rejected because the RCPT command requires an ASCII
   address, the reply-code 553 is returned with the meaning "mailbox
   name not allowed".  When messages are rejected because the MAIL
   command requires an ASCII address, the reply-code 550 is returned
   with the meaning "mailbox unavailable".  When the EAI-aware SMTP
   server supports enhanced mail system status codes [RFC3463], reply-
   code "X.6.7" [RFC5248] is used, meaning that "non-ASCII addresses not
   permitted for that sender/recipient".

   When messages are rejected for other reasons, the server follows the
   model of the base email specifications in RFC 5321; this extension
   does not change those circumstances or reply messages.

   If the reply-code is issued after the final "." of the DATA command,
   the reply-code "554" is used with the meaning "Transaction failed".
   When the EAI-aware SMTP server supports enhanced mail system status
   codes [RFC3463], reply-code "X.6.9" [RFC5248] is used, meaning that
   "UTF-8 header message can not be transmitted to one or more
   recipients, so the message MUST be rejected".

3.6.  Body Parts and SMTP Extensions

   The MAIL command parameter UTF8SMTPbis asserts that a message is an
   internationalized message or the message being sent needs the
   UTF8SMTPbis support.  The message being sent via the MAIL command
   with the UTF8SMTPbis parameter has still a chance of that the message
   transmitted is not an internationalized message.  An EAI-aware SMTP
   client or server that requires accurate knowledge of whether a
   message is internationalized needs to parse all message header fields
   and MIME header fields [RFC2045] and [RFC2047] in the message body.
   However, this specification does not require that the EAI-aware SMTP
   client or server inspects the message.

   While this specification requires that EAI-aware SMTP servers support
   the 8BITMIME extension [RFC6152] 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 email addresses
   obviously does not require non-ASCII body parts in the MIME message
   in RFC 2045 and RFC 2047.  The UTF8SMTPbis extension MAY be used with
   the BODY=8BITMIME parameter [RFC6152] if that is appropriate given
   the body content or, with the BODY=BINARYMIME parameter, if the SMTP
   server advertises BINARYMIME [RFC3030] and that is appropriate.

3.7.  Additional ESMTP Changes and Clarifications

   The information carried in the mail transport process involves
   addresses ("mailboxes") and domain names in various contexts in
   addition to the MAIL and RCPT commands and extended alternatives to
   them.  In general, the rule is that, when RFC 5321 specifies a
   mailbox, this SMTP extension requires UTF-8 form to be used for the
   entire string; when RFC 5321 specifies a domain name, the name SHOULD
   be in the form of A-label if this domain name is an internationalized
   domain name[RFC5890].

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

3.7.1.  The Initial SMTP Exchange

   When an SMTP connection is opened, the SMTP server sends a "greeting"
   response consisting of the 220 reply-code and some information.  The
   SMTP client then sends the EHLO command.  Since the SMTP client
   cannot know whether the SMTP server supports UTF8SMTPbis until after
   it receives the response from EHLO, the EAI-aware SMTP client MUST
   send only ASCII (LDH label or A-label [RFC5890] ) domains in the EHLO
   command and that, if the EAI-aware SMTP server provides domain names
   in the EHLO response, they MUST be in the form of LDH labels or
   A-labels.

3.7.2.  Mail eXchangers

   If multiple DNS MX records are used to specify multiple servers for a
   domain in section 5 of [RFC5321], it is strongly advised that all or
   none of them SHOULD support the UTF8SMTPbis extension.  Otherwise,
   surprising rejections can happen during temporary or permanent
   failures, which users might perceive as serious reliability issues.
   In order to avoid the possible surprising rejections, the EAI-aware
   email system may also implement the advice in EAI addresses advice
   document [EAI addresses] and EAI deployment advice document [EAI
   Deployment].

3.7.3.  Trace Information

   For the

   The trace information [RFC5321], this memo updates the time stamp
   line <Return-path-line>, <Time-stamp-line> and the return path line [RFC5321] formally defined as follows:

   uReturn-path-line = "Return-Path:" FWS uReverse-path CRLF
    ; Replaces Return-path-line in Section 4.4 of RFC 5321

   uReverse-path = uPath / "<>"
    ; Replace Reverse-path in RFC 5321, section 4.1.2

   uPath = "<" [ A-d-l ":" ] uMailbox ">"
    ; Replace Path in RFC 5321, section 4.1.2
    ; uMailbox is their
   related rules have been defined in section 3.3 of this document

   A-d-l = <Defined in section 4.1.2 of RFC 5321>

   uTime-stamp-line = "Received:" FWS uStamp CRLF
    ; Replaces Time-stamp-line in Section 4.4 of RFC 5321

   uStamp = From-domain By-domain uOpt-info [CFWS] ";" FWS date-time
    ; Replaces Stamp in Section 4.4 of RFC 5321

   From-domain = <Defined in section 4.4 of RFC 5321>

   By-domain = <Defined in section 4.4 of RFC 5321>

   date-time = <Defined in section 3.3 of RFC 5322>
    ; Same definition with date-time in Section 4.4 of RFC 5321

   uOpt-info = [Via] [With] [ID] [uFor]
         [Additional-Registered-Clauses]
    ; Replaces Opt-info in Section 4.4 of RFC 5321
    ; The protocol value for With will allow a UTF8SMTPbis value

   Via = <Defined in section 4.4 of RFC 5321>

   With = <Defined in section 4.4 of RFC 5321>

   ID = <Defined in section 4.4 of RFC 5321>

   Additional-Registered-Clauses = <Defined in section 4.4 of RFC 5321>

   uFor = CFWS "FOR" FWS ( uPath / uMailbox)
    ; Replaces For in Section 4.4 of RFC 5321
    ; uMailbox is defined in section 3.3 of this
   [RFC5321].  This document has updated <Mailbox> and <Domain> to
   support non-ASCII characters.  So Return-path-line may include the
   'Reverse-path' clause where internationalized domain name with the
   U-label form may be used.  Time-stamp-line may include the 'For'
   clause where the internationalized domain name with the U-label form
   may be used.

   Except in the 'uFor' 'For' clause and 'uReverse-path' value 'Reverse-path' clause where
   internationalized domain name with the U-label form MAY be used,
   internationalized domain names in Received fields MUST be transmitted
   in the form of A-labels.  The protocol value of the WITH 'WITH' clause
   when this extension is used is one of the UTF8SMTPbis values
   specified in the "IANA Considerations" section of this document.

3.7.4.  UTF-8 Strings in Replies

3.7.4.1.  MAIL Command

   If an SMTP client follows this specification and sends any MAIL
   commands containing the UTF8SMTPbis parameter, the EAI-aware SMTP
   server is permitted to use UTF-8 characters in the email address
   associated with 251 and 551 reply-codes, and the SMTP client MUST be
   able to accept and process them.  If a given MAIL command does not
   include the UTF8SMTPbis parameter, the EAI-aware SMTP server MUST NOT
   return a 251 or 551 response containing a non-ASCII mailbox.
   Instead, it MUST transform such responses into 250 or 550 responses
   that do not contain non-ASCII addresses.

3.7.4.2.  VRFY and EXPN Commands and the UTF8SMTPbis Parameter

   If the VRFY and EXPN commands are transmitted with the parameter
   "UTF8SMTPbis", it indicates the SMTP client can accept UTF-8 strings
   in replies to those commands.  This parameter for the VRFY and EXPN
   commands SHOULD only be used after the SMTP client sees the EHLO
   response with the UTF8SMTPbis keyword.  This allows the EAI-aware
   SMTP server to use UTF-8 strings in mailbox names and full names that
   occur in replies without concern that the SMTP client might be
   confused by them.  An SMTP client that conforms to this specification
   MUST accept and correctly process replies from the VRFY and EXPN
   commands that contain UTF-8 strings.  However, the EAI-aware SMTP
   server MUST NOT use UTF-8 strings in replies if the SMTP client does
   not specifically allow such replies by transmitting this parameter.
   Most replies do not require that a mailbox name be included in the
   returned text, and therefore UTF-8 string is not needed in them.
   Some replies, notably those resulting from successful execution of
   the VRFY and EXPN commands, do include the mailbox.

   VERIFY (VRFY) and EXPAND (EXPN) command syntaxes are changed to:

   vrfy = "VRFY" SP uString String
     [ SP "UTF8SMTPbis" ] CRLF
    ; String may include UTF-8 characters

   expn = "EXPN" SP uString String
     [ SP "UTF8SMTPbis" ] CRLF

   uString = uAtom / uQuoted-string
    ; uAtom and uQuoted-string are defined in
    ; Section 3.3 of this document. String may include UTF-8 characters

   The "UTF8SMTPbis" parameter does not use a value.  If the reply to a
   VERIFY (VRFY) or EXPAND (EXPN) command requires UTF-8 string, but the
   SMTP client did not use the "UTF8SMTPbis" parameter, then the EAI-
   aware SMTP server MUST use either the reply-code 252 or 550.  Reply-
   code 252, defined in [RFC5321], means "Cannot VRFY user, but will
   accept the message and attempt the delivery".  Reply-code 550, also
   defined in [RFC5321], means "Requested action not taken: mailbox
   unavailable".  When the EAI-aware SMTP server supports enhanced mail
   system status codes [RFC3463], the enhanced reply-code as specified
   below is used.  Using the "UTF8SMTPbis" parameter with a VERIFY
   (VRFY) or EXPAND (EXPN) command enables UTF-8 replies for that
   command only.

   If a normal success response (i.e., 250) is returned, the response
   MAY include the full name of the user and MUST include the mailbox of
   the user.  It MUST be in either of the following forms:

   User Name <uMailbox> <Mailbox>
    ; uMailbox Mailbox is defined in Section section 3.3 of this document.
    ; User Name can contain non-ASCII characters.

   uMailbox

   Mailbox
    ; uMailbox Mailbox is defined in Section section 3.3 of this document.

   If the SMTP reply requires UTF-8 strings, but UTF-8 string is not
   allowed in the reply, and the EAI-aware SMTP server supports enhanced
   mail system status codes [RFC3463], the enhanced reply-code is
   "X.6.8" [RFC5248], meaning "A reply containing a UTF-8 string is
   REQUIRED to show the mailbox name, but that form of response is not
   permitted by the SMTP client".

   If the SMTP client does not support the UTF8SMTPbis extension, but
   receives a UTF-8 string in a reply, it may not be able to properly
   report the reply to the user, and some clients might crash.
   Internationalized messages in replies are only allowed in the
   commands under the situations described above.  Under any other
   circumstances, UTF-8 string MUST NOT appear in the reply.

   Although UTF-8 form is needed to represent email addresses in
   responses under the rules specified in this section, this extension
   does not permit the use of UTF-8 string for any other purposes.  EAI-
   aware SMTP servers MUST NOT include non-ASCII characters in replies
   except in the limited cases specifically permitted in this section.

4.  IANA Considerations

   IANA is requested to add a new value "UTF8SMTPbis" to the SMTP
   Service Extension subregistry of the Mail Parameters registry,
   according to the following data:

       +-------------+---------------------------------+-----------+
       | Keywords    | Description                     | Reference |
       +-------------+---------------------------------+-----------+
       | UTF8SMTPbis | Internationalized email address | [RFCXXXX] |
       +-------------+---------------------------------+-----------+

   This document updates the values to the SMTP Enhanced Status Code
   subregistry of the Mail Parameters registry, following the guidance
   in Sections 3.5 and 3.7.4.2 of this document, and being based on
   [RFC5248].  The registration data is as follows:

    Code:       X.6.7
    Sample Text:    non-ASCII addresses not permitted
          for that sender/recipient
    Associated basic status code: 550, 553
    Description:    This indicates the reception of a MAIL or RCPT
              command that non-ASCII addresses are not permitted
    Defined:      RFC XXXX (Standard track)
    Submitter:     Jiankang YAO
    Change controller: ima@ietf.org

    Code:       X.6.8
    Sample Text:    UTF-8 string reply is required,
              but not permitted by the SMTP client
    Associated basic status code: 252, 550, 553
    Description:    This indicates that a reply containing a UTF-8
              string is required to show the mailbox name,
    but that form of response is not
    permitted by the SMTP client.
    Defined:      RFC XXXX (Standard track)
    Submitter:     Jiankang YAO
    Change controller: ima@ietf.org

    Code:       X.6.9
    Sample Text:    UTF-8 header message can not be transferred
              to one or more recipient so the message
    must be rejected
    Associated basic status code: 550
    Description:    This indicates that transaction failed
          after the final "." of the DATA command.
    Defined:      RFC XXXX (Standard track)
    Submitter:     Jiankang YAO
    Change controller: ima@ietf.org

    Code:       X.6.10
    Description:    This is a duplicate of X.6.8 and
          is thus deprecated.

   The following entries SHOULD be updated or added in the "Mail
   Transmission Types" registry under the Mail Parameters registry.

   +--------------+-------------------------------+--------------------+
   | WITH         | Description                   | Reference          |
   | protocol     |                               |                    |
   | types        |                               |                    |
   +--------------+-------------------------------+--------------------+
   | UTF8SMTP     | ESMTP with UTF8SMTP           | [RFCXXXX]          |
   | UTF8SMTPA    | ESMTP with UTF8SMTP and SMTP  | [RFC4954]          |
   |              | AUTH                          | [RFCXXXX]          |
   | UTF8SMTPS    | ESMTP with UTF8SMTP and       | [RFC3207]          |
   |              | STARTTLS                      | [RFCXXXX]          |
   | UTF8SMTPSA   | ESMTP with UTF8SMTP and both  | [RFC3207]          |
   |              | STARTTLS and SMTP AUTH        | [RFC4954]          |
   |              |                               | [RFCXXXX]          |
   | UTF8LMTP     | LMTP with UTF8SMTP            | [RFCXXXX]          |
   | UTF8LMTPA    | LMTP with UTF8SMTP and SMTP   | [RFC4954]          |
   |              | AUTH                          | [RFCXXXX]          |
   | UTF8LMTPS    | LMTP with UTF8SMTP and        | [RFC3207]          |
   |              | STARTTLS                      | [RFCXXXX]          |
   | UTF8LMTPSA   | LMTP with UTF8SMTP and both   | [RFC3207]          |
   |              | STARTTLS and LMTP AUTH        | [RFC4954]          |
   |              |                               | [RFCXXXX]          |
   +--------------+-------------------------------+--------------------+

5.  Security Considerations

   The extended security considerations discussion in the framework
   document [RFC4952bis] will be applied here.

   More security considerations are discussed below:

   Beyond the use inside the email global system (in SMTP envelopes and
   message headers), internationalized email addresses will also show up
   inside other cases, in particular:

   o  the logging systems of SMTP transactions and other logs to monitor
      the email systems;
   o  the trouble ticket systems used by Security Teams to manage
      security incidents, when an email address is involved;

   In order to avoid problems that could cause loss of data, this will
   likely require extending these systems to support full UTF-8, or to
   require to provide an adequate mechanisms for mapping non-ASCII
   strings to ASCII.

   Another security aspect to be considered is related to the ability by
   security team members to quickly understand, read and identify email
   addresses from the logs, when they are tracking an incident.

   Mechanims to automatically and quickly provide the origin or
   ownership of an internationalized email address SHALL be implemented
   for use also by log readers which cannot read easily non-ASCII
   information.

   The SMTP commands VRFY and EXPN are sometimes used in SMTP
   transactions where there is no message to transfer (by tools used to
   take automated actions in case potential spam messages are
   identified).  RFC 5321 section 3.5 and 7.3 give some detailed
   description of use and possible behaviours.  Implementation of
   internationalized addrsses can affect also logs and actions by these
   tools.

6.  Acknowledgements

   This document revised the [RFC5336]document based on the EAI WG's
   discussion result.  Many EAI WG members did some tests and
   implementations to move this document to the Standard Track document.
   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 specification.  Additional
   important comments and suggestions, and often specific text, were
   contributed by many members of the WG and design team.  Those
   contributions include material from 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, Alexey Melnikov, Pete Resnick, S. Moonesamy, Soobok
   Lee, Shawn Steele, Alfred Hoenes, Miguel Garcia, Magnus Westerlund,
   and Lars Eggert.  Of course, none of the individuals are necessarily
   responsible for the combination of ideas represented here.

   Thanks a lot to Dave Crocker for his comments and helping of ABNF
   refinement.

7.  Change History

   [[anchor14: RFC Editor: Please remove this section.]]

7.1.  draft-yao-eai-rfc5336bis: Version 00

   Applied errata suggested by Alfred Hoenes.

7.2.  draft-ietf-eai-rfc5336bis: Version 00

   Applied the changes suggested by the EAI new charter.

7.3.  draft-ietf-eai-rfc5336bis: Version 01

   Applied the changes suggested by 78 IETF EAI meeting.

7.4.  draft-ietf-eai-rfc5336bis: Version 02

   remove the appendix since rfc4952bis has added this material

   improve the text

   remove the text about no body parameter

7.5.  draft-ietf-eai-rfc5336bis: Version 03

   improve the text

7.6.  draft-ietf-eai-rfc5336bis: Version 04

   update the abstract

   improve the text

7.7.  draft-ietf-eai-rfc5336bis: Version 05

   improve the text based on AD and Co-chairs

7.8.  draft-ietf-eai-rfc5336bis: Version 06

   update the iana consideration

7.9.  draft-ietf-eai-rfc5336bis: Version 07

   improve the iana consideration

7.10.  draft-ietf-eai-rfc5336bis: Version 08

   improve the texts

   add the mail parameter

   add the new section about mail command parameter usage

   update the security consideration

7.11.  draft-ietf-eai-rfc5336bis: Version 09

   improve the texts

7.12.  draft-ietf-eai-rfc5336bis: Version 10

   refine the ABNF definitions

   improve the texts

8.  References

8.1.  Normative References

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

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

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

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

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

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

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

   [RFC4952bis]
              Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", I-D rfc4952bis, September 2010.

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

   [RFC5248]  Hansen  , T. and J. Klensin, "A Registry for SMTP Enhanced
              Mail System Status Codes", RFC 5248, June 2008.

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

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

   [RFC5335bis]
              Abel, Y. and S. Steel, "Internationalized Email Headers",
              I-D rfc5335bis, March 2011.

   [RFC5337bis]
              Hansen, T., Ed., Newman, C., and A. Melnikov, Ed.,
              "Internationalized Delivery Status and Disposition
              Notifications", I-D 5337bis, October 2010.

   [RFC5890]  Klensin, J., "Internationalizing Domain Names in
              Applications (IDNA definitions)", RFC 5890, June 2010.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.

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

8.2.  Informative References

   [EAI Deployment]
              Yao, J., Lee, X., and S. Steel, "Advice for EAI
              deployment", draft eai-deployment, December 2010.

   [EAI addresses]
              Steel, S., Yao, J., and Mark. Davis, "Advice for non-ASCII
              & ASCII addresses", draft eai-address-advice,
              December 2010.

   [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.

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

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.

   [RFC4954]  Siemborski, R. and A. Melnikov, "SMTP Service Extension
              for Authentication", RFC 4954, July 2007.

   [RFC5336]  Yao, J. and W. Mao, "SMTP Extension for Internationalized
              Email Addresses", RFC 5336, September 2008.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
              July 2009.

Authors' Addresses

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

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

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

   Phone: +86 10 58812230
   Email: maowei_ietf@cnnic.cn