Network Working Group                                             J. Yao
Internet-Draft                                                    W. Mao
Obsoletes: 5336 (if approved)                                      CNNIC
Intended status: Standards Track                            July 7,                         August 11, 2011
Expires: January 8, February 12, 2012

               SMTP Extension for Internationalized Email
                    draft-ietf-eai-rfc5336bis-11.txt
                    draft-ietf-eai-rfc5336bis-12.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 January 8, February 12, 2012.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Role of This Specification . . . . . . . . . . . . . . . .  4
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.
     1.2.  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 . . . . . . . . . . . . . . .  9  8
     3.5.  Non-ASCII addresses and Reply-codes  . . . . . . . . . . .  9
     3.6.  Body Parts and SMTP Extensions . . . . . . . . . . . . . . 10
     3.7.  Additional ESMTP Changes and Clarifications  . . . . . . . 10
       3.7.1.  The Initial SMTP Exchange  . . . . . . . . . . . . . . 10
       3.7.2.  Mail eXchangers  . . . . . . . . . . . . . . . . . . . 11
       3.7.3.  Trace Information  . . . . . . . . . . . . . . . . . . 11
       3.7.4.  UTF-8 Strings in Replies . . . . . . . . . . . . . . . 11
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   7.  Change History . . . . . . . . . . . . . . . . . . . . . . . . 16
     7.1.  draft-yao-eai-rfc5336bis: Version 00 . . . . . . . . . . . 16
     7.2.  draft-ietf-eai-rfc5336bis: Version 00  . . . . . . . . . . 17
     7.3.  draft-ietf-eai-rfc5336bis: Version 01  . . . . . . . . . . 17
     7.4.  draft-ietf-eai-rfc5336bis: Version 02  . . . . . . . . . . 17
     7.5.  draft-ietf-eai-rfc5336bis: Version 03  . . . . . . . . . . 17
     7.6.  draft-ietf-eai-rfc5336bis: Version 04  . . . . . . . . . . 17
     7.7.  draft-ietf-eai-rfc5336bis: Version 05  . . . . . . . . . . 17
     7.8.  draft-ietf-eai-rfc5336bis: Version 06  . . . . . . . . . . 17
     7.9.  draft-ietf-eai-rfc5336bis: Version 07  . . . . . . . . . . 17
     7.10. draft-ietf-eai-rfc5336bis: Version 08  . . . . . . . . . . 17
     7.11. draft-ietf-eai-rfc5336bis: Version 09  . . . . . . . . . . 18
     7.12. draft-ietf-eai-rfc5336bis: Version 10  . . . . . . . . . . 18
     7.13. draft-ietf-eai-rfc5336bis: Version 11  . . . . . . . . . . 18
     7.14. draft-ietf-eai-rfc5336bis: Version 12  . . . . . . . . . . 18
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 18
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 19 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20

1.  Introduction

   The document defines a Simple Mail Transfer Protocol [RFC5321] provides a negotiation
   mechanism about service
   extension by which SMTP clients so servers can discover
   SMTP server capabilities and make decisions for further processing.
   This document uses this mechanism and specifies an SMTP extension advertise the ability to
   permit accept and process
   internationalized email addresses (see section 1.2) in the
   SMTP envelope, 1.1), and Unicode characters encoded in UTF-8 [RFC3629] in
   the headers.
   internationalized email headers [RFC5335bis].

   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.  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.

   [[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].  Some
   basic rules in this document can be found from [RFC5234] or [RFC5321]
   under the same names.

1.3.

1.2.  Updates to Other Specifications

   This specification extends some syntax rules defined in RFC 5321 and
   permits internationalized email address addresses in the envelope, but it
   does not modify RFC 5321.  It permits data formats defined in
   [RFC5335bis], but it does not modify RFC 5322.  It does require that
   the 8BITMIME extension [RFC6152] be announced by the UTF8SMTPbis-
   aware SMTP server and used with "BODY=8BMITMIME" by the UTF8SMTPbis-
   aware SMTP client, but it does not modify the 8BITMIME specification
   in any way.

2.  Overview of Operation

   This specification describes document specifies an optional extension to element of the email
   transport mechanism that permits non-ASCII characters in both internationalization
   work, specifically the
   envelope and header fields definition of messages, which are encoded in UTF-8. an SMTP extension for
   internationalized email.  The extension is identified with the token
   "UTF8SMTPbis".

   The internationalized email headers 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 "Internationalized
        Email".
   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 UTF8SMTPbis-aware SMTP client MUST ignore any parameters if
        they appear for this keyword; that is, the UTF8SMTPbis-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
        UTF8SMTPbis-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 13
        characters by the possible addition of the UTF8SMTPbis
        parameter. [[anchor6: [[anchor5: RFC Editor: the number '13' will be
        replaced by the new number (2 spaces + 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 name to be looked up in the DNS MUST allow for [RFC5890]
   behavior.  When doing lookups, the UTF8SMTPbis-aware SMTP client or
   server MUST either use a Unicode aware DNS library, or transform it the
   internationalized domain name to A-label defined as described 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 UTF8SMTPbis-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 a
   UTF8SMTPbis-aware SMTP client (UTF8SMTPbis-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
       internationalized delivery status and disposition notifications
       specification [RFC5337bis].
   2.  If and only if the UTF8SMTPbis-aware SMTP client (sender) is a
       Message Submission Agent ("MSA") [RFC4409] [RFC5598], it may
       choose its own way to deal with this scenario according to the
       provisions of [RFC4409] or its future versions.  But the detailed
       specification of this process and its results are 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 UTF8SMTPbis-aware
       SMTP client.

   This document applies when an a UTF8SMTPbis-aware SMTP client or server
   supports the UTF8SMTPbis extension.  For all other cases, and for
   addresses and messages that do not require an a UTF8SMTPbis extension,
   UTF8SMTPbis-aware SMTP clients and servers do not change the behavior
   specified in [RFC5321].

   If a UTF8SMTPbis-aware SMTP server advertises the Delivery Status
   Notification (DSN) [RFC3461] extension, it MUST implement
   [RFC5337bis].

3.3.  Extended Mailbox Address Syntax

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

   The key changes made by this specification include:
   o  In order to update the <Mailbox> to support the internationalized
      email address, the <Mailbox> ABNF rule will be imported from RFC
      5321 directly, and other related rules are imported from RFC 5321,
      RFC 5234, RFC 5890 or RFC 5335bis, or are extended in this
      document.

   o  Extend the definition of <sub-domain> to permit both the RFC 5321
      definition and a UTF-8 string in a DNS label that is conforming
      with IDNA definitions [RFC5890].
   o  Extend the definition of <atext> to permit both the RFC 5321
      definition and a UTF-8 string.  That string MUST NOT contain any
      of the ASCII graphics or controls characters.

   The following ABNF rules will be imported from RFC 5321, section
   4.1.2 directly:
   o  <Mailbox>
   o  <Local-part>
   o  <Dot-string>
   o  <Quoted-string>
   o  <QcontentSMTP>
   o  <Domain>
   o  <Atom>

   The following ABNF rule will be imported from RFC 5335bis, section
   4.1 directly:
   o  <UTF8-non-ascii>

   The following ABNF rule will be imported from RFC 5234, appendix B.1
   directly:
   o  <DQUOTE>

   The following ABNF rule will be imported 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
    ; extend the definition of sub-domain in RFC5321, section 4.1.2

   atext   =/  UTF8-non-ascii
    ; extend the definition of atext in RFC5321, section 4.1.2

   quoted-pairSMTP  =/ %d92 UTF8-non-ascii
    ; extend the definition of quoted-pairSMTP in RFC5321, section 4.1.2

   qtextSMTP  =/ UTF8-non-ascii
    ; extend the definition of qtextSMTP in RFC5321, section 4.1.2

3.4.  MAIL Command Parameter Usage

   If the envelope or message being sent requires the capabilities of
   the UTF8SMTPbis extension, the UTF8SMTPbis-aware SMTP client MUST
   supply the UTF8SMTPbis parameter with the MAIL command.  If this
   parameter is provided, it MUST have no value.  If the UTF8SMTPbis-
   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 UTF8SMTPbis-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

   A UTF8SMTPbis-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 UTF8SMTPbis-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 UTF8SMTPbis-aware
   SMTP server supports enhanced mail system status codes [RFC3463],
   reply-code "X.6.7" [RFC5248] is (see section 4)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 a message is issued rejected after the final "." of the DATA command, command
   because one or more recipient is unable to accept and process a
   message with internationalized email headers, the reply-code "554" is
   used with the meaning "Transaction failed".
   When  If the UTF8SMTPbis-aware
   SMTP server supports enhanced mail system status codes [RFC3463], reply-code
   reply code "X.6.9" [RFC5248] (see section 4) is used, used to indicate this
   condition, meaning that "UTF-8 header message can not be transmitted
   to one or more recipients, so the message MUST be rejected".

   The UTF8SMTPbis-aware SMTP servers are encouraged to detect that
   recipients can not accept internationalized messages and generate an
   error after the RCPT command rather than waiting until after the DATA
   command to issue an error.

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 UTF8SMTPbis-
   aware  A UTF8SMTPbis-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
   UTF8SMTPbis-aware UTF8SMTPbis-
   aware SMTP client or server inspects the message.

   While this specification requires that UTF8SMTPbis-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 string.  When RFC 5321 specifies a domain name, the
   internationalized domain name SHOULD be in the form of A-label U-label if this domain name the
   UTF8SMTPbis extension is an internationalized
   domain name[RFC5890]. supported; otherwise, it SHOULD be in the
   form of A-label.

   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 UTF8SMTPbis-aware SMTP client
   MUST send only ASCII (LDH label or A-label [RFC5890] ) domains in the
   EHLO command and that, if the UTF8SMTPbis-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.

3.7.3.  Trace Information

   The trace information <Return-path-line>, <Time-stamp-line> and their
   related rules are defined in in section 4.4 of RFC 5321 [RFC5321].
   This document updates <Mailbox> and <Domain> to support non-ASCII
   characters.  When the UTF8SMTPbis extension is used, the 'Reverse-
   path' clause of the Return-path-line may include an internationalized
   domain name that uses the U-label form; The 'Stamp' clause ot the
   Time-stamp-line may include an internationalized domain name that
   uses the U-label form.

   If the messages that include trace fields are sent by an UTF8SMTPbis-
   aware SMTP client or relay server without the UTF8SMTPbis parameter
   at MAIL commands, trace field values must conform to RFC 5321
   regardless of the SMTP server's capability.

   If the messages that include trace fields are sent by an UTF8SMTPbis-
   aware

   When a UTF8SMTPbis-aware SMTP client or relay server adds a trace field to a message
   that was or will be transmitted with the UTF8SMTPbis parameter at
   MAIL commands, trace field values that server SHOULD use the U-label form for the
   internationalized domain name. names in that new trace field.

   The protocol value of the '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 UTF8SMTPbis-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 UTF8SMTPbis-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 UTF8SMTPbis-
   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
   UTF8SMTPbis-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 String
     [ SP "UTF8SMTPbis" ] CRLF
    ; String may include Non-ASCII characters

   expn = "EXPN" SP String
     [ SP "UTF8SMTPbis" ] CRLF
    ; String may include Non-ASCII characters

   The "UTF8SMTPbis" parameter does not use have a value.  If the reply to a
   VERIFY (VRFY) or EXPAND (EXPN) command requires a UTF-8 string, but
   the SMTP client did not use the "UTF8SMTPbis" parameter, then the
   UTF8SMTPbis-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 UTF8SMTPbis-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 <Mailbox>
    ; Mailbox is defined in section 3.3 of this document.
    ; User Name can contain non-ASCII characters.

   Mailbox
    ; Mailbox is defined in 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 UTF8SMTPbis-aware SMTP server supports
   enhanced mail system status codes [RFC3463], the enhanced reply-code
   is "X.6.8" [RFC5248], [RFC5248] (see section 4), 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. mishandle that
   reply.  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.
   UTF8SMTPbis-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 apply 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

   Mechanisms 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 addresses can affect also logs and actions by these
   tools.

6.  Acknowledgements

   This document revised the [RFC5336]document based on the Email
   Address Internationalization (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:

   [[anchor12: 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

7.13.  draft-ietf-eai-rfc5336bis: Version 11

   remove the update of RFC5321 and RFC5322

   change the title from "SMTP Extension for Internationalized Email
   Address" to "SMTP Extension for Internationalized Email" based on
   Ernie's comment

   the trace field of section 3.7.3 is updated to reflect the WG's
   conclusion

   improve the texts

7.14.  draft-ietf-eai-rfc5336bis: Version 12

   Update according to Chris Newman's comments

   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.

   [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.

   [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.

   [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

   [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