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rfc:rfc6531

Internet Engineering Task Force (IETF) J. Yao Request for Comments: 6531 W. Mao Obsoletes: 5336 CNNIC Category: Standards Track February 2012 ISSN: 2070-1721

             SMTP Extension for Internationalized Email

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 is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6531.

Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow

Yao & Mao Standards Track [Page 1] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   1.2.  Changes Made to Other Specifications . . . . . . . . . . .  3
 2.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  4
 3.  Mail Transport-Level Protocol  . . . . . . . . . . . . . . . .  4
   3.1.  Framework for the Internationalization Extension . . . . .  4
   3.2.  The SMTPUTF8 Extension . . . . . . . . . . . . . . . . . .  5
   3.3.  Extended Mailbox Address Syntax  . . . . . . . . . . . . .  7
   3.4.  MAIL Command Parameter Usage . . . . . . . . . . . . . . .  8
   3.5.  Non-ASCII Addresses and Reply-Codes  . . . . . . . . . . .  9
   3.6.  Body Parts and SMTP Extensions . . . . . . . . . . . . . .  9
   3.7.  Additional ESMTP Changes and Clarifications  . . . . . . . 10
     3.7.1.  The Initial SMTP Exchange  . . . . . . . . . . . . . . 10
     3.7.2.  Mail eXchangers  . . . . . . . . . . . . . . . . . . . 10
     3.7.3.  Trace Information  . . . . . . . . . . . . . . . . . . 11
     3.7.4.  UTF-8 Strings in Replies . . . . . . . . . . . . . . . 11
 4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   4.1.  SMTP Service Extensions Registry . . . . . . . . . . . . . 13
   4.2.  SMTP Enhanced Status Code Registry . . . . . . . . . . . . 13
   4.3.  WITH Protocol Types Sub-Registry of the Mail
         Transmission Types Registry  . . . . . . . . . . . . . . . 15
 5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
 6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
 7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   7.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
   7.2.  Informative References . . . . . . . . . . . . . . . . . . 18

Yao & Mao Standards Track [Page 2] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

1. Introduction

 The document defines a Simple Mail Transfer Protocol [RFC5321]
 extension so servers can advertise the ability to accept and process
 internationalized email addresses (see Section 1.1) and
 internationalized email headers [RFC6532].
 An extended overview of the extension model for internationalized
 email addresses and the email header appears in RFC 6530 [RFC6530],
 referred to as "the framework document" 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.

1.1. 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, which may or may not be members of the ASCII
 subset, in UTF-8 [RFC3629], a standard Unicode Encoding Form.  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", "SMTPUTF8", "internationalized
 message", and "message" are used in this document according to the
 definitions in the framework document [RFC6530].
 Strings referred to in this document, including ASCII strings, MUST
 be expressed in UTF-8.
 This specification uses Augmented BNF (ABNF) rules [RFC5234].  Some
 basic rules in this document are identified in Section 3.3 as being
 defined (under the same names) in RFC 5234 [RFC5234], RFC 5321
 [RFC5321], RFC 5890 [RFC5890], or RFC 6532 [RFC6532].

1.2. Changes Made to Other Specifications

 This specification extends some syntax rules defined in RFC 5321 and
 permits internationalized email addresses in the envelope and in
 trace fields, but it does not modify RFC 5321.  It permits data
 formats defined in RFC 6532 [RFC6532], but it does not modify RFC
 5322.  It does require that the 8BITMIME extension [RFC6152] be
 announced by the SMTPUTF8-aware SMTP server and used with
 "BODY=8BITMIME" by the SMTPUTF8-aware SMTP client, but it does not
 modify the 8BITMIME specification in any way.

Yao & Mao Standards Track [Page 3] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

 This specification replaces an earlier, experimental, approach to the
 same problem [RFC5336].  Section 6 of RFC 6530 [RFC6530] describes
 the changes in approach between RFC 5336 [RFC5336] and this
 specification.  Anyone trying to convert an implementation from the
 experimental specification to the specification in this document will
 need to review those changes carefully.

2. Overview of Operation

 This document specifies an element of the email internationalization
 work, specifically the definition of an SMTP extension for
 internationalized email.  The extension is identified with the token
 "SMTPUTF8".
 The internationalized email headers specification [RFC6532] 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
      "SMTPUTF8".
 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 SMTPUTF8-aware SMTP client MUST ignore any parameters if
      they appear for this keyword; that is, the SMTPUTF8-aware SMTP
      client MUST behave as if the parameters do not appear.  If an
      SMTP server includes SMTPUTF8 in its EHLO response, it MUST be
      fully compliant with this version of this specification.
 4.   One OPTIONAL parameter, SMTPUTF8, is added to the MAIL command.
      The parameter does not accept a value.  If this parameter is set
      in the MAIL command, it indicates that the SMTP client is
      SMTPUTF8-aware.  Its presence also asserts that the envelope
      includes the non-ASCII address, the message being sent is an
      internationalized message, or the message being sent needs the
      SMTPUTF8 support.

Yao & Mao Standards Track [Page 4] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

 5.   The maximum length of a MAIL command line is increased by 10
      characters to accommodate the possible addition of the SMTPUTF8
      parameter.
 6.   One OPTIONAL parameter, SMTPUTF8, is added to the VERIFY (VRFY)
      and EXPAND (EXPN) commands.  The SMTPUTF8 parameter does not
      accept a 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 RFC 6532
      [RFC6532].
 11.  The SMTPUTF8 extension is valid on the submission port
      [RFC6409].  It may also be used with the Local Mail Transfer
      Protocol (LMTP) [RFC2033].  When these protocols are used, their
      use should be reflected in the trace field WITH keywords as
      appropriate [RFC3848].

3.2. The SMTPUTF8 Extension

 An SMTP server that announces the SMTPUTF8 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, the
 actual parsing of the <local-part> and the delimiters used are
 unchanged from the base email specification [RFC5321].  Any domain
 name to be looked up in the DNS MUST conform to and be processed as
 specified for Internationalizing Domain Names in Applications (IDNA)
 [RFC5890].  When doing lookups, the SMTPUTF8-aware SMTP client or
 server MUST either use a Unicode-aware DNS library, or transform the
 internationalized domain name to A-label form (i.e., a fully-
 qualified domain name that contains one or more A-labels but no
 U-labels) as specified in RFC 5890 [RFC5890].
 An SMTP client that receives the SMTPUTF8 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 [RFC6532] (which may also include mailbox names in

Yao & Mao Standards Track [Page 5] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

 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 SMTPUTF8 extension does not change
 the server-relaying behaviors described in RFC 5321.
 If the SMTPUTF8 SMTP extension is not offered by the SMTP server, the
 SMTPUTF8-aware SMTP client MUST NOT transmit an internationalized
 email address and MUST NOT transmit a mail message containing
 internationalized mail headers as described in RFC 6532 [RFC6532] at
 any level within its MIME structure [RFC2045].  (For this paragraph,
 the internationalized domain name in A-label form as specified in
 IDNA definitions [RFC5890] is not considered to be
 "internationalized".)  Instead, if an SMTPUTF8-aware SMTP client
 (sender) attempts to transfer an internationalized message and
 encounters an SMTP server that does not support the extension, the
 best action for it to take depends on other conditions.  In
 particular:
 o  If it is a Message Submission Agent (MSA) [RFC6409] [RFC5598], it
    MAY choose its own way to deal with this scenario using the wide
    discretion for changing addresses or otherwise fixing up and
    transforming messages allowed by RFC 6409.  As long as the
    resulting message conforms to the requirements of RFC 5321 (i.e.,
    without the SMTPUTF8 extension), the details of that
    transformation are outside the scope of this document.
 o  If it is not an MSA or is an MSA and does not choose to transform
    the message to one that does not require the SMTPUTF8 extension,
    it SHOULD reject the message.  As usual, this can be done either
    by generating an appropriate reply during the SMTP transaction or
    by accepting the message and then generating and transmitting a
    non-delivery notification.  If the latter choice is made, the
    notification process MUST conform to the requirements of RFC 5321,
    RFC 3464 [RFC3464], and RFC 6533 [RFC6533].
 o  As specified in Section 2.2.3 of RFC 5321, an SMTP client with
    additional information and/or knowledge of special circumstances
    MAY choose to requeue the message and try later and/or try an
    alternate MX host as specified in that section.
 This document applies when an SMTPUTF8-aware SMTP client or server
 supports the SMTPUTF8 extension.  For all other cases, and for
 addresses and messages that do not require an SMTPUTF8 extension,
 SMTPUTF8-aware SMTP clients and servers do not change the behavior
 specified in RFC 5321 [RFC5321].

Yao & Mao Standards Track [Page 6] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

 If an SMTPUTF8-aware SMTP server advertises the Delivery Status
 Notification (DSN) [RFC3461] extension, it MUST implement RFC 6533
 [RFC6533].

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  The <Mailbox> ABNF rule is imported from RFC 5321 and updated in
    order to support the internationalized email address.  Other
    related rules are imported from RFC 5321, RFC 5234, RFC 5890, and
    RFC 6532, or are extended in this document.
 o  The definition of <sub-domain> is extended to permit both the RFC
    5321 definition and a UTF-8 string in a DNS label that conforms
    with IDNA definitions [RFC5890].
 o  The definition of <atext> is extended to permit both the RFC 5321
    definition and a UTF-8 string.  That string MUST NOT contain any
    of the ASCII graphics or control characters.
 The following ABNF rules imported from RFC 5321, Section 4.1.2, are
 updated directly or indirectly by this document:
 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 6532, Section 3.1,
 directly:
 o  <UTF8-non-ascii>

Yao & Mao Standards Track [Page 7] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

 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 RFC 5321, Section 4.1.2
 atext   =/  UTF8-non-ascii
  ; extend the implicit definition of atext in
  ; RFC 5321, Section 4.1.2, which ultimately points to
  ; the actual definition in RFC 5322, Section 3.2.3
 qtextSMTP  =/ UTF8-non-ascii
  ; extend the definition of qtextSMTP in RFC 5321, Section 4.1.2
 esmtp-value  =/ UTF8-non-ascii
  ; extend the definition of esmtp-value in RFC 5321, Section 4.1.2

3.4. MAIL Command Parameter Usage

 If the envelope or message being sent requires the capabilities of
 the SMTPUTF8 extension, the SMTPUTF8-aware SMTP client MUST supply
 the SMTPUTF8 parameter with the MAIL command.  If this parameter is
 provided, it MUST not accept a value.  If the SMTPUTF8-aware SMTP
 client is aware that neither the envelope nor the message being sent
 requires any of the SMTPUTF8 extension capabilities, it SHOULD NOT
 supply the SMTPUTF8 parameter with the MAIL command.
 Because there is no guarantee that a next-hop SMTP server will
 support the SMTPUTF8 extension, use of the SMTPUTF8 extension always
 carries a risk of transmission failure.  In fact, during the early
 stages of deployment for the SMTPUTF8 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.

Yao & Mao Standards Track [Page 8] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

3.5. Non-ASCII Addresses and Reply-Codes

 An SMTPUTF8-aware SMTP client MUST NOT send an internationalized
 message to an SMTP server that does not support SMTPUTF8.  If the
 SMTP server does not support this option, then the SMTPUTF8-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 SMTPUTF8-aware SMTP
 server supports enhanced mail system status codes [RFC3463], reply-
 code "X.6.7" [RFC5248] (see Section 4) is used, meaning "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 specification in RFC 5321; this extension
 does not change those circumstances or reply messages.
 If a message is rejected after the final "." of the DATA command
 because one or more recipients are unable to accept and process a
 message with internationalized email headers, the reply-code "554" is
 used with the meaning "Transaction failed".  If the SMTPUTF8-aware
 SMTP server supports enhanced mail system status codes [RFC3463],
 reply code "X.6.9" [RFC5248] (see Section 4) is used to indicate this
 condition, meaning "UTF-8 header message cannot be transmitted to one
 or more recipients, so the message must be rejected".
 The SMTPUTF8-aware SMTP servers are encouraged to detect that
 recipients cannot 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 SMTPUTF8 asserts that a message is an
 internationalized message or the message being sent needs the
 SMTPUTF8 support.  There is still a chance that a message being sent
 via the MAIL command with the SMTPUTF8 parameter is not an
 internationalized message.  An SMTPUTF8-aware SMTP client or server
 that requires accurate knowledge of whether a message is
 internationalized needs to parse all message header fields and MIME

Yao & Mao Standards Track [Page 9] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

 header fields [RFC2045] in the message body.  However, this
 specification does not require that the SMTPUTF8-aware SMTP client or
 server inspects the message.
 Although this specification requires that SMTPUTF8-aware SMTP servers
 support the 8BITMIME extension [RFC6152] to ensure that servers have
 adequate handling capability for 8-bit data, it does not require non-
 ASCII body parts in the MIME message as specified in RFC 2045.  The
 SMTPUTF8 extension MAY be used as follows (assuming it is appropriate
 given the body content):
  1. with the BODY=8BITMIME parameter [RFC6152], or
  1. with the BODY=BINARYMIME parameter, if the SMTP server advertises

BINARYMIME [RFC3030].

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
 internationalized domain name SHOULD be in U-label form if the
 SMTPUTF8 extension is supported; otherwise, it SHOULD be in A-label
 form.
 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 SMTPUTF8 until after it
 receives the response to the EHLO, the SMTPUTF8-aware SMTP client
 MUST send only ASCII (LDH label or A-label [RFC5890]) domains in the
 EHLO command.  If the SMTPUTF8-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 (as described in Section 5 of RFC 5321 [RFC5321]), it is
 strongly advised that all or none of them SHOULD support the SMTPUTF8

Yao & Mao Standards Track [Page 10] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

 extension.  Otherwise, unexpected 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 Section 4.4 of RFC 5321 [RFC5321].
 This document updates <Mailbox> and <Domain> to support non-ASCII
 characters.  When the SMTPUTF8 extension is used, the 'Reverse-path'
 clause of the Return-path-line may include an internationalized
 domain name that uses the U-label form.  Also, the 'Stamp' clause of
 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 SMTPUTF8-
 aware SMTP client or relay server without the SMTPUTF8 parameter
 included in the MAIL commands, trace field values must conform to RFC
 5321 regardless of the SMTP server's capability.
 When an SMTPUTF8-aware SMTP server adds a trace field to a message
 that was or will be transmitted with the SMTPUTF8 parameter included
 in the MAIL commands, that server SHOULD use the U-label form for
 internationalized domain names in the new trace field.
 The protocol value of the 'WITH' clause when this extension is used
 is one of the SMTPUTF8 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 SMTPUTF8 parameter, the SMTPUTF8-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 SMTPUTF8 parameter, the SMTPUTF8-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 SMTPUTF8 Parameter

 If the SMTPUTF8 parameter is transmitted with the VRFY and EXPN
 commands, it indicates that the SMTP client can accept UTF-8 strings
 in replies to those commands.  The parameter with the VRFY and EXPN

Yao & Mao Standards Track [Page 11] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

 commands SHOULD only be used after the SMTP client sees the EHLO
 response with the SMTPUTF8 keyword.  This allows an SMTPUTF8-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 to the VRFY and EXPN
 commands that contain UTF-8 strings.  However, an SMTPUTF8-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
 with the VRFY and EXPN commands.
 Most replies do not require that a mailbox name be included in the
 returned text, and therefore a 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 "SMTPUTF8" ] CRLF
  ; String may include Non-ASCII characters
 expn = "EXPN" SP String
   [ SP "SMTPUTF8" ] CRLF
  ; String may include Non-ASCII characters
 The SMTPUTF8 parameter does not accept a value.  If the reply to a
 VRFY or EXPN command requires a UTF-8 string, but the SMTP client did
 not use the SMTPUTF8 parameter, then the SMTPUTF8-aware SMTP server
 MUST use either the reply-code 252 or 550.  Reply-code 252, defined
 in RFC 5321 [RFC5321], means "Cannot VRFY user, but will accept the
 message and attempt the delivery".  Reply-code 550, also defined in
 RFC 5321 [RFC5321], means "Requested action not taken: mailbox
 unavailable".  When the SMTPUTF8-aware SMTP server supports enhanced
 mail system status codes [RFC3463], the enhanced reply-code as
 specified below is used.  Using the SMTPUTF8 parameter with a VRFY or
 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.

Yao & Mao Standards Track [Page 12] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

 If the SMTP reply requires UTF-8 strings, but a UTF-8 string is not
 allowed in the reply, and the SMTPUTF8-aware SMTP server supports
 enhanced mail system status codes [RFC3463], the enhanced reply-code
 is "X.6.8" [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 SMTPUTF8 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 mishandle that
 reply.  Internationalized messages in replies are only allowed in the
 commands under the situations described above.
 Although UTF-8 strings are needed to represent email addresses in
 responses under the rules specified in this section, this extension
 does not permit the use of UTF-8 strings for any other purposes.
 SMTPUTF8-aware SMTP servers MUST NOT include non-ASCII characters in
 replies except in the limited cases specifically permitted in this
 section.

4. IANA Considerations

4.1. SMTP Service Extensions Registry

 IANA has added a new value "SMTPUTF8" to the "SMTP Service Extension"
 registry of the "Mail Parameters" registry, according to the
 following data:
      +----------+---------------------------------+-----------+
      | Keywords | Description                     | Reference |
      +----------+---------------------------------+-----------+
      | SMTPUTF8 | Internationalized email address | [RFC6531] |
      +----------+---------------------------------+-----------+

4.2. SMTP Enhanced Status Code Registry

 The code definitions in this document replace those specified in RFC
 5336, following the guidance in Sections 3.5 and 3.7.4.2 of this
 document, and based on RFC 5248 [RFC5248].  IANA has updated the
 "Simple Mail Transfer Protocol (SMTP) Enhanced Status Code Registry"
 with the following data:

Yao & Mao Standards Track [Page 13] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

  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 6531 (Standards 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 6531 (Standards Track)
  Submitter:   Jiankang YAO
  Change controller: ima@ietf.org
  Code:        X.6.9
  Sample Text: UTF-8 header message cannot be transferred to one or
               more recipients, 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 6531 (Standards 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.

Yao & Mao Standards Track [Page 14] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

4.3. WITH Protocol Types Sub-Registry of the Mail Transmission Types

    Registry
 IANA has modified or added the following entries in the "WITH
 protocol types" sub-registry under the "Mail Transmission Types"
 registry.
 +--------------+------------------------------+---------------------+
 | WITH         | Description                  | Reference           |
 | protocol     |                              |                     |
 | types        |                              |                     |
 +--------------+------------------------------+---------------------+
 | UTF8SMTP     | ESMTP with SMTPUTF8          | [RFC6531]           |
 | UTF8SMTPA    | ESMTP with SMTPUTF8 and AUTH | [RFC4954] [RFC6531] |
 | UTF8SMTPS    | ESMTP with SMTPUTF8 and      | [RFC3207] [RFC6531] |
 |              | STARTTLS                     |                     |
 | UTF8SMTPSA   | ESMTP with SMTPUTF8 and both | [RFC3207] [RFC4954] |
 |              | STARTTLS and AUTH            | [RFC6531]           |
 | UTF8LMTP     | LMTP with SMTPUTF8           | [RFC6531]           |
 | UTF8LMTPA    | LMTP with SMTPUTF8 and AUTH  | [RFC4954] [RFC6531] |
 | UTF8LMTPS    | LMTP with SMTPUTF8 and       | [RFC3207] [RFC6531] |
 |              | STARTTLS                     |                     |
 | UTF8LMTPSA   | LMTP with SMTPUTF8 and both  | [RFC3207] [RFC4954] |
 |              | STARTTLS and AUTH            | [RFC6531]           |
 +--------------+------------------------------+---------------------+

5. Security Considerations

 The extended security considerations discussion in the framework
 document [RFC6530] applies 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
 require providing an adequate mechanism for mapping non-ASCII strings
 to ASCII.

Yao & Mao Standards Track [Page 15] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

 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.
 Mechanisms to automatically and quickly provide the origin or
 ownership of an internationalized email address SHALL be implemented
 for use by log readers that cannot easily read 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).  Sections 3.5 and 7.3 of RFC 5321 give detailed
 descriptions of use and possible behaviors.  Implementation of
 internationalized addresses can also affect logs and actions by these
 tools.

6. Acknowledgements

 This document revises RFC 5336 [RFC5336] based on the result of the
 Email Address Internationalization (EAI) working group's discussion.
 Many EAI working group members did tests and implementations to move
 this document to the Standards Track.  Significant comments and
 suggestions were received from Xiaodong LEE, Nai-Wen HSU, Yangwoo KO,
 Yoshiro YONEYA, and other members of JET and were incorporated into
 the specification.  Additional important comments and suggestions,
 and often specific text, were contributed by many members of the
 working group 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, Joseph Yee, 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 with ABNF
 refinement.

7. References

7.1. Normative References

 [ASCII]    American National Standards Institute  (formerly United
            States of America Standards Institute), "USA Code for
            Information Interchange", ANSI X3.4-1968, 1968.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.

Yao & Mao Standards Track [Page 16] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

 [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.
 [RFC3848]  Newman, C., "ESMTP and LMTP Transmission Types
            Registration", RFC 3848, July 2004.
 [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.
 [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.
 [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
            STD 72, RFC 6409, November 2011.
 [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
            Internationalized Email", RFC 6530, February 2012.
 [RFC6532]  Yang, A., Steele, S., and N. Freed, "Internationalized
            Email Headers", RFC 6532, February 2012.
 [RFC6533]  Hansen, T., Ed., Newman, C., and A. Melnikov, Ed.,
            "Internationalized Delivery Status and Disposition
            Notifications", RFC RFC6533, February 2012.

Yao & Mao Standards Track [Page 17] RFC 6531 SMTP Extension for SMTPUTF8 February 2012

7.2. Informative References

 [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.
 [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
 China
 Phone: +86 10 58813007
 EMail: yaojk@cnnic.cn
 Wei MAO
 CNNIC
 No.4 South 4th Street, Zhongguancun
 Beijing
 China
 Phone: +86 10 58812230
 EMail: maowei_ietf@cnnic.cn

Yao & Mao Standards Track [Page 18]

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