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rfc:rfc5336

Network Working Group J. Yao, Ed. Request for Comments: 5336 W. Mao, Ed. Updates: 2821, 2822, 4952 CNNIC Category: Experimental September 2008

        SMTP Extension for Internationalized Email Addresses

Status of This Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Abstract

 This document specifies an SMTP extension for transport and delivery
 of email messages with internationalized email addresses or header
 information.  Communication with systems that do not implement this
 specification is specified in another document.  This document
 updates some syntaxes and rules defined in RFC 2821 and RFC 2822, and
 has some material updating RFC 4952.

Yao & Mao Experimental [Page 1] RFC 5336 EAI SMTP Extension September 2008

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.  Role of This Specification . . . . . . . . . . . . . . . .  3
   1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
 2.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  4
 3.  Mail Transport-Level Protocol  . . . . . . . . . . . . . . . .  4
   3.1.  Framework for the Internationalization Extension . . . . .  4
   3.2.  The UTF8SMTP Extension . . . . . . . . . . . . . . . . . .  5
   3.3.  Extended Mailbox Address Syntax  . . . . . . . . . . . . .  7
   3.4.  The ALT-ADDRESS Parameter  . . . . . . . . . . . . . . . .  9
   3.5.  ALT-ADDRESS Parameter Usage and Response Codes . . . . . . 10
   3.6.  Body Parts and SMTP Extensions . . . . . . . . . . . . . . 11
   3.7.  Additional ESMTP Changes and Clarifications  . . . . . . . 11
     3.7.1.  The Initial SMTP Exchange  . . . . . . . . . . . . . . 12
     3.7.2.  Mail eXchangers  . . . . . . . . . . . . . . . . . . . 12
     3.7.3.  Trace Information  . . . . . . . . . . . . . . . . . . 12
     3.7.4.  UTF-8 Strings in Replies . . . . . . . . . . . . . . . 13
 4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
 5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
 6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
 7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   7.1.  Normative References . . . . . . . . . . . . . . . . . . . 18
   7.2.  Informative References . . . . . . . . . . . . . . . . . . 19
 Appendix A.  Material Updating RFC 4952  . . . . . . . . . . . . . 20
   A.1.  Conventional Message and Internationalized Message . . . . 20
   A.2.  LMTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   A.3.  SMTP Service Extension for DSNs  . . . . . . . . . . . . . 20
   A.4.  Implementation Advice  . . . . . . . . . . . . . . . . . . 20
   A.5.  Applicability of SMTP Extension to Additional Uses . . . . 21

Yao & Mao Experimental [Page 2] RFC 5336 EAI SMTP Extension September 2008

1. Introduction

 An internationalized email address includes two parts, the local part
 and the domain part.  The ways email addresses are used by protocols
 are different from the ways domain names are used.  The most critical
 difference is that emails are delivered through a chain of clients
 and servers, while domain names are resolved by name servers looking
 up those names in their own tables.  In addition to this, the Simple
 Mail Transfer Protocol [RFC2821] provides a negotiation mechanism
 about service extension with which clients can discover server
 capabilities and make decisions for further processing.  An extended
 overview of the extension model for internationalized addresses and
 headers appears in [RFC4952], referred to as "the framework document"
 or just as "Framework" elsewhere in this specification.  This
 document specifies an SMTP extension to permit internationalized
 email addresses in envelopes, and UNICODE characters (encoded in
 UTF-8) [RFC3629] in headers.

1.1. Role of This Specification

 The framework document 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 [RFC2821] [RFC2822] 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 [RFC2821] 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 "conventional message" and "internationalized message" are
 defined in an appendix to this specification.  The terms "UTF-8
 string" or "UTF-8 character" are used informally to refer to Unicode
 characters encoded in UTF-8 [RFC3629].  All other specialized terms
 used in this specification are defined in the framework document or
 in the base Internet email specifications [RFC2821] [RFC2822].  In
 particular, the terms "ASCII address", "internationalized email
 address", "non-ASCII address", "i18mail address", "UTF8SMTP",
 "message", and "mailing list" are used in this document according to
 the definitions in the framework document.

Yao & Mao Experimental [Page 3] RFC 5336 EAI SMTP Extension September 2008

 This specification defines only those Augmented BNF (ABNF) [RFC5234]
 syntax rules that are different from those of the base email
 specifications [RFC2821][RFC2822] and, where the earlier rules are
 upgraded or extended, gives them new names.  When the new rule is a
 small modification to the older one, it is typically given a name
 starting with "u".  Rules that are undefined here may be found in the
 base email specifications under the same names.

2. Overview of Operation

 This specification describes an optional extension to the email
 transport mechanism that permits non-ASCII [ASCII] characters in both
 the envelope and header fields of messages, which are encoded with
 UTF-8 [RFC3629] characters.  The extension is identified with the
 token "UTF8SMTP".  In order to provide information that may be needed
 in downgrading, an optional alternate ASCII address may be needed if
 an SMTP client attempts to transfer an internationalized message and
 encounters a server that does not support this extension.
 The EAI UTF-8 header specification [RFC5335] provides the details of
 how and where non-ASCII characters are permitted in the header fields
 of messages.  The context for this specification is described in the
 framework document.

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
      "UTF8SMTP".
 3.   No parameter values are defined for this EHLO keyword value.  In
      order to permit future (although unanticipated) extensions, the
      EHLO response MUST NOT contain any parameters for that keyword.
      Clients MUST ignore any parameters; that is, clients MUST behave
      as if the parameters do not appear.  If a server includes
      UTF8SMTP in its EHLO response, it MUST be fully compliant with
      this version of this specification.

Yao & Mao Experimental [Page 4] RFC 5336 EAI SMTP Extension September 2008

 4.   One optional parameter, ALT-ADDRESS, is added to the MAIL and
      RCPT commands of SMTP.  ALT-ADDRESS specifies an all-ASCII
      address which can be used as a substitute for the corresponding
      primary (i18mail) address when downgrading.  More discussion of
      the use of this parameter appears in [RFC4952] and [Downgrade].
 5.   One optional parameter "UTF8REPLY" is added to the VRFY and EXPN
      commands.  The parameter UTF8REPLY 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.
 6.   No additional SMTP verbs are defined by this extension.
 7.   Servers offering this extension MUST provide support for, and
      announce, the 8BITMIME extension [RFC1652].
 8.   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).
 9.   The mail message body is extended as specified in [RFC5335].
 10.  The maximum length of MAIL and RCPT command lines is increased
      by 460 characters by the possible addition of the ALT-ADDRESS
      keyword and value.
 11.  The UTF8SMTP extension is valid on the submission port
      [RFC4409].

3.2. The UTF8SMTP Extension

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

Yao & Mao Experimental [Page 5] RFC 5336 EAI SMTP Extension September 2008

 An SMTP client that receives the UTF8SMTP 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 [RFC5335] (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 either ACE (ASCII Compatible
 Encoding) labels (as specified in IDNA [RFC3490]) or UTF-8 strings.
 All labels in domain parts of mailbox names which are IDNs (either
 UTF-8 or ACE strings) MUST be valid.  If the original client submits
 a message to a Message Submission Server ("MSA") [RFC4409], it is the
 responsibility of the MSA that all domain labels are valid;
 otherwise, it is the original client's responsibility.  The presence
 of the UTF8SMTP extension does not change the requirement of RFC 2821
 that servers relaying mail MUST NOT attempt to parse, evaluate, or
 transform the local part in any way.
 If the UTF8SMTP SMTP extension is not offered by the Server, the SMTP
 client MUST NOT transmit an internationalized address and MUST NOT
 transmit a mail message containing internationalized mail headers as
 described in [RFC5335] at any level within its MIME structure.  (For
 this paragraph, the internationalized domain name in the form of ACE
 labels as specified in IDNA [RFC3490] is not considered as
 "internationalized".)  Instead, if an SMTP client (SMTP sender)
 attempts to transfer an internationalized message and encounters a
 server that does not support the extension, it MUST make one of the
 following four choices:
 1.  If and only if the SMTP client (sender) is a Message Submission
     Server ("MSA") [RFC4409], it MAY, consistent with the general
     provisions for changes by such servers, rewrite the envelope,
     headers, or message material to make them entirely ASCII and
     consistent with the provisions of RFC 2821 [RFC2821] and RFC 2822
     [RFC2822].
 2.  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 2821 [RFC2821], RFC 3464 [RFC3464], and the EAI
     delivery status notification (DSN) specification [RFC5337].
 3.  It may find an alternate route to the destination that permits
     UTF8SMTP.  That route may be discovered by trying alternate Mail
     eXchanger (MX) hosts (using preference rules as specified in RFC
     2821) or using other means available to the SMTP-sender.
 4.  If and only if ASCII addresses are available for all addresses
     that appear in the return path and the specific forward paths
     being attempted, it may downgrade the message to an all-ASCII

Yao & Mao Experimental [Page 6] RFC 5336 EAI SMTP Extension September 2008

     form as specified in [Downgrade].  An ASCII address is considered
     to be "available" for a particular address if the original
     address in the envelope is in ASCII or if an ALT-ADDRESS
     parameter is specified for a UTF8SMTP address.
 The difference between choice 1 and choice 4 is that choice 1 is
 constrained by Message Submission [RFC4409], while choice 4 is
 constrained by [Downgrade].

3.3. Extended Mailbox Address Syntax

 RFC 2821, Section 4.1.2, defines the syntax of a mailbox entirely in
 terms of ASCII characters, using the production for a mailbox and
 those productions on which it depends.
 The key changes made by this specification are, informally, to
 o  Change the definition of "sub-domain" to permit either the
    definition above or a UTF-8 string representing a DNS label that
    is conformant with IDNA [RFC3490].
 o  Change the definition of "Atom" to permit either the definition
    above or a UTF-8 string.  That string MUST NOT contain any of the
    ASCII characters (either graphics or controls) that are not
    permitted in "atext"; it is otherwise unrestricted.
 According to the description above, the syntax of an
 internationalized email mailbox name (address) is defined in ABNF
 [RFC5234] as follows.

Yao & Mao Experimental [Page 7] RFC 5336 EAI SMTP Extension September 2008

         uMailbox = uLocal-part "@" uDomain
           ; Replace Mailbox in RFC 2821, Section 4.1.2
         uLocal-part = uDot-string / uQuoted-string
           ; MAY be case-sensitive
           ; Replace Local-part in RFC 2821, Section 4.1.2
         uDot-string = uAtom *("." uAtom)
           ; Replace Dot-string in RFC 2821, Section 4.1.2
         uAtom = 1*ucharacter
               ; Replace Atom in RFC 2821, Section 4.1.2
         ucharacter = atext / UTF8-non-ascii
         atext = <See Section 3.2.4 of RFC 2822>
         uQuoted-string = DQUOTE *uqcontent DQUOTE
           ; Replace Quoted-string in RFC 2821, Section 4.1.2
         DQUOTE = <See appendix B.1 of RFC 5234>
         uqcontent = qcontent / UTF8-non-ascii
         qcontent = <See Section 3.2.5 of RFC 2822>
         uDomain = (sub-udomain 1*("." sub-udomain)) / address-literal
           ; Replace Domain in RFC 2821, Section 4.1.2
         address-literal = <See Section 4.1.2 of RFC 2822>
         sub-udomain = uLet-dig [uLdh-str]
           ; Replace sub-domain in RFC 2821, Section 4.1.2
         uLet-dig = Let-dig / UTF8-non-ascii
         Let-dig = <See Section 4.1.3 of RFC 2821>
         uLdh-str = *( ALPHA / DIGIT / "-" / UTF8-non-ascii) uLet-dig
           ; Replace Ldh-str in RFC 2821, Section 4.1.3
         UTF8-non-ascii = UTF8-2 / UTF8-3 / UTF8-4
         UTF8-2 =  <See Section 4 of RFC 3629>
         UTF8-3 =  <See Section 4 of RFC 3629>
         UTF8-4 =  <See Section 4 of RFC 3629>

Yao & Mao Experimental [Page 8] RFC 5336 EAI SMTP Extension September 2008

 The value of "uDomain" SHOULD be verified by applying the tests
 specified as part of IDNA [RFC3490].  If that verification fails, the
 email address with that uDomain MUST NOT be regarded as a valid email
 address.

3.4. The ALT-ADDRESS Parameter

 If the UTF8SMTP extension is offered, the syntax of the SMTP MAIL and
 RCPT commands is extended to support the optional esmtp-keyword "ALT-
 ADDRESS".  That keyword specifies an alternate all-ASCII address that
 may be used when downgrading.  If the ALT-ADDRESS esmtp-keyword is
 used, it MUST have an associated esmtp-value (ALT-ADDRESS-esmtp-
 value, which is defined below).
 While it may be tempting to consider ALT-ADDRESS as a general-purpose
 second-chance address, such behavior is not defined here.  Instead,
 in this specification ALT-ADDRESS only has meaning when the
 associated primary address is non-ASCII and the message is
 downgraded.  This restriction allows for future extension of the
 specification even though no such extensions are currently
 anticipated.
 Based on the definition of mail-parameters in [RFC2821], the ALT-
 ADDRESS parameter usage in the commands of MAIL and RCPT is defined
 as follows.  The following definitions are given in the same format
 as used in RFC 2821.
      "MAIL FROM:" ("<>" / uReverse-path) [ SP Mail-parameters ] CRLF
         ; Update the MAIL command in RFC 2821, Section 4.1.1.2.
         ; A new parameter defined by the ABNF non-terminal
         ; <ALT-ADDRESS-parameter> is added.  It complies
         ; with the syntax specified for <esmtp-param> in RFC 2821.
      "RCPT TO:" ("<Postmaster@" uDomain ">" / "<Postmaster>" /
            uForward-path) [ SP Rcpt-parameters ] CRLF
             ; Update RCPT command in RFC 2821, Section 4.1.1.3.
             ; A new parameter defined by the ABNF non-terminal
             ; <ALT-ADDRESS-parameter> is added.  It complies
             ; with the syntax specified for <esmtp-param>.
             ; uDomain is defined in Section 3.3 of this document.
      uReverse-path = uPath
         ; Replace Reverse-path in RFC 2821, Section 4.1.2.
      uForward-path = uPath
         ; Replace Forward-path in RFC 2821, Section 4.1.2.

Yao & Mao Experimental [Page 9] RFC 5336 EAI SMTP Extension September 2008

      uPath = "<" [ A-d-l ":" ] uMailbox ">"
         ; Replace Path in RFC 2821, Section 4.1.2.
         ; uMailbox is defined in Section 3.3 of this document.
      A-d-l = <See Section 4.1.2 of RFC 2821>
      ALT-ADDRESS-parameter = "ALT-ADDRESS=" ALT-ADDRESS-value
      ALT-ADDRESS-value = xtext
             ; The value is a mailbox name encoded as xtext.
      xtext = <See Section 4.2 of RFC 3461>
 The ALT-ADDRESS-parameter MUST NOT appear more than once in any MAIL
 or RCPT command.  ALT-ADDRESS-esmtp-value MUST be an all-ASCII email
 address before xtext encoding.

3.5. ALT-ADDRESS Parameter Usage and Response Codes

 An "internationalized message" as defined in the appendix of this
 specification MUST NOT be sent to an SMTP server that does not
 support UTF8SMTP.  Such a message MAY be rejected by a server if it
 lacks ALT-ADDRESSes as discussed in Section 3.2 of this
 specification.
 The three-digit reply codes used in this section are consistent with
 their meanings as defined in RFC 2821.
 When messages are rejected because the RCPT command requires an ALT-
 ADDRESS, the response code 553 is used with the meaning "mailbox name
 not allowed".  When messages are rejected for other reasons, such as
 the MAIL command requiring an ALT-ADDRESS, the response code 550 is
 used with the meaning "mailbox unavailable".  When the server
 supports enhanced mail system status codes [RFC3463], response code
 "X.6.7" [RFC5248] is used, meaning that "The ALT-ADDRESS is required
 but not specified".
 If the response code is issued after the final "." of the DATA
 command, the response code "554" is used with the meaning
 "Transaction failed".  When the server supports enhanced mail system
 status codes [RFC3463], response code "X.6.9" [RFC5248] is used,
 meaning that "UTF8SMTP downgrade failed".

Yao & Mao Experimental [Page 10] RFC 5336 EAI SMTP Extension September 2008

3.6. Body Parts and SMTP Extensions

 There is no ESMTP parameter to assert that a message is an
 internationalized message.  An SMTP server that requires accurate
 knowledge of whether a message is internationalized is required to
 parse all message header fields and MIME header fields in the message
 body.
 While this specification requires that servers support the 8BITMIME
 extension [RFC1652] to ensure that servers have adequate handling
 capability for 8-bit data and to avoid a number of complex encoding
 problems, the use of internationalized addresses obviously does not
 require non-ASCII body parts in the MIME message.  The UTF8SMTP
 extension MAY be used with the BODY=8BITMIME parameter if that is
 appropriate given the body content or, with the BODY=BINARYMIME
 parameter, if the server advertises BINARYMIME [RFC3030] and that is
 appropriate.
 Assuming that the server advertises UTF8SMTP and 8BITMIME, and
 receives at least one non-ASCII address, with or without ALT-ADDRESS,
 the precise interpretation of 'No BODY parameter', "BODY=8BITMIME",
 and "BODY=BINARYMIME" in the MAIL command is:
 1.  If there is no BODY parameter, the header contains UTF-8
     characters, but all the body parts are in ASCII (possibly as the
     result of a content-transfer-encoding).
 2.  If a BODY=8BITMIME parameter is present, the header contains
     UTF-8 characters, and some or all of the body parts contain 8-bit
     line-oriented data.
 3.  If a BODY=BINARYMIME parameter is present, the header contains
     UTF-8 characters, and some or all body parts contain binary data
     without restriction as to line lengths or delimiters.

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 2821 specifies a
 mailbox, this specification expects UTF-8 to be used for the entire
 string; when RFC 2821 specifies a domain name, the name SHOULD be in
 the form of ACE labels if its raw form is non-ASCII.
 The following subsections list and discuss all of the relevant cases.

Yao & Mao Experimental [Page 11] RFC 5336 EAI SMTP Extension September 2008

3.7.1. The Initial SMTP Exchange

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

3.7.2. Mail eXchangers

 Organizations often authorize multiple servers to accept mail
 addressed to them.  For example, the organization may itself operate
 more than one server, and may also or instead have an agreement with
 other organizations to accept mail as a backup.  Authorized servers
 are generally listed in MX records as described in RFC 2821.  When
 more than one server accepts mail for the domain-part of a mailbox,
 it is strongly advised that either all or none of them support the
 UTF8SMTP extension.  Otherwise, surprising downgrades can happen
 during temporary failures, which users might perceive as a serious
 reliability issue.

3.7.3. Trace Information

 When an SMTP server receives a message for delivery or further
 processing, it MUST insert trace ("time stamp" or "Received")
 information at the beginning of the message content.  "Time stamp" or
 "Received" appears in the form of "Received:" lines.  The most
 important use of Received: lines is for debugging mail faults.  When
 the delivery SMTP server makes the "final delivery" of a message, it
 inserts a Return-path line at the beginning of the mail data.  The
 primary purpose of the Return-path is to designate the address to
 which messages indicating non-delivery or other mail system failures
 are to be sent.  For the trace information, this memo updates the
 time stamp line and the return path line [RFC2821] formally defined
 as follows:
    uReturn-path-line = "Return-Path:" FWS uReverse-path <CRLF>
        ; Replaces Return-path-line in Section 4.4 of RFC 2821
        ; uReverse-path is defined in Section 3.3 of this document
    uTime-stamp-line = "Received:" FWS uStamp <CRLF>
        ; Replaces Time-stamp-line in Section 4.4 of RFC 2821
    uStamp = From-domain By-domain uOpt-info ";"  FWS date-time
        ; Replaces Stamp in Section 4.4 of RFC 2821

Yao & Mao Experimental [Page 12] RFC 5336 EAI SMTP Extension September 2008

     uOpt-info = [Via] [With] [ID] [uFor]
        ; Replaces Opt-info in Section 4.4 of RFC 2821
        ; The protocol value for With will allow a UTF8SMTP value
       uFor = "FOR" ( FWS (uPath / uMailbox) ) CFWS
        ; Replaces For in Section 4.4 of RFC 2821
        ; uPath and uMailbox are defined in Sections 2.4 and
        ; 2.3, respectively, of this document
 Note: The FOR parameter has been changed to match the definition in
 [RFC2821bis], permitting only one address in the For clause.  The
 group working on that document reached mailing list consensus that
 the syntax in [RFC2821] that permitted more than one address was
 simply a mistake.
 Except in the 'uFor' clause and 'uReverse-path' value where non-ASCII
 domain names may be used, internationalized domain names in Received
 fields MUST be transmitted in the form of ACE labels.  The protocol
 value of the WITH clause when this extension is used is one of the
 UTF8SMTP values specified in the "IANA Considerations" section of
 this document.

3.7.4. UTF-8 Strings in Replies

3.7.4.1. MAIL and RCPT Commands

 If the client issues a RCPT command containing non-ASCII characters,
 the SMTP server is permitted to use UTF-8 characters in the email
 address associated with 251 and 551 response codes.
 If an SMTP client follows this specification and sends any RCPT
 commands containing non-ASCII addresses, it MUST be able to accept
 and process 251 or 551 responses containing UTF-8 email addresses.
 If a given RCPT command does not include a non-ASCII envelope
 address, the 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 addresses.

3.7.4.2. VRFY and EXPN Commands and the UTF8REPLY Parameter

 If the VRFY and EXPN commands are transmitted with an optional
 parameter "UTF8REPLY", it indicates the client can accept UTF-8
 strings in replies from those commands.  This allows the server to
 use UTF-8 strings in mailbox names and full names that occur in
 replies without concern that the 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 SMTP server MUST NOT use UTF-8

Yao & Mao Experimental [Page 13] RFC 5336 EAI SMTP Extension September 2008

 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 is not needed in them.  Some replies, notably those
 resulting from successful execution of the VRFY and EXPN commands, do
 include the mailbox, making the provisions of this section important.
 VERIFY (VRFY) and EXPAND (EXPN) command syntaxes are changed to:
     "VRFY" SP (uLocal-part / uMailbox) [SP "UTF8REPLY"] CRLF
            ; uLocal-part and uMailbox are defined in
            ; Section 3.3 of this document.
     "EXPN" SP ( uLocal-part / uMailbox ) [ SP "UTF8REPLY" ] CRLF
            ; uLocal-part and uMailbox are defined in
            ; Section 3.3 of this document.
 The "UTF8REPLY" parameter does not use a value.  If the reply to a
 VERIFY (VRFY) or EXPAND (EXPN) command requires UTF-8, but the SMTP
 client does not use the "UTF8REPLY" parameter, then the server MUST
 use either the response code 252 or 550.  Response code 252, defined
 in [RFC2821], means "Cannot VRFY user, but will accept the message
 and attempt the delivery".  Response code 550, also defined in
 [RFC2821], means "Requested action not taken: mailbox unavailable".
 When the server supports enhanced mail system status codes [RFC3463],
 the enhanced response code as specified below is used.  Using the
 "UTF8REPLY" 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>
          ; uMailbox is defined in Section 3.3 of this document.
          ; User Name can contain non-ASCII characters.
       uMailbox
          ; uMailbox is defined in Section 3.3 of this document.
 If the SMTP reply requires UTF-8 strings, but UTF-8 is not allowed in
 the reply, and the server supports enhanced mail system status codes
 [RFC3463], the enhanced response code is either "X.6.8" or "X.6.10"
 [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 client".

Yao & Mao Experimental [Page 14] RFC 5336 EAI SMTP Extension September 2008

 If the SMTP client does not support the UTF8SMTP 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 text MUST NOT appear in the reply.
 Although UTF-8 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 for any other purposes.  SMTP servers MUST
 NOT include non-ASCII characters in replies except in the limited
 cases specifically permitted in this section.

4. IANA Considerations

 IANA has added a new value "UTF8SMTP" to the SMTP Service Extension
 subregistry of the Mail Parameters registry, according to the
 following data:
      +----------+---------------------------------+-----------+
      | Keywords | Description                     | Reference |
      +----------+---------------------------------+-----------+
      | UTF8SMTP | Internationalized email address | [RFC5336] |
      +----------+---------------------------------+-----------+
 This document adds new 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:

Yao & Mao Experimental [Page 15] RFC 5336 EAI SMTP Extension September 2008

    Code:               X.6.7
    Sample Text:        The ALT-ADDRESS is required but not specified
    Associated basic status code:  553, 550
    Description:        This indicates the reception of a MAIL or RCPT
                        command that required an ALT-ADDRESS parameter
                        but such parameter was not present.
    Defined:            RFC 5336  (Experimental track)
    Submitter:          Jiankang YAO
    Change controller:  IESG.
    Code:               X.6.8
    Sample Text:        UTF-8 string reply is required,
                        but not permitted by the client
    Associated basic status code:  553, 550
    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 client.
    Defined:            RFC  5336.  (Experimental track)
    Submitter:          Jiankang YAO
    Change controller:  IESG.
     Code:               X.6.9
     Sample Text:        UTF8SMTP downgrade failed
     Associated basic status code:  550
     Description:        This indicates that transaction failed
                         after the final "." of the DATA command.
     Defined:            RFC  5336.  (Experimental track)
     Submitter:          Jiankang YAO
     Change controller:  IESG.
    Code:               X.6.10
    Sample Text:        UTF-8 string reply is required,
                        but not permitted by the client
    Associated basic status code:  252
    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 client.
    Defined:            RFC 5336.  (Experimental track)
    Submitter:          Jiankang YAO
    Change controller:  IESG.

Yao & Mao Experimental [Page 16] RFC 5336 EAI SMTP Extension September 2008

 The "Mail Transmission Types" registry under the Mail Parameters
 registry is requested to be updated to include the following new
 entries:
 +---------------+----------------------------+----------------------+
 | WITH protocol | Description                | Reference            |
 | types         |                            |                      |
 +---------------+----------------------------+----------------------+
 | UTF8SMTP      | UTF8SMTP with Service      | [RFC5336]            |
 |               | Extensions                 |                      |
 | UTF8SMTPA     | UTF8SMTP with SMTP AUTH    | [RFC4954] [RFC5336]  |
 | UTF8SMTPS     | UTF8SMTP with STARTTLS     | [RFC3207] [RFC5336]  |
 | UTF8SMTPSA    | UTF8SMTP with both         | [RFC3207] [RFC4954]  |
 |               | STARTTLS and SMTP AUTH     | [RFC5336]            |
 +---------------+----------------------------+----------------------+

5. Security Considerations

 See the extended security considerations discussion in the framework
 document [RFC4952].

6. Acknowledgements

 Much of the text in the initial version of this specification was
 derived or copied from [Emailaddr] with the permission of the author.
 Significant comments and suggestions were received from Xiaodong LEE,
 Nai-Wen Hsu, Yangwoo KO, Yoshiro YONEYA, and other members of the JET
 team and were incorporated into the 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.

Yao & Mao Experimental [Page 17] RFC 5336 EAI SMTP Extension September 2008

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.
 [RFC1652]     Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
               Crocker, "SMTP Service Extension for 8bit-
               MIMEtransport", RFC 1652, July 1994.
 [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2821]     Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
               April 2001.
 [RFC2822]     Resnick, P., "Internet Message Format", RFC 2822,
               April 2001.
 [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.
 [RFC3490]     Faltstrom, P., Hoffman, P., and A. Costello,
               "Internationalizing Domain Names in Applications
               (IDNA)", RFC 3490, March 2003.
 [RFC3629]     Yergeau, F., "UTF-8, a transformation format of ISO
               10646", STD 63, RFC 3629, November 2003.
 [RFC4409]     Gellens, R. and J. Klensin, "Message Submission for
               Mail", RFC 4409, April 2006.
 [RFC4952]     Klensin, J. and Y. Ko, "Overview and Framework for
               Internationalized Email", RFC 4952, July 2007.
 [RFC5234]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
               Specifications: ABNF", STD 68, RFC 5234, January 2008.

Yao & Mao Experimental [Page 18] RFC 5336 EAI SMTP Extension September 2008

 [RFC5248]     Hansen, T. and J. Klensin, "A Registry for SMTP
               Enhanced Mail System Status Codes", BCP 138, RFC 5248,
               June 2008.
 [RFC5335]     Abel, Y., Ed., "Internationalized Email Headers",
               RFC 5335, September 2008.
 [RFC5337]     Newman, C. and A. Melnikov, Ed., "Internationalized
               Delivery Status and Disposition Notifications",
               RFC 5337, September 2008.

7.2. Informative References

 [Downgrade]   Fujiwara, K. and Y. Yoneya, "Downgrading mechanism for
               Email Address Internationalization", Work in Progress,
               July 2008.
 [Emailaddr]   Klensin, J., "Internationalization of Email Addresses",
               Work in Progress, July 2005.
 [RFC0974]     Partridge, C., "Mail routing and the domain system",
               RFC 974, January 1986.
 [RFC2033]     Myers, J., "Local Mail Transfer Protocol", RFC 2033,
               October 1996.
 [RFC2821bis]  Klensin, J., "Simple Mail Transfer Protocol", Work
               in Progress, July 2008.
 [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., Ed. and A. Melnikov, Ed., "SMTP Service
               Extension for Authentication", RFC 4954, July 2007.

Yao & Mao Experimental [Page 19] RFC 5336 EAI SMTP Extension September 2008

Appendix A. Material Updating RFC 4952

 RFC 4952, the overview and framework document covering this set of
 extensions for internationalized email, was completed before this
 specification, which specifies a particular part of the protocol set.
 This appendix, which is normative, contains material that would have
 been incorporated into RFC 4952 had it been delayed until the work
 described in the rest of this specification was completed.  This
 material should be included in any update to RFC 4952.

A.1. Conventional Message and Internationalized Message

 o  A conventional message is one that does not use any extension
    defined in this document or in the UTF-8 header specification
    [RFC5335], and which is strictly conformant to RFC 2822 [RFC2822].
 o  An internationalized message is a message utilizing one or more of
    the extensions defined in this specification or in the UTF-8
    header specification [RFC5335], so that it is no longer conformant
    to the RFC 2822 specification of a message.

A.2. LMTP

 LMTP [RFC2033] may be used as the final delivery agent.  In such
 cases, LMTP may be arranged to deliver the mail to the mail store.
 The mail store may not have UTF8SMTP capability.  LMTP needs to be
 updated to deal with these situations.

A.3. SMTP Service Extension for DSNs

 The existing Draft Standard regarding delivery status notifications
 (DSNs) [RFC3461] is limited to ASCII text in the machine readable
 portions of the protocol.  "International Delivery Status and
 Disposition Notifications" [RFC5337] adds a new address type for
 international email addresses so an original recipient address with
 non-ASCII characters can be correctly preserved even after
 downgrading.  If an SMTP server advertises both the UTF8SMTP and the
 DSN extension, that server MUST implement EAI DSN [RFC5337] including
 support for the ORCPT parameter.

A.4. Implementation Advice

 In the absence of this extension, SMTP clients and servers are
 constrained to using only those addresses permitted by RFC 2821.  The
 local parts of those addresses MAY be made up of any ASCII
 characters, although some of them MUST be quoted as specified there.
 It is notable in an internationalization context that there is a long
 history on some systems of using overstruck ASCII characters (a

Yao & Mao Experimental [Page 20] RFC 5336 EAI SMTP Extension September 2008

 character, a backspace, and another character) within a quoted string
 to approximate non-ASCII characters.  This form of
 internationalization SHOULD be phased out as this extension becomes
 widely deployed, but backward-compatibility considerations require
 that it continue to be supported.

A.5. Applicability of SMTP Extension to Additional Uses

 Among other protocol changes, the SMTP extension allows an optional
 alternate address to be supplied with the MAIL and RCPT commands.
 For the purposes of this set of specifications, this alternate
 address only has meaning when the primary address contains UTF-8
 characters and the message is downgraded.  While it may be tempting
 to consider the alternate address as a general-purpose second-chance
 address to be used whenever the primary address is rejected, such
 behavior is not defined here.  This restriction allows for future
 extensions to be developed which create such a general-purpose
 second-chance address, although no specific work on such an extension
 is currently anticipated.  Note that any such extension needs to
 consider the question of what the [RFC0974] sequencing rules mean
 when different possible servers support different sets of ESMTP
 options (or, in this case, addresses).  The answer to this question
 may also imply updates to [RFC2821].

Authors' Addresses

 Jiankang YAO (editor)
 CNNIC
 No.4 South 4th Street, Zhongguancun
 Beijing
 Phone: +86 10 58813007
 EMail: yaojk@cnnic.cn
 Wei MAO (editor)
 CNNIC
 No.4 South 4th Street, Zhongguancun
 Beijing
 Phone: +86 10 58812230
 EMail: maowei_ietf@cnnic.cn

Yao & Mao Experimental [Page 21] RFC 5336 EAI SMTP Extension September 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Yao & Mao Experimental [Page 22]

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