Internationalized Delivery Status and Disposition Notifications

Network Working Group                                          C. Newman
Request for Comments: 5337                              Sun Microsystems
Updates: 3461, 3464, 3798                               A. Melnikov, Ed.
Category: Experimental                                         Isode Ltd
                                                          September 2008


    Internationalized Delivery Status and Disposition Notifications

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

   Delivery status notifications (DSNs) are critical to the correct
   operation of an email system.  However, the existing Draft Standards
   (RFC 3461, RFC 3462, RFC 3464) are presently limited to US-ASCII text
   in the machine-readable portions of the protocol.  This specification
   adds a new address type for international email addresses so an
   original recipient address with non-US-ASCII characters can be
   correctly preserved even after downgrading.  This also provides
   updated content return media types for delivery status notifications
   and message disposition notifications to support use of the new
   address type.

   This document experimentally extends RFC 3461, RFC 3464, and RFC
   3798.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  3
   3.  UTF-8 Address Type . . . . . . . . . . . . . . . . . . . . . .  3
   4.  UTF-8 Delivery Status Notifications  . . . . . . . . . . . . .  6
     4.1.  Additional Requirements on SMTP Servers  . . . . . . . . .  8
   5.  UTF-8 Message Disposition Notifications  . . . . . . . . . . .  9
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
     6.1.  UTF-8 Mail Address Type Registration . . . . . . . . . . . 10
     6.2.  Update to 'smtp' Diagnostic Type Registration  . . . . . . 11
     6.3.  message/global-headers . . . . . . . . . . . . . . . . . . 11
     6.4.  message/global-delivery-status . . . . . . . . . . . . . . 12
     6.5.  message/global-disposition-notification  . . . . . . . . . 13
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 16
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 17

1.  Introduction

   When an email message is transmitted using the UTF8SMTP [RFC5336]
   extension and Internationalized Email Headers [RFC5335], it is
   sometimes necessary to return that message or generate a Message
   Disposition Notification (MDN) [RFC3798].  As a message sent to
   multiple recipients can generate a status and disposition
   notification for each recipient, it is helpful if a client can
   correlate these notifications based on the recipient address it
   provided; thus, preservation of the original recipient is important.
   This specification describes how to preserve the original recipient
   and updates the MDN and DSN formats to support the new address types.

2.  Conventions Used in This Document

   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 [RFC2119].

   The formal syntax use the Augmented Backus-Naur Form (ABNF) [RFC5234]
   notation including the core rules defined in Appendix B of RFC 5234
   [RFC5234] and the UTF-8 syntax rules in Section 4 of [RFC3629].

3.  UTF-8 Address Type

   An Extensible Message Format for Delivery Status Notifications
   [RFC3464] defines the concept of an address type.  The address format
   introduced in Internationalized Email Headers [RFC5335] is a new
   address type.  The syntax for the new address type in the context of
   status notifications is specified at the end of this section.

   An SMTP [RFC2821] server that advertises both the UTF8SMTP extension
   [RFC5336] and the DSN extension [RFC3461] MUST accept a UTF-8 address
   type in the ORCPT parameter including 8-bit UTF-8 characters.  This
   address type also includes a 7-bit encoding suitable for use in a
   message/delivery-status body part or an ORCPT parameter sent to an
   SMTP server that does not advertise UTF8SMTP.

   This address type has 3 forms: utf-8-addr-xtext, utf-8-addr-unitext,
   and utf-8-address.  The first 2 forms are 7-bit safe.

   The utf-8-address form is only suitable for use in newly defined
   protocols capable of native representation of 8-bit characters.  That
   is, the utf-8-address form MUST NOT be used in the ORCPT parameter
   when the SMTP server doesn't advertise support for UTF8SMTP or the
   SMTP server supports UTF8SMTP, but the address contains US-ASCII
   characters not permitted in the ORCPT parameter (e.g., the ORCPT
   parameter forbids unencoded SP and the = character), or in a 7-bit
   transport environment including a message/delivery-status Original-
   Recipient or Final-Recipient field.  In the former case, the utf-8-
   addr-xtext form (see below) MUST be used instead; in the latter case,
   the utf-8-addr-unitext form MUST be used.  The utf-8-address form MAY
   be used in the ORCPT parameter when the SMTP server also advertises
   support for UTF8SMTP and the address doesn't contain any US-ASCII
   characters not permitted in the ORCPT parameter.  It SHOULD be used
   in a message/global-delivery-status Original-Recipient or Final-
   Recipient DSN field, or in an Original-Recipient header field
   [RFC3798] if the message is a UTF8SMTP message.

   In addition, the utf-8-addr-unitext form can be used anywhere where
   the utf-8-address form is allowed.

   When using in the ORCPT parameter, the UTF-8 address type requires
   that US-ASCII CTLs, SP, \, +, and = be encoded using xtext encoding
   as described in [RFC3461].  This is described by the utf-8-addr-xtext
   form in the ABNF below.  Unicode characters MAY be included in a
   UTF-8 address type using a "\x{HEXPOINT}" syntax
   (EmbeddedUnicodeChar), where HEXPOINT is 2 to 6 hexadecimal digits.
   When sending data to a UTF8SMTP-capable server, native UTF-8
   characters SHOULD be used instead of the EmbeddedUnicodeChar syntax
   described in details below.  When sending data to an SMTP server that
   does not advertise UTF8SMTP, then the EmbeddedUnicodeChar syntax MUST
   be used instead of UTF-8.

   When the ORCPT parameter is placed in a message/
   global-delivery-status Original-Recipient field, the utf-8-addr-xtext
   form of the UTF-8 address type SHOULD be converted to the utf-8-
   address form (see the ABNF below) by removing all xtext encoding
   first (which will result in the utf-8-addr-unitext form), followed by
   removal of the unitext encoding.  However, if an address is labeled
   with the UTF-8 address type but does not conform to utf-8 syntax,
   then it MUST be copied into the message/global-delivery-status field
   without alteration.

   The ability to encode characters with the EmbeddedUnicodeChar
   encodings should be viewed as a transitional mechanism.  It is hoped
   that as systems lacking support for UTF8SMTP become less common over
   time, these encodings can eventually be phased out.

   In the ABNF below, all productions not defined in this document are
   defined in Appendix B of [RFC5234], in Section 4 of [RFC3629], or in
   [RFC3464].

  utf-8-type-addr     = "utf-8;" utf-8-enc-addr

  utf-8-address       = uMailbox [ 1*WSP "<" Mailbox ">" ]
    ; uMailbox is defined in [RFC5336].
    ; Mailbox is defined in [RFC2821].

  utf-8-enc-addr      = utf-8-addr-xtext /
                        utf-8-addr-unitext /
                        utf-8-address

  utf-8-addr-xtext    = xtext
                    ; xtext is defined in [RFC3461].
                    ; When xtext encoding is removed,
                    ; the syntax MUST conform to
                    ; utf-8-addr-unitext.

  utf-8-addr-unitext  = 1*(QUCHAR / EmbeddedUnicodeChar)
                      ; MUST follow utf-8-address ABNF when
                      ; dequoted

  QUCHAR              = %x21-2a / %x2c-3c / %x3e-5b / %x5d-7e /
                        UTF8-2 / UTF8-3 / UTF8-4
                      ; US-ASCII printable characters except
                      ; CTLs, SP, '\', '+' and '=', plus
                      ; other Unicode characters in UTF-8

  EmbeddedUnicodeChar =   %x5C.78 "{" HEXPOINT "}"
                      ; starts with "\x"

  HEXPOINT = "5C" / (HEXDIG8 HEXDIG) /    ; 2 digit forms
             ( NZHEXDIG 2(HEXDIG) ) /     ; 3 digit forms
             ( NZDHEXDIG 3(HEXDIG) ) /
             ( "D" %x30-37 2(HEXDIG) ) /
                      ; 4 digit forms excluding surrogate
             ( NZHEXDIG 4(HEXDIG) ) /     ; 5 digit forms
                     ( "10" 4*HEXDIG )    ; 6 digit forms
             ; represents either "\" or a Unicode code point outside the
             ; US-ASCII repertoire

  HEXDIG8             = %x38-39 / "A" / "B" / "C" / "D" / "E" / "F"
                      ; HEXDIG excluding 0-7
  NZHEXDIG            = %x31-39 / "A" / "B" / "C" / "D" / "E" / "F"
                      ; HEXDIG excluding "0"
  NZDHEXDIG           = %x31-39 / "A" / "B" / "C" / "E" / "F"
                      ; HEXDIG excluding "0" and "D"

4.  UTF-8 Delivery Status Notifications

   A traditional delivery status notification [RFC3464] comes in a
   three-part multipart/report [RFC3462] container, where the first part
   is human-readable text describing the error, the second part is a
   7-bit-only message/delivery-status, and the optional third part is
   used for content (message/rfc822) or header (text/rfc822-headers)
   return.  As the present DSN format does not permit returning of
   undeliverable UTF8SMTP messages, three new media types are needed.

   The first type, message/global-delivery-status, has the syntax of
   message/delivery-status with three modifications.  First, the charset
   for message/global-delivery-status is UTF-8, and thus any field MAY
   contain UTF-8 characters when appropriate (see the ABNF below).  In
   particular, the Diagnostic-Code field MAY contain UTF-8 as described
   in UTF8SMTP [RFC5336]; the Diagnostic-Code field SHOULD be in
   i-default language [DEFAULTLANG].  Second, systems generating a
   message/global-delivery-status body part SHOULD use the utf-8-address
   form of the UTF-8 address type for all addresses containing
   characters outside the US-ASCII repertoire.  These systems SHOULD up-
   convert the utf-8-addr-xtext or the utf-8-addr-unitext form of a
   UTF-8 address type in the ORCPT parameter to the utf-8-address form
   of a UTF-8 address type in the Original-Recipient field.  Third, a
   new optional field called Localized-Diagnostic is added.  Each
   instance includes a language tag [LANGTAGS] and contains text in the
   specified language.  This is equivalent to the text part of the
   Diagnostic-Code field.  All instances of Localized-Diagnostic MUST
   use different language tags.  The ABNF for message/
   global-delivery-status is specified below.

   In the ABNF below, all productions not defined in this document are
   defined in Appendix B of [RFC5234], in Section 4 of [RFC3629], or in
   [RFC3464].

   utf-8-delivery-status-content = per-message-fields
                         1*( CRLF utf-8-per-recipient-fields )
        ; "per-message-fields" remains unchanged from the definition
            ; in RFC 3464, except for the "extension-field"
            ; which is updated below.

    utf-8-per-recipient-fields =
         [ original-recipient-field CRLF ]
         final-recipient-field CRLF
         action-field CRLF
         status-field CRLF
         [ remote-mta-field CRLF ]
         [ diagnostic-code-field CRLF
           *(localized-diagnostic-text-field CRLF) ]
         [ last-attempt-date-field CRLF ]
         [ will-retry-until-field CRLF ]
         *( extension-field CRLF )
     ; All fields except for "original-recipient-field",
     ; "final-recipient-field", "diagnostic-code-field"
     ; and "extension-field" remain unchanged from
     ; the definition in RFC 3464.

   generic-address =/ utf-8-enc-addr
     ; Only allowed with the "utf-8" address-type.
     ;
     ; This indirectly updates "original-recipient-field"
     ; and "final-recipient-field"

   diagnostic-code-field =
        "Diagnostic-Code" ":" diagnostic-type ";" *text-fixed

   localized-diagnostic-text-field =
        "Localized-Diagnostic" ":" Language-Tag ";" *utf8-text
     ; "Language-Tag" is a language tag as defined in [LANGTAGS].

   extension-field =/ extension-field-name ":" *utf8-text

   text-fixed = %d1-9 /      ; Any Unicode character except for NUL,
               %d11 /       ; CR and LF, encoded in UTF-8
               %d12 /
               %d14-127
     ; Same as <text> from [RFC2822], but without <obs-text>.
     ; If/when RFC 2822 is updated to disallow <obs-text>,
     ; this should become just <text>
     ; Also, if/when RFC 2822 is updated to disallow control characters
     ; this should become a reference to RFC 2822upd instead.

   utf8-text = text-fixed / UTF8-non-ascii

   UTF8-non-ascii   = UTF8-2 / UTF8-3 / UTF8-4
   The second type, used for returning the content, is message/global
   which is similar to message/rfc822, except it contains a message with
   UTF-8 headers.  This media type is described in [RFC5335].

   The third type, used for returning the headers, is message/
   global-headers and contains only the UTF-8 header fields of a message
   (all lines prior to the first blank line in a UTF8SMTP message).
   Unlike message/global, this body part provides no difficulties for
   the present infrastructure.

   Note that as far as multipart/report [RFC3462] container is
   concerned, message/global-delivery-status, message/global, and
   message/global-headers MUST be treated as equivalent to message/
   delivery-status, message/rfc822, and text/rfc822-headers.  That is,
   implementations processing multipart/report MUST expect any
   combinations of the 6 MIME types mentioned above inside a multipart/
   report MIME type.

   All three new types will typically use the "8bit" Content-Transfer-
   Encoding.  (In the event all content is 7-bit, the equivalent
   traditional types for delivery status notifications MAY be used.  For
   example, if information in message/global-delivery-status part can be
   represented without any loss of information as message/
   delivery-status, then the message/delivery-status body part may be
   used.)  Note that [RFC5335] relaxed restriction from MIME [RFC2046]
   regarding use of Content-Transfer-Encoding in new "message" subtypes.
   This specification explicitly allows use of Content-Transfer-Encoding
   in message/global-headers and message/global-delivery-status.  This
   is not believed to be problematic as these new MIME types are
   intended primarily for use by newer systems with full support for
   8-bit MIME and UTF-8 headers.

4.1.  Additional Requirements on SMTP Servers

   If an SMTP server that advertises both UTF8SMTP and DSN needs to
   return an undeliverable UTF8SMTP message, then it MUST NOT downgrade
   [DOWNGRADE] the UTF8SMTP message when generating the corresponding
   multipart/report.  If the return path SMTP server does not support
   UTF8SMTP, then the undeliverable body part and headers MUST be
   encoded using a 7-bit Content-Transfer-Encoding such as "base64" or
   "quoted-printable" [RFC2045], as detailed in Section 4.  Otherwise,
   "8bit" Content-Transfer-Encoding can be used.

5.  UTF-8 Message Disposition Notifications

   Message Disposition Notifications [RFC3798] have a similar design and
   structure to DSNs.  As a result, they use the same basic return
   format.  When generating an MDN for a UTF-8 header message, the third
   part of the multipart/report contains the returned content (message/
   global) or header (message/global-headers), same as for DSNs.  The
   second part of the multipart/report uses a new media type, message/
   global-disposition-notification, which has the syntax of message/
   disposition-notification with two modifications.  First, the charset
   for message/global-disposition-notification is UTF-8, and thus any
   field MAY contain UTF-8 characters when appropriate (see the ABNF
   below).  (In particular, the failure-field, the error-field, and the
   warning-field MAY contain UTF-8.  These fields SHOULD be in i-default
   language [DEFAULTLANG].)  Second, systems generating a message/
   global-disposition-notification body part (typically a mail user
   agent) SHOULD use the UTF-8 address type for all addresses containing
   characters outside the US-ASCII repertoire.

   The MDN specification also defines the Original-Recipient header
   field, which is added with a copy of the contents of ORCPT at
   delivery time.  When generating an Original-Recipient header field, a
   delivery agent writing a UTF-8 header message in native format SHOULD
   convert the utf-8-addr-xtext or the utf-8-addr-unitext form of a
   UTF-8 address type in the ORCPT parameter to the corresponding utf-8-
   address form.

   The MDN specification also defines the Disposition-Notification-To
   header, which is an address header and thus follows the same 8-bit
   rules as other address headers such as "From" and "To" when used in a
   UTF-8 header message.

     ; ABNF for "original-recipient-header", "original-recipient-field",
     ; and "final-recipient-field" from RFC 3798 is implicitly updated
     ; as they use the updated "generic-address" as defined in
     ; Section 4 of this document.

   failure-field = "Failure" ":" *utf8-text
     ; "utf8-text" is defined in Section 4 of this document.

   error-field = "Error" ":" *utf8-text
     ; "utf8-text" is defined in Section 4 of this document.

   warning-field = "Warning" ":" *utf8-text
     ; "utf8-text" is defined in Section 4 of this document.

6.  IANA Considerations

   This specification does not create any new IANA registries.  However,
   the following items have been registered as a result of this
   document.

6.1.  UTF-8 Mail Address Type Registration

   The mail address type registry was created by RFC 3464.  The
   registration template response follows:

   (a) The proposed address-type name.

       UTF-8

   (b) The syntax for mailbox addresses of this type, specified using
       BNF, regular expressions, ASN.1, or other non-ambiguous language.

       See Section 3.

   (c) If addresses of this type are not composed entirely of graphic
       characters from the US-ASCII repertoire, a specification for how
       they are to be encoded as graphic US-ASCII characters in a DSN
       Original-Recipient or Final-Recipient DSN field.

       This address type has 3 forms (as defined in Section 3): utf-8-
       addr-xtext, utf-8-addr-unitext, and utf-8-address.  The first 2
       forms are 7-bit safe.

   The utf-8-address form MUST NOT be used

   1.  in the ORCPT parameter when the SMTP server doesn't advertise
       support for UTF8SMTP;

   2.  or the SMTP server supports UTF8SMTP, but the address contains
       US-ASCII characters not permitted in the ORCPT parameter (e.g.,
       the ORCPT parameter forbids SP and the = characters);

   3.  or in a 7-bit transport environment including a message/
       delivery-status Original-Recipient or Final-Recipient field.

   The utf-8-addr-xtext form MUST be used instead in the first case; the
   utf-8-addr-unitext form MUST be used in the other two cases.  The
   utf-8-address form MAY be used in the ORCPT parameter when the SMTP
   server also advertises support for UTF8SMTP and the address doesn't
   contain any US-ASCII characters not permitted in the ORCPT parameter;

   in a message/global-delivery-status Original-Recipient or Final-
   Recipient DSN field; or in an Original-Recipient header field
   [RFC3798] if the message is a UTF8SMTP message.

   In addition, the utf-8-addr-unitext form can be used anywhere where
   the utf-8-address form is allowed.

6.2.  Update to 'smtp' Diagnostic Type Registration

   The mail diagnostic type registry was created by RFC 3464.  The
   registration for the 'smtp' diagnostic type should be updated to
   reference RFC 5337 in addition to RFC 3464.

   When the 'smtp' diagnostic type is used in the context of a message/
   delivery-status body part, it remains as presently defined.  When the
   'smtp' diagnostic type is used in the context of a message/
   global-delivery-status body part, the codes remain the same, but the
   text portion MAY contain UTF-8 characters.

6.3.  message/global-headers

   Type name:  message

   Subtype name:  global-headers

   Required parameters:  none

   Optional parameters:  none

   Encoding considerations:  This media type contains Internationalized
      Email Headers [RFC5335] with no message body.  Whenever possible,
      the 8-bit content transfer encoding SHOULD be used.  When this
      media type passes through a 7-bit-only SMTP infrastructure it MAY
      be encoded with the base64 or quoted-printable content transfer
      encoding.

   Security considerations:  See Section 7.

   Interoperability considerations:  It is important that this media
      type is not converted to a charset other than UTF-8.  As a result,
      implementations MUST NOT include a charset parameter with this
      media type.  Although it might be possible to downconvert this
      media type to the text/rfc822-header media type, such conversion
      is discouraged as it loses information.

   Published specification:  RFC 5337
   Applications that use this media type:  UTF8SMTP servers and email
      clients that support multipart/report generation or parsing.

   Additional information:

   Magic number(s):  none

   File extension(s):  In the event this is saved to a file, the
      extension ".u8hdr" is suggested.

   Macintosh file type code(s):  The 'TEXT' type code is suggested as
      files of this type are typically used for diagnostic purposes and
      suitable for analysis in a UTF-8 aware text editor.  A uniform
      type identifier (UTI) of "public.utf8-email-message-header" is
      suggested.  This type conforms to "public.utf8-plain-text" and
      "public.plain-text".

   Person & email address to contact for further information:  See the
      Authors' Addresses section of this document.

   Intended usage:  COMMON

   Restrictions on usage:  This media type contains textual data in the
      UTF-8 charset.  It typically contains octets with the 8th bit set.
      As a result, a transfer encoding is required when a 7-bit
      transport is used.

   Author:  See the Authors' Addresses section of this document.

   Change controller:  IETF Standards Process

6.4.  message/global-delivery-status

   Type name:  message

   Subtype name:  global-delivery-status

   Required parameters:  none

   Optional parameters:  none

   Encoding considerations:  This media type contains delivery status
      notification attributes in the UTF-8 charset.  The 8-bit content
      transfer encoding MUST be used with this content-type, unless it
      is sent over a 7-bit transport environment in which case quoted-
      printable or base64 may be necessary.

   Security considerations:  See Section 7
   Interoperability considerations:  This media type provides
      functionality similar to the message/delivery-status content-type
      for email message return information.  Clients of the previous
      format will need to be upgraded to interpret the new format;
      however, the new media type makes it simple to identify the
      difference.

   Published specification:  RFC 5337

   Applications that use this media type:  SMTP servers and email
      clients that support delivery status notification generation or
      parsing.

   Additional information:

   Magic number(s):  none

   File extension(s):  The extension ".u8dsn" is suggested.

   Macintosh file type code(s):  A uniform type identifier (UTI) of
      "public.utf8-email-message-delivery-status" is suggested.  This
      type conforms to "public.utf8-plain-text".

   Person & email address to contact for further information:  See the
      Authors' Addresses section of this document.

   Intended usage:  COMMON

   Restrictions on usage:  This is expected to be the second part of a
      multipart/report.

   Author:  See the Authors' Addresses section of this document.

   Change controller:  IETF Standards Process

6.5.  message/global-disposition-notification

   Type name:  message

   Subtype name:  global-disposition-notification

   Required parameters:  none

   Optional parameters:  none
   Encoding considerations:  This media type contains disposition
      notification attributes in the UTF-8 charset.  The 8-bit content
      transfer encoding MUST be used with this content-type, unless it
      is sent over a 7-bit transport environment in which case quoted-
      printable or base64 may be necessary.

   Security considerations:  See Section 7.

   Interoperability considerations:  This media type provides
      functionality similar to the message/disposition-notification
      content-type for email message disposition information.  Clients
      of the previous format will need to be upgraded to interpret the
      new format; however, the new media type makes it simple to
      identify the difference.

   Published specification:  RFC 5337

   Applications that use this media type:  Email clients or servers that
      support message disposition notification generation or parsing.

   Additional information:

   Magic number(s):  none

   File extension(s):  The extension ".u8mdn" is suggested.

   Macintosh file type code(s):  A uniform type identifier (UTI) of
      "public.utf8-email-message-disposition-notification" is suggested.
      This type conforms to "public.utf8-plain-text".

   Person & email address to contact for further information:  See the
      Authors' Addresses section of this document.

   Intended usage:  COMMON

   Restrictions on usage:  This is expected to be the second part of a
      multipart/report.

   Author:  See the Authors' Addresses section of this document.

   Change controller:  IETF Standards Process

7.  Security Considerations

   Automated use of report types without authentication presents several
   security issues.  Forging negative reports presents the opportunity
   for denial-of-service attacks when the reports are used for automated
   maintenance of directories or mailing lists.  Forging positive
   reports may cause the sender to incorrectly believe a message was
   delivered when it was not.

   Malicious users can generate report structures designed to trigger
   coding flaws in report parsers.  Report parsers need to use secure
   coding techniques to avoid the risk of buffer overflow or denial-of-
   service attacks against parser coding mistakes.  Code reviews of such
   parsers are also recommended.

   Malicious users of the email system regularly send messages with
   forged envelope return paths, and these messages trigger delivery
   status reports that result in a large amount of unwanted traffic on
   the Internet.  Many users choose to ignore delivery status
   notifications because they are usually the result of "blowback" from
   forged messages and thus never notice when messages they sent go
   undelivered.  As a result, support for correlation of delivery status
   and message disposition notification messages with sent-messages has
   become a critical feature of mail clients and possibly mail stores if
   the email infrastructure is to remain reliable.  In the short term,
   simply correlating message-IDs may be sufficient to distinguish true
   status notifications from those resulting from forged originator
   addresses.  But in the longer term, including cryptographic signature
   material that can securely associate the status notification with the
   original message is advisable.

   As this specification permits UTF-8 in additional fields, the
   security considerations of UTF-8 [RFC3629] apply.

8.  References

8.1.  Normative References

   [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.

   [RFC3462]      Vaudreuil, G., "The Multipart/Report Content Type for
                  the Reporting of Mail System Administrative Messages",
                  RFC 3462, 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", STD 63, RFC 3629, November 2003.

   [RFC3798]      Hansen, T. and G. Vaudreuil, "Message Disposition
                  Notification", RFC 3798, May 2004.

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

   [RFC5335]      Yang, A., Ed., "Internationalized Email Headers",
                  RFC 5335, September 2008.

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

   [LANGTAGS]     Phillips, A. and M. Davis, "Tags for Identifying
                  Languages", RFC 4646, September 2006.

   [DEFAULTLANG]  Alvestrand, H., "IETF Policy on Character Sets and
                  Languages", RFC 2277, January 1998.

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.

   [RFC2046]      Freed, N. and N. Borenstein, "Multipurpose Internet
                  Mail Extensions (MIME) Part Two: Media Types",
                  RFC 2046, November 1996.

   [DOWNGRADE]    Fujiwara, K. and Y. Yoneya, "Downgrading mechanism for
                  Email Address Internationalization", Work in Progress,
                  July 2008.

Appendix A.  Acknowledgements

   Many thanks for input provided by Pete Resnick, James Galvin, Ned
   Freed, John Klensin, Harald Alvestrand, Frank Ellermann, SM, and
   members of the EAI WG to help solidify this proposal.

Authors' Addresses

   Chris Newman
   Sun Microsystems
   800 Royal Oaks
   Monrovia, CA  91016-6347
   US

   EMail: chris.newman@sun.com


   Alexey Melnikov (editor)
   Isode Ltd
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK

   EMail: Alexey.Melnikov@isode.com

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