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rfc:rfc5337

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.

Newman & Melnikov Experimental [Page 1] RFC 5337 Internationalized DSN and MDNs September 2008

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

Newman & Melnikov Experimental [Page 2] RFC 5337 Internationalized DSN and MDNs September 2008

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

Newman & Melnikov Experimental [Page 3] RFC 5337 Internationalized DSN and MDNs September 2008

 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].

Newman & Melnikov Experimental [Page 4] RFC 5337 Internationalized DSN and MDNs September 2008

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"

Newman & Melnikov Experimental [Page 5] RFC 5337 Internationalized DSN and MDNs September 2008

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.

Newman & Melnikov Experimental [Page 6] RFC 5337 Internationalized DSN and MDNs September 2008

  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

Newman & Melnikov Experimental [Page 7] RFC 5337 Internationalized DSN and MDNs September 2008

 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.

Newman & Melnikov Experimental [Page 8] RFC 5337 Internationalized DSN and MDNs September 2008

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.

Newman & Melnikov Experimental [Page 9] RFC 5337 Internationalized DSN and MDNs September 2008

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;

Newman & Melnikov Experimental [Page 10] RFC 5337 Internationalized DSN and MDNs September 2008

 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

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 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

Newman & Melnikov Experimental [Page 12] RFC 5337 Internationalized DSN and MDNs September 2008

 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

Newman & Melnikov Experimental [Page 13] RFC 5337 Internationalized DSN and MDNs September 2008

 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

Newman & Melnikov Experimental [Page 14] RFC 5337 Internationalized DSN and MDNs September 2008

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.

Newman & Melnikov Experimental [Page 15] RFC 5337 Internationalized DSN and MDNs September 2008

 [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.

Newman & Melnikov Experimental [Page 16] RFC 5337 Internationalized DSN and MDNs September 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

Newman & Melnikov Experimental [Page 17] RFC 5337 Internationalized DSN and MDNs September 2008

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