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⇱ RFC 6533: Internationalized Delivery Status and Disposition Notifications | RFC Editor

RFC 6533 Internationalized Delivery Status and Disposition Notifications

  • T. Hansen, Ed.,  
  • C. Newman,  
  • A. Melnikov
Proposed Standard
Internet Engineering Task Force (IETF) T. Hansen, Ed.
Request for Comments: 6533 AT&T Laboratories
Obsoletes:  C. Newman
Updates: , , ,  Oracle
Category: Standards Track A. Melnikov
ISSN: 2070-1721 Isode Ltd
 February 2012


 Internationalized Delivery Status and Disposition Notifications

Abstract

 Delivery status notifications (DSNs) are critical to the correct
 operation of an email system. However, the existing Draft Standards
 (, , ) are presently limited to 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-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 extends , , , and .

Status of This Memo

 This is an Internet Standards Track document.

 This document is a product of the Internet Engineering Task Force
 (IETF). It represents the consensus of the IETF community. It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG). Further information on
 Internet Standards is available in .

 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 .













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Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors. All rights reserved.

 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document. Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document. Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

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. The message/global-delivery-status Media Type . . . . . . 6
 4.2. The message/global Media Type . . . . . . . . . . . . . . 8
 4.3. The message/global-headers Media Type . . . . . . . . . . 8
 4.4. Using These Media Types with multipart/report . . . . . . 8
 4.5. Additional Requirements on SMTP Servers . . . . . . . . . 9
 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 . . . . . . . . . 14
 7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
 8.1. Normative References . . . . . . . . . . . . . . . . . . . 16
 8.2. Informative References . . . . . . . . . . . . . . . . . . 17
 Appendix A. Changes since  . . . . . . . . . . . . . . . 18
 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 18












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1. Introduction

 When an email message is transmitted using the SMTPUTF8 []
 extension and Internationalized Email Headers [], it is
 sometimes necessary to return that message or generate a Message
 Disposition Notification (MDN) []. 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.

 NOTE: While this specification updates the experimental versions of
 this protocol by removing certain constructs (e.g., the "<addr
 <addr>>" address syntax is no longer permitted), the name of the
 Address Type "UTF-8" and the media type names message/global,
 message/global-delivery-status, and message/global-headers have not
 been changed.

 This specification is a revision of and replacement for [].
  describes the change in approach between this
 specification and the previous version.

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 [].

 The formal syntax uses the Augmented Backus-Naur Form (ABNF)
 [] notation including the core rules defined in  and the UTF-8 syntax rules in .

3. UTF-8 Address Type

 "An Extensible Message Format for Delivery Status Notifications"
 [] defines the concept of an address type. The address format
 introduced in "Internationalized Email Headers" [] 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 [] server that advertises both the SMTPUTF8 extension
 [] and the DSN extension [] 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 SMTPUTF8.



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 This address type has 3 forms: utf-8-addr-xtext, utf-8-addr-unitext,
 and utf-8-address. Only the first form is 7-bit safe (only uses
 ASCII characters [ASCII]).

 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:

 1. in the ORCPT parameter when the SMTP server doesn't advertise
 support for SMTPUTF8 (utf-8-addr-xtext MUST be used instead); or

 2. if the SMTP server supports SMTPUTF8, but the address contains
 ASCII characters not permitted in the ORCPT parameter (e.g., the
 ORCPT parameter forbids unencoded SP and the '=' character),
 (either utf-8-addr-unitext or utf-8-addr-xtext MUST be used
 instead); or

 3. in a 7-bit transport environment including a message/
 delivery-status "Original-Recipient:" or "Final-Recipient:"
 field, (utf-8-addr-xtext MUST be used instead).

 The utf-8-address form MAY be used in the ORCPT parameter when the
 SMTP server also advertises support for SMTPUTF8 and the address
 doesn't contain any 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 [] if the message is a
 SMTPUTF8 message.

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

 When used in the ORCPT parameter, the UTF-8 address type requires
 that ASCII CTLs, SP, '\', '+', and '=' be encoded using 'unitext'
 encoding (see below). This is described by the utf-8-addr-xtext and
 utf-8-addr-unitext forms in the ABNF below. The 'unitext' encoding
 uses "\x{HEXPOINT}" syntax (EmbeddedUnicodeChar in the ABNF below)
 for encoding any Unicode character outside of ASCII range, as well as
 for encoding CTLs, SP, '\', '+', and '='. HEXPOINT is 2 to 6
 hexadecimal digits. This encoding avoids the need to use the xtext
 encoding described in [], as any ASCII characters that need to
 be escaped using xtext encoding never appear in any unitext-encoded
 string. When sending data to a SMTPUTF8-capable server, native UTF-8
 characters SHOULD be used instead of the EmbeddedUnicodeChar syntax
 described below. When sending data to an SMTP server that does not
 advertise SMTPUTF8, then the EmbeddedUnicodeChar syntax MUST be used
 instead of UTF-8.




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 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 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 and avoided
 when possible. It is hoped that as systems lacking support for
 SMTPUTF8 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 , in , or in
 [].

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

 utf-8-address = Mailbox
 ; Mailbox as defined in [].

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

 utf-8-addr-xtext = 1*(QCHAR / EmbeddedUnicodeChar)
 ; 7bit form of utf-8-addr-unitext.
 ; Safe for use in the ORCPT []
 ; parameter even when SMTPUTF8 SMTP
 ; extension is not advertised.

 utf-8-addr-unitext = 1*(QUCHAR / EmbeddedUnicodeChar)
 ; MUST follow utf-8-address ABNF when
 ; dequoted.
 ; Safe for using in the ORCPT []
 ; parameter when SMTPUTF8 SMTP extension
 ; is also advertised.

 QCHAR = %x21-2a / %x2c-3c / %x3e-5b / %x5d-7e
 ; ASCII printable characters except
 ; CTLs, SP, '\', '+', '='.







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 QUCHAR = QCHAR / UTF8-2 / UTF8-3 / UTF8-4
 ; ASCII printable characters except
 ; CTLs, SP, '\', '+' and '=', plus
 ; other Unicode characters encoded in UTF-8

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

 HEXPOINT = ( ( "0"/"1" ) %x31-39 ) / "10" / "20" /
 "2B" / "3D" / "7F" / ; all xtext-specials
 "5C" / (HEXDIG8 HEXDIG) / ; 2-digit forms
 ( NZHEXDIG 2(HEXDIG) ) / ; 3-digit forms
 ( NZDHEXDIG 3(HEXDIG) ) / ; 4-digit forms excluding
 ( "D" %x30-37 2(HEXDIG) ) / ; ... surrogate
 ( NZHEXDIG 4(HEXDIG) ) / ; 5-digit forms
 ( "10" 4*HEXDIG ) ; 6-digit forms
 ; represents either "\" or a Unicode code point outside
 ; the 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 [] comes in a
 three-part multipart/report [] 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 standard DSN format does not permit the
 return of undeliverable SMTPUTF8 messages, three new media types have
 been defined. ([] introduced experimental versions of these
 media types.)

4.1. The message/global-delivery-status Media Type

 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 SMTPUTF8 []; the "Diagnostic-Code:" field SHOULD
 be in i-default language []. Second, systems generating a
 message/global-delivery-status body part SHOULD use the utf-8-address



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 form of the UTF-8 address type for all addresses containing
 characters outside the 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,
 an optional field called "Localized-Diagnostic:" is added. Each
 instance includes a language tag [] 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 , in , or in
 []. Note that <text-fixed> is the same as <text> from
 [], but without <obs-text>. If or when  is updated to
 disallow <obs-text>, <text-fixed> should become just <text>. Also,
 if or when  is updated to disallow control characters in
 <text>, <text-fixed> should become a reference to that update
 instead.

 utf-8-delivery-status-content = per-message-fields
 1*( CRLF utf-8-per-recipient-fields )
 ; "per-message-fields" remains unchanged from the definition
 ; in , 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 ]
 [ final-log-id-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 .








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 generic-address =/ utf-8-enc-addr
 ; Only allowed with the "utf-8" address-type.
 ; Updates .
 ;
 ; 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 [].

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

 text-fixed = %d1-9 / ; Any ASCII character except for NUL,
 %d11 / ; CR, and LF.
 %d12 / ; See note above about <text-fixed>
 %d14-127

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

 UTF8-non-ascii = UTF8-2 / UTF8-3 / UTF8-4

4.2. The message/global Media Type

 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 [].

4.3. The message/global-headers Media Type

 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 SMTPUTF8 message).
 Unlike message/global, this body part provides no difficulties for
 the present infrastructure.

4.4. Using These Media Types with multipart/report

 Note that as far as a multipart/report [] 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,





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 implementations processing multipart/report MUST expect any
 combinations of the 6 media types mentioned above inside a multipart/
 report media 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 a 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 [] relaxed a restriction from MIME []
 regarding the use of Content-Transfer-Encoding in new "message"
 subtypes. This specification explicitly allows the use of Content-
 Transfer-Encoding in message/global-headers and message/
 global-delivery-status. This is not believed to be problematic as
 these new media types are intended primarily for use by newer systems
 with full support for 8-bit MIME and UTF-8 headers.

4.5. Additional Requirements on SMTP Servers

 If an SMTP server that advertises both SMTPUTF8 and DSN needs to
 return an undeliverable SMTPUTF8 message, then it has two choices for
 encapsulating the SMTPUTF8 message when generating the corresponding
 multipart/report:

 If the return-path SMTP server does not support SMTPUTF8, then the
 undeliverable body part and headers MUST be encoded using a 7-bit
 Content-Transfer-Encoding such as "base64" or "quoted-printable"
 [], as detailed in Section 4.

 Otherwise, "8bit" Content-Transfer-Encoding can be used.

5. UTF-8 Message Disposition Notifications

 Message Disposition Notifications [] 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





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 language [].) 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 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 field, which is an address header field and thus follows the
 same 8-bit rules as other address header fields 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  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 []. The
 registration template response follows:

 (a) The address-type name.

 UTF-8





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 (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 ASCII repertoire, a specification for how
 they are to be encoded as graphic ASCII characters in an
 "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. Only
 the first form is 7-bit safe.

6.2. Update to 'smtp' Diagnostic Type Registration

 The mail diagnostic type registry was created by [] and
 updated by []. This specification replaces []. The
 registration for the 'smtp' diagnostic type has been updated to
 reference  in addition to [] and to remove the
 reference to [].

 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 [] 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.





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 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 down-convert this
 media type to the text/rfc822-header media type, such conversion
 is discouraged as it loses information.

 Published specification: 

 Applications that use this media type: SMTPUTF8 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




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 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: 

 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







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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: 

 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.




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 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 [] apply.














Hansen, et al. Standards Track [Page 15]
 Internationalized DSN and MDNs February 2012


8. References

8.1. Normative References

 [] American National Standards Institute (formerly United
 States of America Standards Institute), "USA Code for
 Information Interchange", ANSI X3.4-1968, 1968.

 ANSI X3.4-1968 has been replaced by newer versions with
 slight modifications, but the 1968 version remains
 definitive for the Internet.

 [] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, , March 1997.

 [] Alvestrand, H., "IETF Policy on Character Sets and
 Languages", BCP 18, , January 1998.

 [] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
 Extension for Delivery Status Notifications (DSNs)",
 , January 2003.

 [] Moore, K. and G. Vaudreuil, "An Extensible Message Format
 for Delivery Status Notifications", ,
 January 2003.

 [] Yergeau, F., "UTF-8, a transformation format of ISO
 10646", STD 63, , November 2003.

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

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

 [] Klensin, J., "Simple Mail Transfer Protocol", ,
 October 2008.

 [] Resnick, P., Ed., "Internet Message Format", ,
 October 2008.

 [] Phillips, A. and M. Davis, "Tags for Identifying
 Languages", BCP 47, , September 2009.

 [] Kucherawy, M., Ed., "The Multipart/Report Media Type for
 the Reporting of Mail System Administrative Messages", STD
 73, , January 2012.




Hansen, et al. Standards Track [Page 16]
 Internationalized DSN and MDNs February 2012


 [] Klensin, J. and Y. Ko, "Overview and Framework for
 Internationalized Email", , February 2012.

 [] Yao, J. and W. Mao, "SMTP Extension for Internationalized
 Email", , February 2012.

 [] Yang, A., Steele, S., and N. Freed, "Internationalized
 Email Headers", , February 2012.

8.2. Informative References

 [] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
 Extensions (MIME) Part One: Format of Internet Message
 Bodies", , November 1996.

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

 [] Newman, C. and A. Melnikov, "Internationalized Delivery
 Status and Disposition Notifications", ,
 September 2008.





























Hansen, et al. Standards Track [Page 17]
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Appendix A. Changes since 

 Changes were made to move from Experimental to Standards Track. The
 most significant was the removal of an embedded alternative ASCII
 address within a utf-8-address, and the reflections of the ABNF
 changes in [].

 Fixed description of utf-8-addr-xtext and utf-8-addr-unitext.

 References to Downgrade and uMailbox removed/fixed.

 ABNF changes and fixed errata submitted by Alfred Hoenes.

 Minor changes to MIME type references.

 Other minor corrections.

Appendix B. Acknowledgements

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




























Hansen, et al. Standards Track [Page 18]
 Internationalized DSN and MDNs February 2012


Authors' Addresses

 Tony Hansen (editor)
 AT&T Laboratories
 200 Laurel Ave.
 Middletown, NJ 07748
 US

 EMail: tony+eaidsn@maillennium.att.com


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

 EMail: chris.newman@oracle.com


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

 EMail: Alexey.Melnikov@isode.com























Hansen, et al. Standards Track [Page 19]
RFC 6533: Internationalized Delivery Status and Disposition Notifications
Proposed Standard