Internet Engineering Task Force (IETF) T. Hansen, Ed.

Request for Comments: 6533 AT&T Laboratories

Obsoletes: 5337 C. Newman

Updates: 3461, 3464, 3798, 6522 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

(RFC 3461, RFC 3464, RFC 6522) 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 RFC 3461, RFC 3464, RFC 3798, and RFC 6522.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force

(IETF). It represents the consensus of the IETF community. It has

received public review and has been approved for publication by the

Internet Engineering Steering Group (IESG). Further information on

Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,

and how to provide feedback on it may be obtained at

http://www.rfc-editor.org/info/rfc6533.

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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 RFC 5337 . . . . . . . . . . . . . . . 18

Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 18

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1. Introduction

When an email message is transmitted using the SMTPUTF8 [RFC6531]

extension and Internationalized Email Headers [RFC6532], 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.

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 [RFC5337].

Section 6 of [RFC6530] 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 [RFC2119].

The formal syntax uses the Augmented Backus-Naur Form (ABNF)

[RFC5234] notation including the core rules defined in Appendix B of

[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" [RFC6532] 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 [RFC5321] server that advertises both the SMTPUTF8 extension

[RFC6531] 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 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 [RFC3798] 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 [RFC3461], 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 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 = Mailbox

; Mailbox as defined in [RFC6531].

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 [RFC3461]

; 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 [RFC3461]

; 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 [RFC3464] comes in a

three-part multipart/report [RFC6522] 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. ([RFC5337] 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 [RFC6531]; the "Diagnostic-Code:" field SHOULD

be in i-default language [RFC2277]. 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 [RFC5646] 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]. Note that <text-fixed> is the same as <text> from

[RFC5322], but without <obs-text>. If or when RFC 5322 is updated to

disallow <obs-text>, <text-fixed> should become just <text>. Also,

if or when RFC 5322 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 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 ]

[ 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 RFC 3464.

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generic-address =/ utf-8-enc-addr

; Only allowed with the "utf-8" address-type.

; Updates Section 3.2.3 of RFC 3798.

;

; 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 [RFC5646].

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

; Updates Section 7 of RFC3798

text-fixed = �-9 / ; Any ASCII character except for NUL,

�1 / ; CR, and LF.

�2 / ; See note above about <text-fixed>

�4-127

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 [RFC6532].

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 [RFC6522] 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 [RFC6532] relaxed a restriction from MIME [RFC2046]

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"

[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

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language [RFC2277].) 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 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 [RFC3464]. 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 [RFC3464] and

updated by [RFC5337]. This specification replaces [RFC5337]. The

registration for the 'smtp' diagnostic type has been updated to

reference RFC 6533 in addition to [RFC3464] and to remove the

reference to [RFC5337].

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 [RFC6532] 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: RFC 6533

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: RFC 6533

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: RFC 6533

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 [RFC3629] apply.

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8. References

8.1. Normative References

[ASCII] American National Standards Institute (formerly United

States of America Standards Institute), "USA Code for

Information Interchange", ANSI X3.4-1968, 1968.

ANSI X3.4-1968 has been replaced by newer versions with

slight modifications, but the 1968 version remains

definitive for the Internet.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and

Languages", BCP 18, RFC 2277, January 1998.

[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service

Extension for Delivery Status Notifications (DSNs)",

RFC 3461, 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.

[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,

October 2008.

[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,

October 2008.

[RFC5646] Phillips, A. and M. Davis, "Tags for Identifying

Languages", BCP 47, RFC 5646, September 2009.

[RFC6522] Kucherawy, M., Ed., "The Multipart/Report Media Type for

the Reporting of Mail System Administrative Messages", STD

73, RFC 6522, January 2012.

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[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for

Internationalized Email", RFC 6530, February 2012.

[RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized

Email", RFC 6531, February 2012.

[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized

Email Headers", RFC 6532, February 2012.

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.

[RFC5337] Newman, C. and A. Melnikov, "Internationalized Delivery

Status and Disposition Notifications", RFC 5337,

September 2008.

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Appendix A. Changes since RFC 5337

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 [RFC6531].

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.