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RFC 8098

Message Disposition Notification

Pages: 37
Internet Standard: 85
Obsoletes:  3798
Updates:  20463461
Part 1 of 2 – Pages 1 to 21
None   None   Next

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Internet Engineering Task Force (IETF)                    T. Hansen, Ed.
Request for Comments: 8098                             AT&T Laboratories
STD: 85                                                 A. Melnikov, Ed.
Obsoletes: 3798                                                Isode Ltd
Updates: 2046, 3461                                        February 2017
Category: Standards Track
ISSN: 2070-1721


                    Message Disposition Notification

Abstract

This memo defines a MIME content type that may be used by a Mail User Agent (MUA) or electronic mail gateway to report the disposition of a message after it has been successfully delivered to a recipient. This content type is intended to be machine processable. Additional message header fields are also defined to permit Message Disposition Notifications (MDNs) to be requested by the sender of a message. The purpose is to extend Internet Mail to support functionality often found in other messaging systems, such as X.400 and the proprietary "LAN-based" systems, and are often referred to as "read receipts," "acknowledgements," or "receipt notifications." The intention is to do this while respecting privacy concerns, which have often been expressed when such functions have been discussed in the past. Because many messages are sent between the Internet and other messaging systems (such as X.400 or the proprietary "LAN-based" systems), the MDN protocol is designed to be useful in a multiprotocol messaging environment. To this end, the protocol described in this memo provides for the carriage of "foreign" addresses, in addition to those normally used in Internet Mail. Additional attributes may also be defined to support "tunneling" of foreign notifications through Internet Mail. This document is an Internet Standard. It obsoletes RFC 3798 and updates RFC 2046 (message/partial media type handling) and RFC 3461 (Original-Recipient header field generation requirement).
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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 7841.

   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/rfc8098.

Copyright Notice

   Copyright (c) 2017 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.
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Purposes . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Requirements . . . . . . . . . . . . . . . . . . . . . . 4 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 2. Requesting Message Disposition Notifications . . . . . . . . 5 2.1. The Disposition-Notification-To Header . . . . . . . . . 5 2.2. The Disposition-Notification-Options Header . . . . . . . 8 2.3. The Original-Recipient Header Field . . . . . . . . . . . 9 2.4. Use with the Message/Partial Media Type . . . . . . . . . 10 3. Format of a Message Disposition Notification . . . . . . . . 10 3.1. The Message/Disposition-Notification Media Type . . . . . 12 3.2. Message/Disposition-Notification Content Fields . . . . . 15 3.3. Extension-Fields . . . . . . . . . . . . . . . . . . . . 21 4. Timeline of Events . . . . . . . . . . . . . . . . . . . . . 22 5. Conformance and Usage Requirements . . . . . . . . . . . . . 23 6. Security Considerations . . . . . . . . . . . . . . . . . . . 24 6.1. Forgery . . . . . . . . . . . . . . . . . . . . . . . . . 24 6.2. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 24 6.2.1. Disclosure of Product Information . . . . . . . . . . 25 6.2.2. MUA Fingerprinting . . . . . . . . . . . . . . . . . 25 6.3. Non-repudiation . . . . . . . . . . . . . . . . . . . . . 25 6.4. Mail Bombing . . . . . . . . . . . . . . . . . . . . . . 26 7. Collected ABNF Grammar . . . . . . . . . . . . . . . . . . . 26 8. Guidelines for Gatewaying MDNs . . . . . . . . . . . . . . . 29 8.1. Gatewaying from Other Mail Systems to MDNs . . . . . . . 29 8.2. Gatewaying from MDNs to Other Mail Systems . . . . . . . 29 8.3. Gatewaying of MDN-Requests to Other Mail Systems . . . . 30 9. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 10.1. Disposition-Notification-Options Header Field disposition-notification-parameter Names . . . . . . . . 32 10.2. Disposition Modifier Names . . . . . . . . . . . . . . . 33 10.3. MDN Extension Field Names . . . . . . . . . . . . . . . 33 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 33 11.1. Normative References . . . . . . . . . . . . . . . . . . 33 11.2. Informative References . . . . . . . . . . . . . . . . . 34 Appendix A. Changes from RFC 3798 . . . . . . . . . . . . . . . 36 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 37 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
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1. Introduction

This memo defines a media type [RFC2046] for Message Disposition Notifications (MDNs). An MDN can be used to notify the sender of a message of any of several conditions that may occur after successful delivery, such as display of the message contents, printing of the message, deletion (without display) of the message, or the recipient's refusal to provide MDNs. The "message/disposition- notification" content type defined herein is intended for use within the framework of the "multipart/report" content type defined in RFC-REPORT [RFC6522]. This memo defines the format of the notifications and the RFC-MSGFMT [RFC5322] header fields used to request them.

1.1. Purposes

The MDNs defined in this memo are expected to serve several purposes: a. Inform human beings of the disposition of messages after successful delivery in a manner that is largely independent of human language; b. Allow mail user agents to keep track of the disposition of messages sent by associating returned MDNs with earlier message transmissions; c. Convey disposition notification requests and disposition notifications between Internet Mail and "foreign" mail systems via a gateway; d. Allow "foreign" notifications to be tunneled through a MIME- capable messaging system and back into the original messaging system that issued the original notification, or even to a third messaging system; e. Allow language-independent, yet reasonably precise, indications of the disposition of a message to be delivered.

1.2. Requirements

These purposes place the following constraints on the notification protocol: a. It must be readable by humans and must be machine parsable.
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   b.  It must provide enough information to allow message senders (or
       their user agents) to unambiguously associate an MDN with the
       message that was sent and the original recipient address for
       which the MDN was issued (if such information is available), even
       if the message was forwarded to another recipient address.

   c.  It must also be able to describe the disposition of a message
       independent of any particular human language or of the
       terminology of any particular mail system.

   d.  The specification must be extensible in order to accommodate
       future requirements.

1.3. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-KEYWORDS [RFC2119]. All syntax descriptions use the ABNF specified by RFC-MSGFMT [RFC5322] in which the lexical tokens (used below) are defined: "CRLF", "FWS", "CFWS", "field-name", "mailbox-list", "msg-id", and "text". The following lexical token is defined in RFC-SMTP [RFC5321]: "Atom".

2. Requesting Message Disposition Notifications

Message disposition notifications are requested by including a Disposition-Notification-To header field in the message containing one or more addresses specifying where dispositions should be sent. Further information to be used by the recipient's Mail User Agent (MUA) [RFC5598] in generating the MDN may be provided by also including Original-Recipient and/or Disposition-Notification-Options header fields in the message.

2.1. The Disposition-Notification-To Header

A request for the receiving user agent to issue message disposition notifications is made by placing a Disposition-Notification-To header field into the message. The syntax of the header field is mdn-request-header = "Disposition-Notification-To" ":" mailbox-list CRLF A Disposition-Notification-To header field can appear in a message at most once.
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   The presence of a Disposition-Notification-To header field in a
   message is merely a request for an MDN.  The recipients' user agents
   are always free to silently ignore such a request.

   An MDN MUST NOT itself have a Disposition-Notification-To header
   field.  An MDN MUST NOT be generated in response to an MDN.

   A user agent MUST NOT issue more than one MDN on behalf of each
   particular recipient.  That is, once an MDN has been issued on behalf
   of a recipient, no further MDNs may be issued on behalf of that
   recipient by the same user agent, even if another disposition is
   performed on the message.  However, if a message is forwarded, an MDN
   may have been issued for the recipient doing the forwarding, and the
   recipient of the forwarded message may also cause an MDN to be
   generated.

   It is also possible that if the same message is being accessed by
   multiple user agents (for example, using POP3), then multiple
   dispositions might be generated for the same recipient.  User agents
   SHOULD leverage support in the underlying message access protocol to
   prevent multiple MDNs from being generated.  In particular, when the
   user agent is accessing the message using RFC-IMAP [RFC3501], it
   SHOULD implement the procedures specified in RFC-IMAP-MDN [RFC3503].

   While Internet standards normally do not specify the behavior of user
   interfaces, it is strongly recommended that the user agent obtain the
   user's consent before sending an MDN.  This consent could be obtained
   for each message through some sort of prompt or dialog box, or
   globally through the user's setting of a preference.  The user might
   also indicate globally that MDNs are to never be sent.  The purpose
   of obtaining user's consent is to protect user's privacy.  The
   default value should be not to send MDNs.

   MDNs MUST NOT be sent automatically if the address in the
   Disposition-Notification-To header field differs from the address in
   the Return-Path header field (see RFC-MSGFMT [RFC5322]).  In this
   case, confirmation from the user MUST be obtained, if possible.  If
   obtaining consent is not possible (e.g., because the user is not
   online at the time or the client is not an interactive email client),
   then an MDN MUST NOT be sent.

   Confirmation from the user MUST be obtained (or no MDN sent) if there
   is no Return-Path header field in the message or if there is more
   than one distinct address in the Disposition-Notification-To header
   field.
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   The comparison of the addresses is done using only the addr-spec
   (local-part "@" domain) portion, excluding any angle brackets,
   phrase, and route.  As prescribed by RFC 5322, the comparison is case
   sensitive for the local-part and case insensitive for the domain
   part.  The local-part comparison SHOULD be done after performing
   local-part canonicalization, i.e., after removing the surrounding
   double-quote characters, if any, as well as any escaping "\"
   characters.  (See RFC-MSGFMT [RFC5322] for more details.)
   Implementations MAY treat known domain aliases as equivalent for the
   purpose of comparison.

   Note that use of subaddressing (see [RFC5233]) can result in a
   failure to match two local-parts and thus result in possible
   suppression of the MDN.  This document doesn't recommend special
   handling for this case, as the receiving MUA can't reliably know
   whether or not the sender is using subaddressing.

   If the message contains more than one Return-Path header field, the
   implementation may pick one to use for the comparison or treat the
   situation as a failure of the comparison.

   The reason for not automatically sending an MDN if the comparison
   fails or more than one address is specified is to reduce the
   possibility of mail loops and of MDNs being used for mail bombing.

   It's especially important that a message that contains a Disposition-
   Notification-To header field also contain a Message-ID header field
   to permit user agents to automatically correlate MDNs with their
   original messages.

   If the request for message disposition notifications for some
   recipients and not others is desired, two copies of the message
   should be sent, one with a Disposition-Notification-To header field
   and one without.  Many of the other header fields of the message
   (e.g., To, Cc) will be the same in both copies.  The recipients in
   the respective message envelopes determine from whom message
   disposition notifications are requested and from whom they are not.
   If desired, the Message-ID header field may be the same in both
   copies of the message.  Note that there are other situations (e.g.,
   Bcc) in which it is necessary to send multiple copies of a message
   with slightly different header fields.  The combination of such
   situations and the need to request MDNs for a subset of all
   recipients may result in more than two copies of a message being
   sent, some with a Disposition-Notification-To header field and some
   without.
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   If it is possible to determine that a recipient is a newsgroup, do
   not include a Disposition-Notification-To header field for that
   recipient.  Similarly, if an existing message is resent or gatewayed
   to a newsgroup, the agent that is resending/gatewaying SHOULD strip
   the Disposition-Notification-To header field.  See Section 5 for more
   discussion.  Clients that see an otherwise valid Disposition-
   Notification-To header field in a newsgroup message SHOULD NOT
   generate an MDN.

2.2. The Disposition-Notification-Options Header

Extensions to this specification may require that information be supplied to the recipient's MUA for additional control over how and what MDNs are generated. The Disposition-Notification-Options header field provides an extensible mechanism for such information. The syntax of this header field is as follows: Disposition-Notification-Options = "Disposition-Notification-Options" ":" [FWS] disposition-notification-parameter-list CRLF disposition-notification-parameter-list = disposition-notification-parameter *([FWS] ";" [FWS] disposition-notification-parameter) disposition-notification-parameter = attribute [FWS] "=" [FWS] importance [FWS] "," [FWS] value *([FWS] "," [FWS] value) importance = "required" / "optional" attribute = Atom value = word A Disposition-Notification-Options header field can appear in a message at most once. An importance of "required" indicates that interpretation of the disposition-notification-parameter is necessary for proper generation of an MDN in response to this request. An importance of "optional" indicates that an MUA that does not understand the meaning of this disposition-notification-parameter MAY generate an MDN in response anyway, ignoring the value of the disposition-notification-parameter. No disposition-notification-parameter attribute names are defined in this specification. Attribute names may be defined in the future by later revisions or extensions to this specification. Disposition-
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   notification-parameter attribute names MUST be registered with the
   Internet Assigned Numbers Authority (IANA) using the "Specification
   Required" registration policy [RFC5226].  The "X-" prefix has
   historically been used to denote unregistered "experimental" protocol
   elements that are assumed not to become common use.  Deployment
   experience of this and other protocols has shown that this assumption
   is often false.  This document allows the use of the "X-" prefix
   primarily to allow the registration of attributes that are already in
   common use.  The prefix has no meaning for new attributes.  Its use
   in substantially new attributes may cause confusion and is therefore
   discouraged.  (See Section 10 for a registration form.)

2.3. The Original-Recipient Header Field

Since electronic mail addresses may be rewritten while the message is in transit, it is useful for the original recipient address to be made available by the delivering Message Transfer Agent (MTA) [RFC5598]. The delivering MTA may be able to obtain this information from the ORCPT parameter of the SMTP RCPT TO command, as defined in RFC-SMTP [RFC5321] and RFC-DSN-SMTP [RFC3461]. RFC-DSN-SMTP [RFC3461] is amended as follows: if the ORCPT information is available, the delivering MTA SHOULD insert an Original-Recipient header field at the beginning of the message (along with the Return-Path header field). The delivering MTA MAY delete any other Original-Recipient header fields that occur in the message. The syntax of this header field is as follows: original-recipient-header = "Original-Recipient" ":" OWS address-type OWS ";" OWS generic-address OWS OWS = [CFWS] ; Optional whitespace. ; MDN generators SHOULD use "*WSP" ; (Typically a single space or nothing. ; It SHOULD be nothing at the end of a field.), ; unless an RFC 5322 "comment" is required. ; ; MDN parsers MUST parse it as "[CFWS]". The address-type and generic-address tokens are as specified in the description of the Original-Recipient field in Section 3.2.3. The purpose of carrying the original recipient information and returning it in the MDN is to permit automatic correlation of MDNs with the original message on a per-recipient basis.
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2.4. Use with the Message/Partial Media Type

The use of the header fields Disposition-Notification-To, Disposition-Notification-Options, and Original-Recipient with the MIME message/partial content type (RFC-MIME-MEDIA [RFC2046]) requires further definition. When a message is segmented into two or more message/partial fragments, the three header fields mentioned in the above paragraph SHOULD be placed in the "inner" or "enclosed" message (using the terms of RFC-MIME-MEDIA [RFC2046]). If these header fields are found in the header fields of any of the fragments, they are ignored. When the multiple message/partial fragments are reassembled, the following applies. If these header fields occur along with the other header fields of a message/partial fragment message, they pertain to an MDN that will be generated for the fragment. If these header fields occur in the header fields of the "inner" or "enclosed" message (using the terms of RFC-MIME-MEDIA [RFC2046]), they pertain to an MDN that will be generated for the reassembled message. Section 5.2.2.1 of RFC-MIME-MEDIA [RFC2046]) is amended to specify that, in addition to the header fields specified there, the three header fields described in this specification are to be appended, in order, to the header fields of the reassembled message. Any occurrences of the three header fields defined here in the header fields of the initial enclosing message MUST NOT be copied to the reassembled message.

3. Format of a Message Disposition Notification

A message disposition notification is a MIME message with a top-level content type of multipart/report (defined in RFC-REPORT [RFC6522]). When multipart/report content is used to transmit an MDN: a. The report-type parameter of the multipart/report content is "disposition-notification". b. The first component of the multipart/report contains a human- readable explanation of the MDN, as described in RFC-REPORT [RFC6522]. c. The second component of the multipart/report is of content type message/disposition-notification, described in Section 3.1 of this document.
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   d.  If the original message or a portion of the message is to be
       returned to the sender, it appears as the third component of the
       multipart/report.  The decision of whether or not to return the
       message or part of the message is up to the MUA generating the
       MDN.  However, in the case of encrypted messages requesting MDNs,
       if the original message or a portion thereof is returned, it MUST
       be in its original encrypted form.

   NOTE: For message disposition notifications gatewayed from foreign
   systems, the header fields of the original message may not be
   available.  In this case, the third component of the MDN may be
   omitted, or it may contain "simulated" RFC-MSGFMT [RFC5322] header
   fields that contain equivalent information.  In particular, it is
   very desirable to preserve the subject and date fields from the
   original message.

   The MDN MUST be addressed (in both the message header field and the
   transport envelope) to the address(es) from the Disposition-
   Notification-To header field from the original message for which the
   MDN is being generated.

   The From header field of the MDN MUST contain the address of the
   person for whom the message disposition notification is being issued.

   The envelope sender address (i.e., SMTP "MAIL FROM") of the MDN MUST
   be null (<>), specifying that no Delivery Status Notification
   messages nor other messages indicating successful or unsuccessful
   delivery are to be sent in response to an MDN.

   A message disposition notification MUST NOT itself request an MDN.
   That is, it MUST NOT contain a Disposition-Notification-To header
   field.

   The Message-ID header field (if present) for an MDN MUST be different
   from the Message-ID of the message for which the MDN is being issued.

   A particular MDN describes the disposition of exactly one message for
   exactly one recipient.  Multiple MDNs may be generated as a result of
   one message submission, one per recipient.  However, due to the
   circumstances described in Section 2.1, it's possible that some of
   the recipients for whom MDNs were requested will not generate MDNs.
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3.1. The Message/Disposition-Notification Media Type

The message/disposition-notification media type is defined as follows: Type name: message Subtype name: disposition-notification Required parameters: none Optional parameters: none Encoding considerations: "7bit" encoding is sufficient and MUST be used to maintain readability when viewed by non-MIME mail readers. Security considerations: discussed in Section 6 of RFC 8098. Interoperability considerations: none Published specification: RFC 8098 Applications that use this media type: Mail Transfer Agents and email clients that support multipart/report generation and/or parsing. Fragment identifier considerations: N/A Additional information: Deprecated alias names for this type: N/A Magic number(s): none File extension(s): .disposition-notification 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 text editor. A Uniform Type Identifier (UTI) of "public.utf8- email-message-header" is suggested. This type conforms to "public.plain-text". Person & email address to contact for further information: ART Area Mailing List <art@ietf.org>
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   Intended usage:     COMMON

   Restrictions on usage:  This media type contains textual data in the
                       US-ASCII charset, which is always 7bit.

   Author:             See the Authors' Addresses section of RFC 8098.

   Change controller:  IETF

   Provisional registration?  no

   (While the 7bit restriction applies to the message/disposition-
   notification portion of the multipart/report content, it does not
   apply to the optional third portion of the multipart/report content.)

   The message/disposition-notification report type for use in the
   multipart/report is "disposition-notification".

   The body of a message/disposition-notification consists of one or
   more "fields" formatted according to the ABNF of RFC-MSGFMT [RFC5322]
   header "fields".  The syntax of the message/disposition-notification
   content is as follows:

   disposition-notification-content = [ reporting-ua-field CRLF ]
             [ mdn-gateway-field CRLF ]
             [ original-recipient-field CRLF ]
             final-recipient-field CRLF
             [ original-message-id-field CRLF ]
             disposition-field CRLF
             *( error-field CRLF )
             *( extension-field CRLF )

   extension-field = extension-field-name ":" *([FWS] text)

   extension-field-name = field-name

   Note that the order of the above fields is recommended but not fixed.
   Extension fields can appear anywhere.

3.1.1. General Conventions for Fields

Since these fields are defined according to the rules of RFC-MSGFMT [RFC5322], the same conventions for continuation lines and comments apply. Notification fields may be continued onto multiple lines by beginning each additional line with a SPACE or HTAB. Text that appears in parentheses is considered a comment and not part of the contents of that notification field. Field names are case insensitive, so the names of notification fields may be spelled in
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   any combination of uppercase and lowercase letters.  RFC-MSGFMT
   [RFC5322] comments in notification fields may use the "encoded-word"
   construct defined in RFC-MIME-HEADER [RFC2047].

3.1.2. "*-type" Subfields

Several fields consist of a "-type" subfield, followed by a semi- colon, followed by "*text". For these fields, the keyword used in the address-type or MTA-type subfield indicates the expected format of the address or MTA-name that follows. The "-type" subfields are defined as follows: a. An "address-type" specifies the format of a mailbox address. For example, Internet Mail addresses use the "rfc822" address-type. Other values can appear in this field as specified in the "Address Types" IANA subregistry established by RFC-DSN-FORMAT [RFC3464]. address-type = Atom Atom = <The version from RFC 5321 (not from RFC 5322) is used in this document.> b. An "MTA-name-type" specifies the format of a mail transfer agent name. For example, for an SMTP server on an Internet host, the MTA name is the domain name of that host, and the "dns" MTA-name- type is used. Other values can appear in this field as specified in the "MTA Name Types" IANA subregistry established by RFC-DSN- FORMAT [RFC3464]. mta-name-type = Atom Values for address-type and mta-name-type are case insensitive. Thus, address-type values of "RFC822" and "rfc822" are equivalent. The Internet Assigned Numbers Authority (IANA) maintains a registry of address-type and mta-name-type values, along with descriptions of the meanings of each or a reference to one or more specifications that provide such descriptions. (The "rfc822" address-type is defined in RFC-DSN-SMTP [RFC3461].) Registration forms for address- type and mta-name-type appear in RFC-DSN-FORMAT [RFC3464].
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3.2. Message/Disposition-Notification Content Fields

3.2.1. The Reporting-UA Field

reporting-ua-field = "Reporting-UA" ":" OWS ua-name OWS [ ";" OWS ua-product OWS ] ua-name = *text-no-semi ua-product = *([FWS] text) text-no-semi = %d1-9 / ; "text" characters excluding NUL, CR, %d11 / %d12 / %d14-58 / %d60-127 ; LF, or semi-colon The Reporting-UA field is defined as follows: An MDN describes the disposition of a message after it has been delivered to a recipient. In all cases, the Reporting-UA is the MUA that performed the disposition described in the MDN. The "Reporting-UA" field contains information about the MUA that generated the MDN, which is often used by servers to help identify the scope of reported interoperability problems, to work around or tailor responses to avoid particular MUA limitations, and for analytics regarding MUA or operating system use. An MUA SHOULD send a "Reporting-UA" field unless specifically configured not to do so. If the reporting MUA consists of more than one component (e.g., a base program and plug-ins), this may be indicated by including a list of product names. A reporting MUA SHOULD limit generated product identifiers to what is necessary to identify the product; a sender MUST NOT generate advertising or other nonessential information within the product identifier. A reporting MUA SHOULD NOT generate a "Reporting-UA" field containing needlessly fine-grained detail and SHOULD limit the addition of subproducts by third parties. Overly long and detailed "Reporting- UA" field values increase the risk of a user being identified against their wishes ("fingerprinting"). Likewise, implementations are encouraged not to use the product tokens of other implementations in order to declare compatibility with them, as this circumvents the purpose of the field. If an MUA masquerades as a different MUA, recipients can assume that the user
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   intentionally desires to see responses tailored for that identified
   MUA, even if they might not work as well for the actual MUA being
   used.

   Example:

   Reporting-UA:  Foomail 97.1

3.2.2. The MDN-Gateway Field

The MDN-Gateway field indicates the name of the gateway or MTA that translated a foreign (non-Internet) message disposition notification into this MDN. This field MUST appear in any MDN that was translated by a gateway from a foreign system into MDN format and MUST NOT appear otherwise. mdn-gateway-field = "MDN-Gateway" ":" OWS mta-name-type OWS ";" OWS mta-name OWS mta-name = *text For gateways into Internet Mail, the MTA-name-type will normally be "dns", and the mta-name will be the Internet domain name of the gateway.

3.2.3. Original-Recipient Field

The Original-Recipient field indicates the original recipient address as specified by the sender of the message for which the MDN is being issued. For Internet Mail messages, the value of the Original- Recipient field is obtained from the Original-Recipient header field from the message for which the MDN is being generated. If there is an Original-Recipient header field in the message, or if information about the original recipient is reliably available some other way, then the Original-Recipient field MUST be included. Otherwise, the Original-Recipient field MUST NOT be included. If there is more than one Original-Recipient header field in the message, the MUA may choose the one to use or act as if no Original-Recipient header field is present. original-recipient-field = "Original-Recipient" ":" OWS address-type OWS ";" OWS generic-address OWS generic-address = *text The address-type field indicates the type of the original recipient address. If the message originated within the Internet, the address-
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   type field will normally be "rfc822", and the address will be
   according to the syntax specified in RFC-MSGFMT [RFC5322].  The value
   "unknown" should be used if the Reporting MUA cannot determine the
   type of the original recipient address from the message envelope.
   This address is the same as that provided by the sender and can be
   used to automatically correlate MDN reports with original messages on
   a per-recipient basis.

3.2.4. Final-Recipient Field

The Final-Recipient field indicates the recipient for which the MDN is being issued. This field MUST be present. The syntax of the field is as follows: final-recipient-field = "Final-Recipient" ":" OWS address-type OWS ";" OWS generic-address OWS The generic-address subfield of the Final-Recipient field SHOULD contain the mailbox address of the recipient (which will be the same as the From header field of the MDN) as it was when the MDN was generated by the MUA. One example of when this field might not contain the final recipient address of the message is when an alias (e.g., <customer-support@example.com>) forwards mail to a specific personal address (e.g., <bob@example.com>). Bob might want to be able to send MDNs but not give away his personal email address. In this case, the Final-Recipient field can contain: Final-Recipient: rfc822;customer-support@example.com in place of: Final-Recipient: rfc822;bob@example.com The Final-Recipient address may differ from the address originally provided by the sender, because it may have been transformed during forwarding and gatewaying into a totally unrecognizable mess. However, in the absence of the optional Original-Recipient field, the Final-Recipient field and any returned content may be the only information available with which to correlate the MDN with a particular message recipient. The address-type subfield indicates the type of address expected by the reporting MTA in that context. Recipient addresses obtained via SMTP will normally be of address-type "rfc822", but can be other
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   values from the "Address Types" subregistry of the "Delivery Status
   Notification (DSN) Types" IANA registry.

   Since mailbox addresses (including those used in the Internet) may be
   case sensitive, the case of alphabetic characters in the address MUST
   be preserved.

3.2.5. Original-Message-ID Field

The Original-Message-ID field indicates the message-ID of the message for which the MDN is being issued. It is obtained from the Message-ID header field of the message for which the MDN is issued. This field MUST be present if and only if the original message contained a Message-ID header field. The syntax of the field is as follows: original-message-id-field = "Original-Message-ID" ":" msg-id The msg-id token is as specified in RFC-MSGFMT [RFC5322].

3.2.6. Disposition Field

The Disposition field indicates the action performed by the Reporting MUA on behalf of the user. This field MUST be present. The syntax for the Disposition field is: disposition-field = "Disposition" ":" OWS disposition-mode OWS ";" OWS disposition-type [ OWS "/" OWS disposition-modifier *( OWS "," OWS disposition-modifier ) ] OWS disposition-mode = action-mode OWS "/" OWS sending-mode action-mode = "manual-action" / "automatic-action" sending-mode = "MDN-sent-manually" / "MDN-sent-automatically" disposition-type = "displayed" / "deleted" / "dispatched" / "processed" disposition-modifier = "error" / disposition-modifier-extension disposition-modifier-extension = Atom
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   The disposition-mode, disposition-type, and disposition-modifier
   values may be spelled in any combination of uppercase and lowercase
   US-ASCII characters.

3.2.6.1. Disposition Modes
Disposition mode consists of two parts: action mode and sending mode. The following action modes are defined: "manual-action" The disposition described by the disposition type was a result of an explicit instruction by the user rather than some sort of automatically performed action. (This might include the case when the user has manually configured her MUA to automatically respond to valid MDN requests.) Unless prescribed otherwise in a particular mail environment, in order to preserve the user's privacy, this MUST be the default for MUAs. "automatic-action" The disposition described by the disposition type was a result of an automatic action rather than an explicit instruction by the user for this message. This is typically generated by a Mail Delivery Agent (e.g., MDN generations by Sieve reject action [RFC5429], Fax-over-Email [RFC3249], voice message system (see Voice Profile for Internet Mail (VPIM) [RFC3801]), or upon delivery to a mailing list). "Manual-action" and "automatic-action" are mutually exclusive. One or the other MUST be specified. The following sending modes are defined: "MDN-sent-manually" The user explicitly gave permission for this particular MDN to be sent. Unless prescribed otherwise in a particular mail environment, in order to preserve the user's privacy, this MUST be the default for MUAs. "MDN-sent-automatically" The MDN was sent because the MUA had previously been configured to do so automatically. "MDN-sent-manually" and "MDN-sent-automatically" are mutually exclusive. One or the other MUST be specified.
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3.2.6.2. Disposition Types
The following disposition-types are defined: "displayed" The message has been displayed by the MUA to someone reading the recipient's mailbox. There is no guarantee that the content has been read or understood. "dispatched" The message has been sent somewhere in some manner (e.g., printed, faxed, forwarded) without necessarily having been previously displayed to the user. The user may or may not see the message later. "processed" The message has been processed in some manner (i.e., by some sort of rules or server) without being displayed to the user. The user may or may not see the message later, or there may not even be a human user associated with the mailbox. "deleted" The message has been deleted. The recipient may or may not have seen the message. The recipient might "undelete" the message at a later time and read the message.
3.2.6.3. Disposition Modifiers
Only the extension disposition modifiers are defined: disposition-modifier-extension Disposition modifiers may be defined in the future by later revisions or extensions to this specification. MDN disposition value names MUST be registered with the Internet Assigned Numbers Authority (IANA) using the "Specification Required" registration policy. (See Section 10 for a registration form.) MDNs with disposition modifier names not understood by the receiving MUA MAY be silently ignored or placed in the user's mailbox without special interpretation. They MUST NOT cause any error message to be sent to the sender of the MDN. It is not required that an MUA be able to generate all of the possible values of the Disposition field.
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   A user agent MUST NOT issue more than one MDN on behalf of each
   particular recipient.  That is, once an MDN has been issued on behalf
   of a recipient, no further MDNs may be issued on behalf of that
   recipient, even if another disposition is performed on the message.
   However, if a message is forwarded, a "dispatched" MDN MAY be issued
   for the recipient doing the forwarding and the recipient of the
   forwarded message may also cause an MDN to be generated.

3.2.7. Error Field

The Error field is used to supply additional information in the form of text messages when the "error" disposition modifier appears. The syntax is as follows: error-field = "Error" ":" *([FWS] text) Note that syntax of these header fields doesn't include comments, so the "encoded-word" construct defined in RFC-MIME-HEADER [RFC2047] can't be used to convey non-ASCII text. Applications that need to convey non-ASCII text in these fields should consider implementing the message/global-disposition-notification media type specified in [RFC6533] instead of this specification.

3.3. Extension-Fields

Additional MDN fields may be defined in the future by later revisions or extensions to this specification. MDN field names MUST be registered with the Internet Assigned Numbers Authority (IANA) using the "Specification Required" registration policy. (See Section 10 for a registration form.) MDN Extension-fields may be defined for the following reasons: a. To allow additional information from foreign disposition reports to be tunneled through Internet MDNs. The names of such MDN fields should begin with an indication of the foreign environment name (e.g., X400-Physical-Forwarding-Address). b. To allow transmission of diagnostic information that is specific to a particular Mail User Agent (MUA). The names of such MDN fields should begin with an indication of the MUA implementation that produced the MDN (e.g., Foomail-information).


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