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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).

Hansen & Melnikov Standards Track [Page 1] RFC 8098 MDN February 2017

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.

Hansen & Melnikov Standards Track [Page 2] RFC 8098 MDN February 2017

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

Hansen & Melnikov Standards Track [Page 3] RFC 8098 MDN February 2017

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.

Hansen & Melnikov Standards Track [Page 4] RFC 8098 MDN February 2017

 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.

Hansen & Melnikov Standards Track [Page 5] RFC 8098 MDN February 2017

 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.

Hansen & Melnikov Standards Track [Page 6] RFC 8098 MDN February 2017

 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.

Hansen & Melnikov Standards Track [Page 7] RFC 8098 MDN February 2017

 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-

Hansen & Melnikov Standards Track [Page 8] RFC 8098 MDN February 2017

 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.

Hansen & Melnikov Standards Track [Page 9] RFC 8098 MDN February 2017

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.

Hansen & Melnikov Standards Track [Page 10] RFC 8098 MDN February 2017

 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.

Hansen & Melnikov Standards Track [Page 11] RFC 8098 MDN February 2017

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>

Hansen & Melnikov Standards Track [Page 12] RFC 8098 MDN February 2017

 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

Hansen & Melnikov Standards Track [Page 13] RFC 8098 MDN February 2017

 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].

Hansen & Melnikov Standards Track [Page 14] RFC 8098 MDN February 2017

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

Hansen & Melnikov Standards Track [Page 15] RFC 8098 MDN February 2017

 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-

Hansen & Melnikov Standards Track [Page 16] RFC 8098 MDN February 2017

 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

Hansen & Melnikov Standards Track [Page 17] RFC 8098 MDN February 2017

 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

Hansen & Melnikov Standards Track [Page 18] RFC 8098 MDN February 2017

 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.

Hansen & Melnikov Standards Track [Page 19] RFC 8098 MDN February 2017

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.

Hansen & Melnikov Standards Track [Page 20] RFC 8098 MDN February 2017

 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).

Hansen & Melnikov Standards Track [Page 21] RFC 8098 MDN February 2017

4. Timeline of Events

 The following timeline shows when various events in the processing of
 a message and generation of MDNs take place:
  1. - User composes message.
  1. - User tells MUA to send message.
  1. - MUA passes message to Mail Submission Agent (MSA) and original

recipient information is passed along.

  1. - MSA sends message to next MTA.
  1. - Final MTA receives message.
  1. - Final MTA delivers message to recipient's mailbox (possibly

generating a Delivery Status Notification (DSN)).

  1. - (Recipient's) MUA discovers a new message in recipient's mailbox

and decides whether an MDN should be generated. If the MUA has

    information that an MDN has already been generated for this
    message, no further MDN processing described below is performed.
    If MUA decides that no MDN can be generated, no further MDN
    processing described below is performed.
  1. - MUA performs automatic processing and might generate corresponding

MDNs ("dispatched", "processed", or "deleted" disposition type

    with "automatic-action" and "MDN-sent-automatically" disposition
    modes).  The MUA remembers that an MDN was generated.
  1. - MUA displays list of messages to user.
  1. - User selects a message and requests that some action be performed

on it.

  1. - MUA performs requested action; if an automatic MDN has not already

been generated, with user's permission, sends an appropriate MDN

    ("displayed", "dispatched", "processed", or "deleted" disposition
    type, with "manual-action" and "MDN-sent-manually" or "MDN-sent-
    automatically" disposition mode).  The MUA remembers that an MDN
    was generated.
  1. - User possibly performs other actions on message, but no further

MDNs are generated.

Hansen & Melnikov Standards Track [Page 22] RFC 8098 MDN February 2017

5. Conformance and Usage Requirements

 An MUA or gateway conforms to this specification if it generates MDNs
 according to the protocol defined in this memo.  It is not necessary
 to be able to generate all of the possible values of the Disposition
 field.
 MUAs and gateways MUST NOT generate the Original-Recipient field of
 an MDN unless the mail protocols provide the address originally
 specified by the sender at the time of submission.  Ordinary SMTP
 does not make that guarantee, but the SMTP extension defined in RFC--
 DSN-SMTP [RFC3461] permits such information to be carried in the
 envelope if it is available.  The Original-Recipient header field
 defined in this document provides a way for the MTA to pass the
 original recipient address to the MUA.
 Each sender-specified recipient address may result in more than one
 MDN.  If an MDN is requested for a recipient that is forwarded to
 multiple recipients of an "alias" (as defined in Section 6.2.7.3 of
 RFC-DSN-SMTP [RFC3461]), each of the recipients may issue an MDN.
 Successful distribution of a message to a mailing list exploder or
 gateway to Usenet newsgroup SHOULD be considered the final
 disposition of the message.  A mailing list exploder MAY issue an MDN
 with a disposition type of "processed" and disposition modes of
 "automatic-action" and "MDN-sent-automatically" indicating that the
 message has been forwarded to the list.  In this case, the request
 for MDNs is not propagated to the members of the list.
 Alternatively (if successful distribution of a message to a mailing
 list exploder / Usenet newsgroup is not considered the final
 disposition of the message), the mailing list exploder can issue no
 MDN and propagate the request for MDNs to all members of the list.
 The latter behavior is not recommended for any but small, closely
 knit lists, as it might cause large numbers of MDNs to be generated
 and may cause confidential subscribers to the list to be revealed.
 The mailing list exploder can also direct MDNs to itself, correlate
 them, and produce a report to the original sender of the message.
 This specification places no restrictions on the processing of MDNs
 received by user agents or mailing lists.

Hansen & Melnikov Standards Track [Page 23] RFC 8098 MDN February 2017

6. Security Considerations

 The following security considerations apply when using MDNs.

6.1. Forgery

 MDNs can be (and are, in practice) forged as easily as ordinary
 Internet electronic mail.  User agents and automatic mail handling
 facilities (such as mail distribution list exploders) that wish to
 make automatic use of MDNs should take appropriate precautions to
 minimize the potential damage from denial-of-service attacks.
 Security threats related to forged MDNs include the sending of:
 a.  A falsified disposition notification when the indicated
     disposition of the message has not actually occurred, and
 b.  Unsolicited MDNs.
 Similarly, a forged spam or phishing email message can contain
 Disposition-Notification-To header field that can trick the recipient
 to send an MDN.  MDN processing should only be invoked once
 authenticity of an email message is verified.

6.2. Privacy

 Another dimension of security is privacy.  There may be cases in
 which a message recipient does not wish the disposition of messages
 addressed to him to be known, or is concerned that the sending of
 MDNs may reveal other sensitive information (e.g., when the message
 was read, using which email client, and which OS was used).  In this
 situation, it is acceptable for the MUA to silently ignore requests
 for MDNs.
 If the Disposition-Notification-To header field is passed on
 unmodified when a message is distributed to the subscribers of a
 mailing list, the subscribers to the list may be revealed to the
 sender of the original message by the generation of MDNs.
 Headers of the original message returned in part 3 of the multipart/
 report, as well as content of the message/disposition-notification
 part, could reveal confidential information about host names and/or
 network topology inside a firewall.
 Disposition mode (Section 3.2.6.1) can leak information about
 recipient's MUA configuration, in particular, whether MDNs are

Hansen & Melnikov Standards Track [Page 24] RFC 8098 MDN February 2017

 acknowledged manually or automatically.  If this is a concern, MUAs
 can return "manual-action/MDN-sent-manually" disposition mode in
 generated MDNs.
 In general, any optional MDN field may be omitted if the Reporting
 MUA site or user determines that inclusion of the field would impose
 too great a compromise of site confidentiality.  The need for such
 confidentiality must be balanced against the utility of the omitted
 information in MDNs.
 In some cases, someone with access to the message stream may use the
 MDN request mechanism to monitor the mail reading habits of a target.
 If the target is known to generate MDN reports, they could add a
 Disposition-Notification-To header field containing the envelope from
 address.  This risk can be minimized by not sending MDN's
 automatically.

6.2.1. Disclosure of Product Information

 The "Reporting-UA" field (Section 3.2.1), User-Agent header field,
 and other header fields often reveal information about the respective
 sender's software systems.  In theory, this can make it easier for an
 attacker to exploit known security holes; in practice, attackers tend
 to try all potential holes regardless of the apparent software
 versions being used.  Also note that the "Reporting-UA" field doesn't
 provide any new information in comparison to the "User-Agent" and/or
 (undocumented) "X-Mailer" header fields used by many MUAs.

6.2.2. MUA Fingerprinting

 The "Reporting-UA" field (Section 3.2.1) might contain enough
 information to uniquely identify a specific device, usually when
 combined with other characteristics, particularly if the user agent
 sends excessive details about the user's system or extensions.  Even
 when the guidance in Section 3.2.1 is followed to avoid
 fingerprinting, other sources of unique information may still be
 present, such as the Accept-Language header fields.

6.3. Non-repudiation

 MDNs do not provide non-repudiation with proof of delivery.  Within
 the framework of today's Internet Mail, the MDNs defined in this
 document provide valuable information to the mail user; however, MDNs
 cannot be relied upon as a guarantee that a message was or was not
 seen by the recipient.  Even if MDNs are not actively forged, they
 may be lost in transit.  The recipient may bypass the MDN issuing
 mechanism in some manner.

Hansen & Melnikov Standards Track [Page 25] RFC 8098 MDN February 2017

 One possible solution for this purpose can be found in RFC-SEC-
 SERVICES [RFC2634].

6.4. Mail Bombing

 The MDN request mechanism introduces an additional way of mail
 bombing a mailbox.  The MDN request notification provides an address
 to which MDN's should be sent.  It is possible for an attacking agent
 to send a potentially large set of messages to otherwise unsuspecting
 third party recipients with a false Disposition-Notification-To
 address.  Automatic or simplistic processing of such requests would
 result in a flood of MDN notifications to the target of the attack.
 Additionally, as generated MDN notifications can include the full
 content of messages that caused them and thus they can be bigger than
 such messages, they can be used for bandwidth amplification attacks.
 Such an attack could overrun the storage capacity of the targeted
 mailbox and/or of the mail transport system, and deny service.
 For that reason, MDN's SHOULD NOT be sent automatically where the
 Disposition-Notification-To address is different from the SMTP "MAIL
 FROM" address (which is carried in the Return-Path header field).
 See Section 2.1 for further discussion.

7. Collected ABNF Grammar

 NOTE: The following lexical tokens are defined in RFC-MSGFMT
 [RFC5322]: CRLF, FWS, CFWS, field-name, mailbox-list, msg-id, text,
 comment, and word.  The following lexical tokens are defined in
 RFC-SMTP [RFC5321]: Atom.  (Note that RFC-MSGFMT [RFC5322] also
 defines "atom", but the version from RFC-SMTP [RFC5321] is more
 restrictive and this more restrictive version is used in this
 document.)  The "encoded-word" construct defined in RFC-MIME-HEADER
 [RFC2047] is allowed everywhere where RFC-MSGFMT [RFC5322] "comment"
 is used, for example, in CFWS.
  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]".

Message header fields:

  mdn-request-header =
         "Disposition-Notification-To" ":" mailbox-list CRLF

Hansen & Melnikov Standards Track [Page 26] RFC 8098 MDN February 2017

  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
  original-recipient-header =
         "Original-Recipient" ":" OWS address-type OWS
         ";" OWS generic-address OWS CRLF

Report content:

  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 )
  address-type = Atom
  mta-name-type = Atom
  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

Hansen & Melnikov Standards Track [Page 27] RFC 8098 MDN February 2017

  mdn-gateway-field = "MDN-Gateway" ":" OWS mta-name-type OWS
                      ";" OWS mta-name
  mta-name = *text
  original-recipient-field =
         "Original-Recipient" ":" OWS address-type OWS
         ";" OWS generic-address OWS
  generic-address = *text
  final-recipient-field =
         "Final-Recipient" ":" OWS address-type OWS
         ";" OWS generic-address OWS
  original-message-id-field = "Original-Message-ID" ":" msg-id
  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
  error-field = "Error" ":" *([FWS] text)
  extension-field = extension-field-name ":" *([FWS] text)
  extension-field-name = field-name

Hansen & Melnikov Standards Track [Page 28] RFC 8098 MDN February 2017

8. Guidelines for Gatewaying MDNs

 NOTE: This section provides non-binding recommendations for the
 construction of mail gateways that wish to provide semi-transparent
 disposition notifications between the Internet and another electronic
 mail system.  Specific MDN gateway requirements for a particular pair
 of mail systems may be defined by other documents.

8.1. Gatewaying from Other Mail Systems to MDNs

 A mail gateway may issue an MDN to convey the contents of a "foreign"
 disposition notification over Internet Mail.  When there are
 appropriate mappings from the foreign notification elements to MDN
 fields, the information may be transmitted in those MDN fields.
 Additional information (such as what might be needed to tunnel the
 foreign notification through the Internet) may be defined in
 extension MDN fields.  (Such fields should be given names that
 identify the foreign mail protocol, e.g., X400-* for X.400 protocol
 elements [X.400]).
 The gateway must attempt to supply reasonable values for the
 Reporting-UA, Final-Recipient, and Disposition fields.  These will
 normally be obtained by translating the values from the foreign
 notification into their Internet-style equivalents.  However, some
 loss of information is to be expected.
 The sender-specified recipient address and the original message-id,
 if present in the foreign notification, should be preserved in the
 Original-Recipient and Original-Message-ID fields.
 The gateway should also attempt to preserve the "final" recipient
 address from the foreign system.  Whenever possible, foreign protocol
 elements should be encoded as meaningful printable ASCII strings.
 For MDNs produced from foreign disposition notifications, the name of
 the gateway MUST appear in the MDN-Gateway field of the MDN.

8.2. Gatewaying from MDNs to Other Mail Systems

 It may be possible to gateway MDNs from the Internet into a foreign
 mail system.  The primary purpose of such gatewaying is to convey
 disposition information in a form that is usable by the destination
 system.  A secondary purpose is to allow "tunneling" of MDNs through
 foreign mail systems in case the MDN may be gatewayed back into the
 Internet.

Hansen & Melnikov Standards Track [Page 29] RFC 8098 MDN February 2017

 In general, the recipient of the MDN (i.e., the sender of the
 original message) will want to know, for each recipient: the closest
 available approximation to the original recipient address and the
 disposition (displayed, printed, etc.).
 If possible, the gateway should attempt to preserve the Original-
 Recipient address and Original-Message-ID (if present) in the
 resulting foreign disposition report.
 If it is possible to tunnel an MDN through the destination
 environment, the gateway specification may define a means of
 preserving the MDN information in the disposition reports used by
 that environment.

8.3. Gatewaying of MDN-Requests to Other Mail Systems

 By use of the separate Disposition-Notification-To request header
 field, this specification offers a richer functionality than most, if
 not all, other email systems.  In most other email systems, the
 notification recipient is identical to the message sender as
 indicated in the "from" address.  There are two interesting cases
 when gatewaying into such systems:
 1.  If the address in the Disposition-Notification-To header field is
     identical to the address in the SMTP "MAIL FROM", the expected
     behavior will result, even if the Disposition-Notification-To
     information is lost.  Systems should propagate the MDN request.
 2.  If the address in the Disposition-Notification-To header field is
     different from the address in the SMTP "MAIL FROM", gatewaying
     into a foreign system without a separate notification address
     will result in unintended behavior.  This is especially important
     when the message arrives via a mailing list expansion software
     that may specifically replace the SMTP "MAIL FROM" address with
     an alternate address.  In such cases, the MDN request should not
     be gatewayed and should be silently dropped.  This is consistent
     with other forms of non-support for MDN.

9. Example

 NOTE: This example is provided as illustration only and is not
 considered part of the MDN protocol specification.  If the example
 conflicts with the protocol definition above, the example is wrong.
 Likewise, the use of *-type subfield names or extension fields in
 this example is not to be construed as a definition for those type
 names or extension fields.

Hansen & Melnikov Standards Track [Page 30] RFC 8098 MDN February 2017

 This is an MDN issued after a message has been displayed to the user
 of an Internet Mail user agent.
 Date: Wed, 20 Sep 1995 00:19:00 (EDT) -0400
 From: Joe Recipient <Joe_Recipient@example.com>
 Message-Id: <199509200019.12345@example.com>
 Subject: Disposition notification
 To: Jane Sender <Jane_Sender@example.org>
 MIME-Version: 1.0
 Content-Type: multipart/report; report-type=disposition-notification;
    boundary="RAA14128.773615765/example.com"
  1. -RAA14128.773615765/example.com
 The message sent on 1995 Sep 19 at 13:30:00 (EDT) -0400 to Joe
 Recipient <Joe_Recipient@example.com> with subject "First draft of
 report" has been displayed.
 This is no guarantee that the message has been read or understood.
  1. -RAA14128.773615765/example.com

Content-Type: message/disposition-notification

 Reporting-UA: joes-pc.cs.example.com; Foomail 97.1
 Original-Recipient: rfc822;Joe_Recipient@example.com
 Final-Recipient: rfc822;Joe_Recipient@example.com
 Original-Message-ID: <199509192301.23456@example.org>
 Disposition: manual-action/MDN-sent-manually; displayed
  1. -RAA14128.773615765/example.com

Content-Type: message/rfc822

 [original message optionally goes here]
  1. -RAA14128.773615765/example.com–

10. IANA Considerations

 IANA has completed the following actions:
 1.  IANA has updated the registration template for the message/
     disposition-notification media type to match what appears in
     Section 3.1 of this document and updated the reference for the
     media type to point to this document (instead of to RFC 3798).
 2.  The registries specified here already exist; this section updates
     their documentation.  IANA has changed the reference document for
     the three Message Disposition Notification Parameters registries
     to point to this document (instead of to RFC 3798).

Hansen & Melnikov Standards Track [Page 31] RFC 8098 MDN February 2017

 This document specifies three types of parameters that must be
 registered with the Internet Assigned Numbers Authority (IANA).  All
 of them use the "Specification Required" IANA registration policy
 [RFC5226].
 The forms below are for use when registering a new disposition-
 notification-parameter name for the Disposition-Notification-Options
 header field, a new disposition modifier name, or a new MDN extension
 field.  Each piece of information required by a registration form may
 be satisfied either by providing the information on the form itself
 or by including a reference to a published and publicly available
 specification that includes the necessary information.  IANA MAY
 reject registrations because of incomplete registration forms or
 incomplete specifications.
 To register, complete the following applicable form and send it via
 electronic mail to <IANA@IANA.ORG>.

10.1. Disposition-Notification-Options Header Field

     disposition-notification-parameter Names
 A registration for a Disposition-Notification-Options header field
 disposition-notification-parameter name MUST include the following
 information:
 a.  The proposed disposition-notification-parameter name.
 b.  The syntax for disposition-notification-parameter values,
     specified using BNF, ABNF, regular expressions, or other
     non-ambiguous language.
 c.  If disposition-notification-parameter values are not composed
     entirely of graphic characters from the US-ASCII repertoire, a
     specification for how they are to be encoded as graphic US-ASCII
     characters in a Disposition-Notification-Options header field.
 d.  A reference to a permanent and readily available public
     specification that describes the semantics of the disposition-
     notification-parameter values.

Hansen & Melnikov Standards Track [Page 32] RFC 8098 MDN February 2017

10.2. Disposition Modifier Names

 A registration for a disposition-modifier name (used in the
 Disposition field of a message/disposition-notification) MUST include
 the following information:
 a.  The proposed disposition-modifier name.
 b.  A reference to a permanent and readily available public
     specification that describes the semantics of the disposition
     modifier.

10.3. MDN Extension Field Names

 A registration for an MDN extension-field name MUST include the
 following information:
 a.  The proposed extension field name.
 b.  The syntax for extension values, specified using BNF, ABNF,
     regular expressions, or other non-ambiguous language.
 c.  If extension-field values are not composed entirely of graphic
     characters from the US-ASCII repertoire, a specification for how
     they are to be encoded as graphic US-ASCII characters in a
     Disposition-Notification-Options header field.
 d.  A reference to a permanent and readily available public
     specification that describes the semantics of the extension
     field.

11. References

11.1. Normative References

 [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
            DOI 10.17487/RFC5321, October 2008,
            <http://www.rfc-editor.org/info/rfc5321>.
 [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
            DOI 10.17487/RFC5322, October 2008,
            <http://www.rfc-editor.org/info/rfc5322>.
 [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part One: Format of Internet Message
            Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
            <http://www.rfc-editor.org/info/rfc2045>.

Hansen & Melnikov Standards Track [Page 33] RFC 8098 MDN February 2017

 [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part Two: Media Types", RFC 2046,
            DOI 10.17487/RFC2046, November 1996,
            <http://www.rfc-editor.org/info/rfc2046>.
 [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
            Part Three: Message Header Extensions for Non-ASCII Text",
            RFC 2047, DOI 10.17487/RFC2047, November 1996,
            <http://www.rfc-editor.org/info/rfc2047>.
 [RFC6522]  Kucherawy, M., Ed., "The Multipart/Report Media Type for
            the Reporting of Mail System Administrative Messages",
            STD 73, RFC 6522, DOI 10.17487/RFC6522, January 2012,
            <http://www.rfc-editor.org/info/rfc6522>.
 [RFC3461]  Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
            Extension for Delivery Status Notifications (DSNs)",
            RFC 3461, DOI 10.17487/RFC3461, January 2003,
            <http://www.rfc-editor.org/info/rfc3461>.
 [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format
            for Delivery Status Notifications", RFC 3464,
            DOI 10.17487/RFC3464, January 2003,
            <http://www.rfc-editor.org/info/rfc3464>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC3503]  Melnikov, A., "Message Disposition Notification (MDN)
            profile for Internet Message Access Protocol (IMAP)",
            RFC 3503, DOI 10.17487/RFC3503, March 2003,
            <http://www.rfc-editor.org/info/rfc3503>.

11.2. Informative References

 [RFC2634]  Hoffman, P., Ed., "Enhanced Security Services for S/MIME",
            RFC 2634, DOI 10.17487/RFC2634, June 1999,
            <http://www.rfc-editor.org/info/rfc2634>.
 [RFC3249]  Cancio, V., Moldovan, M., Tamura, H., and D. Wing,
            "Implementers Guide for Facsimile Using Internet Mail",
            RFC 3249, DOI 10.17487/RFC3249, September 2002,
            <http://www.rfc-editor.org/info/rfc3249>.

Hansen & Melnikov Standards Track [Page 34] RFC 8098 MDN February 2017

 [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
            4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
            <http://www.rfc-editor.org/info/rfc3501>.
 [RFC3801]  Vaudreuil, G. and G. Parsons, "Voice Profile for Internet
            Mail - version 2 (VPIMv2)", RFC 3801,
            DOI 10.17487/RFC3801, June 2004,
            <http://www.rfc-editor.org/info/rfc3801>.
 [RFC5233]  Murchison, K., "Sieve Email Filtering: Subaddress
            Extension", RFC 5233, DOI 10.17487/RFC5233, January 2008,
            <http://www.rfc-editor.org/info/rfc5233>.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            DOI 10.17487/RFC5226, May 2008,
            <http://www.rfc-editor.org/info/rfc5226>.
 [RFC5429]  Stone, A., Ed., "Sieve Email Filtering: Reject and
            Extended Reject Extensions", RFC 5429,
            DOI 10.17487/RFC5429, March 2009,
            <http://www.rfc-editor.org/info/rfc5429>.
 [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
            DOI 10.17487/RFC5598, July 2009,
            <http://www.rfc-editor.org/info/rfc5598>.
 [RFC6533]  Hansen, T., Ed., Newman, C., and A. Melnikov,
            "Internationalized Delivery Status and Disposition
            Notifications", RFC 6533, DOI 10.17487/RFC6533, February
            2012, <http://www.rfc-editor.org/info/rfc6533>.
 [X.400]    International Telecommunications Union, "Message handling
            system and service overview", ITU-T Recommendation
            F.400/X.400, June 1999.

Hansen & Melnikov Standards Track [Page 35] RFC 8098 MDN February 2017

Appendix A. Changes from RFC 3798

 Changed IANA registration for different subregistries to
 "Specification Required" to match what is already used by IANA.
 Updated IANA registration template for message/disposition-
 notification.
 "X-" fields no longer reserved for experimental use and can now be
 registered in compliance with RFC 6648.
 Fixed the default MTA-name-type used in "MDN-Gateway" to be "dns".
 Strengthen requirements on obtaining user consent in order to protect
 user privacy.
 Removed discussion of using source routes with MDNs, as source route
 is a deprecated Email feature.
 The values of "dispatched" and "processed" were lost from the ABNF
 for "disposition-type".  (Erratum #691)
 Because the warning disposition modifier was previously removed, the
 warning-field has also been removed.  (Erratum #692)
 Because the failed disposition type was previously removed, the
 failure-field has also been removed.
 The ABNF for ua-name and ua-product included a semi-colon, which
 could not be distinguished from *text in the production.  The ua-name
 was restricted to not include semi-colon.  Semi-colon can still
 appear in the ua-product.
 Removed recommendation to include the MUA DNS host name in the
 "Reporting-UA" MDN field.
 The ABNF did not indicate all places that whitespace was allowable,
 in particular folding whitespace, although all implementations allow
 whitespace and folding in the header fields just like any other
 header field formatted as described in RFC-MSGFMT [RFC5322].  There
 were also a number of places in the ABNF that inconsistently
 permitted comments and whitespace in one leg of the production and
 not another.  The ABNF now specifies FWS and CFWS in several places
 that should have already been specified by the grammar.
 Extension-field was defined in the collected grammar but not in the
 main text.

Hansen & Melnikov Standards Track [Page 36] RFC 8098 MDN February 2017

 The comparison of mailboxes in Disposition-Notification-To to the
 Return-Path addr-spec was clarified.
 The use of the grammar production "parameter" was confusing with the
 RFC 2045 [RFC2045] production of the same name, as well as other uses
 of the same term.  These have been clarified.
 A clarification was added on the extent of the 7bit nature of MDNs.
 Uses of the terms "may" and "might" were clarified.
 A clarification was added on the order of the fields in the message/
 disposition-notification content.

Acknowledgements

 The contributions of Bruce Lilly, Alfred Hoenes, Barry Leiba, Ben
 Campbell, Pete Resnick, Donald Eastlake, and Alissa Cooper are
 gratefully acknowledged for this revision.
 The contributions of Roger Fajman and Greg Vaudreuil to earlier draft
 versions of this document are also gratefully acknowledged.

Authors' Addresses

 Tony Hansen (editor)
 AT&T Laboratories
 200 Laurel Ave. South
 Middletown, NJ  07748
 United States of America
 Email: tony@att.com
 Alexey Melnikov (editor)
 Isode Ltd
 14 Castle Mews
 Hampton, Middlesex  TW12 2NP
 United Kingdom
 Email: Alexey.Melnikov@isode.com

Hansen & Melnikov Standards Track [Page 37]

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