Network Working Group                                     T. Hansen, Ed.
Internet-Draft                                         AT&T Laboratories
Obsoletes: 3798 (if approved)                           A. Melnikov, Ed.
Intended status: Standards Track                               Isode Ltd
Expires: May 31, June 4, 2016                                  November 28,                                   December 2, 2015

                    Message Disposition Notification
                 draft-ietf-appsawg-mdn-3798bis-04.txt
                 draft-ietf-appsawg-mdn-3798bis-05.txt

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 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 multi-
   protocol 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.

Status of This Memo

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   This Internet-Draft will expire on May 31, June 4, 2016.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Purposes  . . . . . . . . . . . . . . . . . . . . . . . .   3
     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 . . . . . . .   7
     2.3.  The Original-Recipient Header Field . . . . . . . . . . .   8
     2.4.  Use with the Message/Partial Content Type . . . . . . . .   8
   3.  Format of a Message Disposition Notification  . . . . . . . .   9
     3.1.  The message/disposition-notification content-type . . . .  10
     3.2.  Message/disposition-notification Fields . . . . . . . . .  12
     3.3.  Extension-fields  . . . . . . . . . . . . . . . . . . . .  18
   4.  Timeline of events  . . . . . . . . . . . . . . . . . . . . .  18
   5.  Conformance and Usage Requirements  . . . . . . . . . . . . .  19
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
     6.1.  Forgery . . . . . . . . . . . . . . . . . . . . . . . . .  20
     6.2.  Privacy . . . . . . . . . . . . . . . . . . . . . . . . .  21  20
     6.3.  Non-Repudiation . . . . . . . . . . . . . . . . . . . . .  22  21
     6.4.  Mail Bombing  . . . . . . . . . . . . . . . . . . . . . .  22
   7.  Collected ABNF Grammar  . . . . . . . . . . . . . . . . . . .  22
   8.  Guidelines for Gatewaying MDNs  . . . . . . . . . . . . . . .  24
     8.1.  Gatewaying from other mail systems to MDNs  . . . . . . .  24
     8.2.  Gatewaying from MDNs to other mail systems  . . . . . . .  25  24
     8.3.  Gatewaying of MDN-requests to other mail systems  . . . .  25
   9.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .  26  25
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27  26
     10.1.  Disposition-Notification-Options header field
            disposition-notification-parameter names . . . . . . . .  27
     10.2.  Disposition modifier names . . . . . . . . . . . . . . .  28  27
     10.3.  MDN extension field names  . . . . . . . . . . . . . . .  28
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  28
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29  28
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  29  28
     12.2.  Informative References . . . . . . . . . . . . . . . . .  30  29
   Appendix A.  Changes from RFC 3798  . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   This memo defines a RFC-MIME-MEDIA [4] content-type 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 [6].

   This memo defines the format of the notifications and the RFC-MSGFMT
   [2] header fields used to request them.

   This memo is an update to RFC 3798 and is intended to be published at
   Internet Standard Level.

   This memo is currently marked with the 'pre5378Trust200902' IPR
   statements until a release has been obtained from all previous
   authors and editors of this text.

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 message 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.

   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 [9].

   All syntax descriptions use the ABNF specified by RFC-MSGFMT [2], in
   which the lexical tokens (used below) are defined: "CRLF", "FWS",
   "CFWS", "field-name", "mailbox", "mailbox-list", "msg-id", and "text".  The
   following lexical tokens are defined in RFC-SMTP [1]: "atom".  The following
   lexical tokens are defined in the definition of the Content-Type
   header field in RFC-MIME-BODY [3]: "attribute" and "value".

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 MUA 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" ":" [FWS]
         mailbox *("," [FWS] mailbox) mailbox-list CRLF

   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 laverage 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 [13], it SHOULD
   implement the procedures specified in RFC-IMAP-MDN [10].

   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.

   MDNs SHOULD 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 [2]).  In this case,
   confirmation from the user SHOULD be obtained, if possible.  If
   obtaining consent is not possible (e.g., because the user is not
   online at the time), then an MDN SHOULD NOT be sent.

   Confirmation from the user SHOULD 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.

   The comparison of the addresses should be done using only the addr-
   spec (local-part "@" domain) portion, excluding any angle brackets,
   phrase and route.  The comparison MUST be 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 [2] for more details.)  Implementations MAY treat known
   domain aliases as equivalent for the purpose of comparison.

   Note that use of subaddressing (see [12]) 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.

   A message that contains a Disposition-Notification-To header field
   SHOULD also contain a Message-ID header field as specified in RFC-
   MSGFMT [2].  This will permit automatic correlation of MDNs with
   their original messages by user agents.

   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 for whom message
   disposition notifications are requested and for 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.

   Messages posted to newsgroups SHOULD NOT have a Disposition-
   Notification-To header field.

2.2.  The Disposition-Notification-Options Header

   Future 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
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

   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-
   notification-parameter attribute names beginning with "X-" will never
   be defined as standard names; such names are reserved for
   experimental use.  disposition-notification-parameter attribute names
   not beginning with "X-" MUST be registered with the Internet Assigned
   Numbers Authority (IANA) and described in a standards-track RFC or an
   experimental RFC approved by the IESG.  [[ more work needed here ]]
   (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 MTA.  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 [1] and RFC-DSN-SMTP [7].

   RFC-DSN-SMTP [7] 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" ":" [FWS] address-type [FWS] ";" [FWS] generic-address

   The address-type and generic-address token 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.

2.4.  Use with the Message/Partial Content 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 [4]]) 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 [4]).  These header fields SHOULD NOT be used
   in the header fields of any of the fragments themselves.

   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 [4]), they pertain to an
   MDN that will be generated for the reassembled message.
   Section 5.2.2.1 of RFC-MIME-MEDIA [4]) 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 [6]).  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 [6].

   c.  The second component of the multipart/report is of content-type
       message/disposition-notification, described in Section 3.1 of
       this document.

   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,
       encrypted message text MUST be returned, if it is returned at
       all, only 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 [2] 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 field of the message 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
   or 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, MDNs may not be generated for
   some recipients for which MDNs were requested.

3.1.  The message/disposition-notification content-type

   The message/disposition-notification content-type is defined as
   follows:

   MIME type name:     message

   MIME subtype name:  disposition-notification

   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 this memo.

   (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 [2]
   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
             *( failure-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 fixed, with the exception
   of the extension fields.

3.1.1.  General conventions for fields

   Since these fields are defined according to the rules of RFC-MSGFMT
   [2], 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 any combination of
   upper and lower case letters.  Comments in notification fields may
   use the "encoded-word" construct defined in RFC-MIME-HEADER [5].

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.

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.

   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 [7].)  Registration forms for address-type
   and mta-name-type appear in RFC-DSN-FORMAT [8].

3.2.  Message/disposition-notification 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
OWS      = *WSP             ; generators
           / [CFWS]         ; parsers
           ; 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 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.  This field is
   optional, but recommended.  For Internet Mail user agents, it is
   recommended that this field contain both: the DNS name of the
   particular instance of the MUA that generated the MDN, and the name
   of the product.  For example,

   Reporting-UA:  pc.example.com; Foomail 97.1

   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.

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
   "smtp", 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
   no Original-Recipient header field in the message, then the Original-
   Recipient field MUST be omitted, unless the same information is
   reliably available some other way.  If there is an Original-Recipient
   header field in the original message (or original recipient
   information is reliably available some other way), then the Original-
   Recipient field must be supplied.  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-
   type field will normally be "rfc822", and the address will be
   according to the syntax specified in RFC-MSGFMT [2].  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 MUST
   contain the mailbox address of the recipient (from the From header
   field of the MDN) as it was when the MDN was generated by the MUA.

   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".

   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 the original message contained a Message-ID
   header field.  The syntax of the field is as follows:

   original-message-id-field =
             "Original-Message-ID" ":" OWS msg-id OWS

   The msg-id token is as specified in RFC-MSGFMT [2].

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

   The disposition-mode, disposition-type, and disposition-modifier may
   be spelled in any combination of upper and lower case characters.

3.2.6.1.  Disposition modes

   The following disposition 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.

   "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.

   "Manual-action" and "automatic-action" are mutually exclusive.  One
   or the other MUST be specified.

   "MDN-sent-manually" The user explicitly gave permission for this
                       particular MDN to be sent.

   "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.

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 is defined:

   disposition-modifier-extension
                       Disposition modifiers may be defined in the
                       future by later revisions or extensions to this
                       specification.  Disposition value names beginning
                       with "X-" will never be defined as standard
                       values; such names are reserved for experimental
                       use.  MDN disposition value names NOT beginning
                       with "X-" MUST be registered with the Internet
                       Assigned Numbers Authority (IANA) and described
                       in a standards-track RFC or an experimental RFC
                       approved by the IESG.  (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.

   If an MUA developer does not wish to register the meanings of such
   disposition modifier extensions, "X-" modifiers may be used for this
   purpose.  To avoid name collisions, the name of the MUA
   implementation should follow the "X-", (e.g., "X-Foomail-").

   It is not required that an MUA be able to generate all of the
   possible values of the Disposition field.

   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.  Failure and Error Fields

   The Failure and Error fields are used to supply additional
   information in the form of text messages when the "failure"
   disposition type or "error" disposition modifier appear.  The syntax
   is as follows:

   failure-field = "Failure" ":" *([FWS] text)
   error-field = "Error" ":" *([FWS] text)

3.3.  Extension-fields

   Additional MDN fields may be defined in the future by later revisions
   or extensions to this specification.  Extension-field names beginning
   with "X-" will never be defined as standard fields; such names are
   reserved for experimental use.  MDN field names NOT beginning with
   "X-" MUST be registered with the Internet Assigned Numbers Authority
   (IANA) and described in a standards-track RFC or an experimental RFC
   approved by the IESG.  (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).

   If an application developer does not wish to register the meanings of
   such extension fields, "X-" fields may be used for this purpose.  To
   avoid name collisions, the name of the application implementation
   should follow the "X-", (e.g., "X-Foomail-Log-ID" or "X-Foomail-EDI-
   info").

4.  Timeline of events

   The following timeline shows when various events in the processing of
   a message and generation of MDNs take place:

   -- User composes message

   -- User tells MUA to send message

   -- MUA passes message to MTA (original recipient information passed
      along)

   -- MTA sends message to next MTA

   -- Final MTA receives message

   -- Final MTA delivers message to MUA (possibly generating a DSN)

   -- MUA performs automatic processing and generates corresponding MDNs
      ("dispatched", "processed" or "deleted" disposition type with
      "automatic-action" and "MDN-sent-automatically" disposition modes)

   -- MUA displays list of messages to user

   -- User selects a message and requests that some action be performed
      on it.

   -- MUA performs requested action and, 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).

   -- User possibly performs other actions on message, but no further
      MDNs are generated.

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 [7] 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 RFC-DSN-SMTP [7],
   section 6.2.7.3), each of the recipients may issue an MDN.

   Successful distribution of a message to a mailing list exploder
   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, the mailing list exploder MAY 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 MAY 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.

6.  Security Considerations

   The following security considerations apply when using MDNs:

6.1.  Forgery

   MDNs may be 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,

   b.  Unsolicited MDNs

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).  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 could reveal confidential information about host names and/or
   network topology inside a firewall.

   An unencrypted MDN could reveal confidential information about an
   encrypted message, especially if all or part of the original message
   is returned in part 3 of the multipart/report.  Encrypted MDNs are
   not defined in this specification.

   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 field containing the envelope from
   address along with a source route.  The source route is ignored in
   the comparison so the addresses will always match.  But if the source
   route is honored when the notification is sent, it could direct the
   message to some other destination.  This risk can be minimized by not
   sending MDN's automatically.

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.

   One possible solution for this purpose can be found in RFC-SEC-
   SERVICES [11].

6.4.  Mail Bombing

   The MDN request mechanism introduces an additional way of mailbombing
   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.  Such an
   attack could overrun the capacity of the targeted mailbox and deny
   service.

   For that reason, MDN's SHOULD NOT be sent automatically where the
   "disposition-notification-to:" address is different from the envelope
   MAIL FROM address.  See Section 2.1 for further discussion.

7.  Collected ABNF Grammar

   NOTE: The following lexical tokens are defined in RFC-MSGFMT [2]:
   CRLF, FWS, CFWS, field-name, mailbox, mailbox-list, msg-id, text, comment. comment,
   word.  The following lexical tokens are defined in RFC-SMTP [1]:
   atom.  (Note that RFC-MSGFMT [2] also defines "atom", but the version
   from RFC-
   SMTP RFC-SMTP [1] is more restrictive and this more restrictive
   version is used in this document.)  The definitions of attribute and value are
   as in the definition of the Content-Type header field in RFC-MIME-
   BODY [3]. "encoded-word" construct defined
   in RFC-MIME-HEADER [5] is allowed everywhere where RFC-MSGFMT [2]
   "comment" is used, for example in CFWS.

Message header fields:
   mdn-request-header =
          "Disposition-Notification-To" ":" [FWS]
                    mailbox *("," [FWS] mailbox) mailbox-list CRLF
   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" ":" [FWS] address-type [FWS] ";" [FWS] generic-address

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
          *( failure-field CRLF )
          *( error-field CRLF )
          *( extension-field CRLF )
   OWS          = *WSP             ; generators
                  / [CFWS]         ; parsers
                  ; 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]".
   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
   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" ":" OWS msg-id OWS
   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
   failure-field = "Failure" ":" *([FWS] text)
   error-field = "Error" ":" *([FWS] text)
   extension-field = extension-field-name ":" *([FWS] text)
   extension-field-name = field-name

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 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).

   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.

   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.

   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"

   --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.

   --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

   --RAA14128.773615765/example.com
   content-type: message/rfc822

   [original message optionally goes here]

   --RAA14128.773615765/example.com--

10.  IANA Considerations

   This document specifies three types of parameters that must be
   registered with the Internet Assigned Numbers Authority (IANA).

   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, 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 standards track RFC or experimental RFC approved
       by the IESG that describes the semantics of the disposition-
       notification-parameter values.

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 standards track RFC or experimental RFC approved
       by the IESG 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 standards track RFC or experimental RFC approved
       by the IESG that describes the semantics of the extension field.

11.  Acknowledgements

   The contributions of Bruce Lilly and Lilly, Alfred Hoenes and Pete Resnick are
   gratefully acknowledged for this revision.

   The contributions of Roger Fajman and Greg Vaudreuil to earlier
   versions of this document are also gratefully acknowledged.

12.  References

12.1.  Normative References

   [1]        Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <http://www.rfc-editor.org/info/rfc5321>.

   [2]        Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <http://www.rfc-editor.org/info/rfc5322>.

   [3]        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>.

   [4]        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>.

   [5]        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>.

   [6]        Vaudreuil, G., "The Multipart/Report Content Type for the
              Reporting of Mail System Administrative Messages",
              RFC 3462, DOI 10.17487/RFC3462, January 2003,
              <http://www.rfc-editor.org/info/rfc3462>.

   [7]        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>.

   [8]        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>.

   [9]        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>.

   [10]       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>.

12.2.  Informative References

   [11]       Hoffman, P., Ed., "Enhanced Security Services for S/MIME",
              RFC 2634, DOI 10.17487/RFC2634, June 1999,
              <http://www.rfc-editor.org/info/rfc2634>.

   [12]       Murchison, K., "Sieve Email Filtering: Subaddress
              Extension", RFC 5233, DOI 10.17487/RFC5233, January 2008,
              <http://www.rfc-editor.org/info/rfc5233>.

   [13]       Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <http://www.rfc-editor.org/info/rfc3501>.

Appendix A.  Changes from RFC 3798

   The values of "dispatched" and "processed" were lost from the ABNF
   for "disposition-type".

   Because the warning disposition modifier was previously removed,
   warning-field has also been removed.

   The ABNF for ua-name and ua-product included 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.

   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
   RFC5322 [2]-formatted header field.  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.

   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
   RFC2045 [3] production of the same name, as well as other uses of the
   same term.  These have been clarified.
   [[CREF1: Not sure what to do with this one: (From Bruce) In the case
   of the message header fields, RFC 2822 also specifies minimum and
   maximum counts for each header field, and similar guidance would
   clarify 3798 (e.g. are multiple Disposition-Notification-Options
   fields permitted in a single message header, and if so, what
   semantics apply?).  ]]

   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.
   [[CREF2: Not sure what to do with this one: (From Bruce) 3.1.1
   explicitly mentions use of RFC 2047 encoded-words in comments
   (however, as noted above there is no explicit provision for
   comments), but fails to mention the other contexts in which encoded-
   words may be used, viz. in an RFC [2]822 "phrase" (e.g. in the
   display name of a name-addr mailbox in Disposition-Notification-To
   (therefore, the discussion of encoded-words should probably be moved
   earlier in the document, prior to the specification of Disposition-
   Notification-To]), and in unstructured text (i.e. every instance of
   *text in the ABNF).  In particular, use of encoded-words might be
   highly desirable in the following places: *) the ua-product portion
   of the Reporting-UA field; *) the generic-address part of the
   Original-Recipient and Final-Recipient fields; *) the (unstructured)
   field bodies of Error, Failure, and Warning fields; in structured
   extension fields where the context (per RFC 2047) is appropriate in
   unstructured extension fields; *) in X- extension fields (see RFC
   2047 for related X- message header fields).  In cases where the field
   syntax is shared with DSN fields, some coordination with the RFC 346x
   authors might be desirable.  ]]

Authors' Addresses

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

   Email: tony+rfc3798@maillennium.att.com

   Alexey Melnikov (editor)
   Isode Ltd
   14 Castle Mews
   Hampton, Middlesex  TW12 2NP
   UK

   Email: Alexey.Melnikov@isode.com