path: root/doc/rfc/rfc9477.txt

Independent Submission                                        J. Benecke
Request for Comments: 9477                     CleverReach GmbH & Co. KG
Category: Experimental                                    September 2023
ISSN: 2070-1721


                 Complaint Feedback Loop Address Header

Abstract

   This document describes a method that allows a Message Originator to
   specify a Complaint Feedback Loop (CFBL) address as a message header
   field.  It also defines the rules for processing and forwarding such
   a complaint.  The motivation for this arises out of the absence of a
   standardized and automated way to provide Mailbox Providers with an
   address for a CFBL.  Currently, providing and maintaining such an
   address is a manual and time-consuming process for Message
   Originators and Mailbox Providers.

   The mechanism specified in this document is being published as an
   experiment to gather feedback and gauge the interest of implementers
   and deployers.  This document is produced through the Independent RFC
   Stream and was not subject to the IETF's approval process.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This is a contribution to the RFC Series, independently
   of any other RFC stream.  The RFC Editor has chosen to publish this
   document at its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not candidates for any level of Internet Standard;
   see 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
   https://www.rfc-editor.org/info/rfc9477.

Copyright Notice

   Copyright (c) 2023 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
   (https://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.

Table of Contents

   1.  Introduction and Motivation
     1.1.  Scope of this Experiment
     1.2.  How CFBL Differs from One-Click-Unsubscribe
   2.  Conventions Used in This Document
   3.  Requirements
     3.1.  Received Message
       3.1.1.  Strict
       3.1.2.  Relaxed
       3.1.3.  Third Party Address
       3.1.4.  DKIM Signature
     3.2.  Multiple CFBL-Address Header Fields
     3.3.  CFBL-Feedback-ID Header Field
     3.4.  Receiving Report Address
     3.5.  Feedback Message
       3.5.1.  XARF Report
   4.  Implementation
     4.1.  Message Originator
     4.2.  Mailbox Provider
   5.  Header Field Syntax
     5.1.  CFBL-Address
     5.2.  CFBL-Feedback-ID
   6.  Security Considerations
     6.1.  Attacks on the Feedback Loop Address
     6.2.  Automatic Suspension of an Account
     6.3.  Enumeration Attacks / Provoking Unsubscription
     6.4.  Data Privacy
     6.5.  Abusing for Validity and Existence Queries
   7.  IANA Considerations
     7.1.  CFBL-Address
     7.2.  CFBL-Feedback-ID
   8.  Examples
     8.1.  Simple
     8.2.  Data Privacy Safe Report
     8.3.  Data Privacy Safe Report with HMAC
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Acknowledgments
   Author's Address

1.  Introduction and Motivation

   This memo extends the CFBL recommendations described in [RFC6449]
   with an automated way to provide the necessary information by the
   Message Originator to Mailbox Providers.  The reader should be
   familiar with the terminology and concepts in that document.  Terms
   beginning with capital letters used in this memo are described in
   that document.

   As described in [RFC6449], the registration for such a CFBL needs to
   be done manually by a human at any Mailbox Provider that provides a
   CFBL.  The key underpinning of [RFC6449] is that access to the CFBL
   is a privilege and Mailbox Providers are not prepared to send
   feedback to anyone they cannot reasonably believe are legitimate.
   However, manual registration and management can be quite time-
   consuming if there are new feedback loops rising up or if the Message
   Originator wants to add new IP addresses, DomainKeys Identified Mail
   (DKIM) domains, or change their complaint address.  In addition, a
   manual process is not well suited or feasible for smaller Mailbox
   Providers.

   Here, we propose that Message Originators add a header field without
   the need to manually register with each Feedback Provider and willing
   Mailbox Providers can use it to send the Feedback Messages to the
   specified complaint address.  This simplification or extension of a
   manual registration and verification process would be another
   advantage for the Mailbox Providers.

   A new message header field, rather than a new DNS record, was chosen
   to easily distinguish between multiple Message Originators without
   requiring user or administrator intervention.  For example, if a
   company uses multiple systems, each system can set this header field
   on its own without requiring users or administrators to make any
   changes to their DNS.  No additional DNS lookup is required of the
   Mailbox Provider side to obtain the complaint address.

   The proposed mechanism is capable of being operated in compliance
   with data privacy laws, e.g., the EU's General Data Protection
   Regulation (GDPR) or the California Consumer Privacy Act (CCPA).  As
   described in Section 6.4, a Feedback Message may contain personal
   data.  This document describes a way to omit this personal data when
   sending the Feedback Message and only send back a header field.

   Nevertheless, the described mechanism below potentially permits a
   kind of person-in-the-middle attack between the domain owner and the
   recipient.  A bad actor can generate forged reports to be "from" a
   domain name the bad actor is attacking and send these reports to the
   CFBL address.  These fake messages can result in a number of actions,
   such as blocking accounts or deactivating recipient addresses.  This
   potential harm and others are described with potential
   countermeasures in Section 6.

   In summary, this document has the following objectives:

   *  Allow Message Originators to signal that a complaint address
      exists without requiring manual registration with all providers.

   *  Allow Mailbox Providers to obtain a complaint address without
      developing their own manual registration process.

   *  Have the ability to provide a complaint address to smaller Mailbox
      Providers who do not have a feedback loop in place

   *  Provide a data privacy safe option for a CFBL.

1.1.  Scope of this Experiment

   The CFBL-Address header field and the CFBL-Feedback-ID header field
   comprise an experiment.  Participation in this experiment consists of
   adding the CFBL-Address header field on the Message Originator side
   or by using the CFBL-Address header field to send Feedback Messages
   to the provided address on the Mailbox Provider side.  Feedback on
   the results of this experiment can be emailed to the author, raised
   as an issue at <https://github.com/jpbede/rfc-cfbl-address-header/>,
   or can be emailed to the IETF cfbl mailing list (cfbl@ietf.org).

   The goal of this experiment is to answer the following questions
   based on real-world deployments:

   *  Is there interest among Message Originators and Mailbox Providers?

   *  If the Mailbox Provider adds this capability, will it be used by
      the Message Originators?

   *  If the Message Originator adds this capability, will it be used by
      the Mailbox Providers?

   *  Does the presence of the CFBL-Address and CFBL-Feedback-ID header
      fields introduce additional security issues?

   *  What additional security measures/checks need to be performed at
      the Mailbox Provider before a Feedback Message is sent?

   *  What additional security measures/checks need to be performed at
      the Message Originator after a Feedback Message is received?

   This experiment will be considered successful if the CFBL-Address
   header field is used by a leading Mailbox Provider and by at least
   two Message Originators within the next two years.  It will also be
   considered a success if these parties successfully use the address
   specified in the header field to exchange Feedback Messages.

   If this experiment is successful and these header fields prove to be
   valuable and popular, the header fields may be taken to the IETF for
   further discussion and revision.

1.2.  How CFBL Differs from One-Click-Unsubscribe

   For good reasons, the One-Click-Unsubscribe [RFC8058] signaling
   already exists and may have several interests in common with this
   document.  However, this header field requires the List-Unsubscribe
   header field.  The purpose of this header field is to provide the
   link to unsubscribe from a list.  For this reason, this header field
   is only used by operators of broadcast marketing lists or mailing
   lists and not in normal email traffic.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   In this document, "CFBL" is the abbreviation for "Complaint Feedback
   Loop" and will hereafter be used.

   Syntax descriptions use ABNF [RFC5234] [RFC7405].

3.  Requirements

3.1.  Received Message

   This section describes the requirements that must be met for the
   following: a received message, the message that is sent from the
   Message Originator to the Mailbox Provider, and a report that is to
   be sent later.

3.1.1.  Strict

   If the domain in the RFC5322.From and the domain in the CFBL-Address
   header fields are identical, this domain MUST be matched by a valid
   [DKIM] signature.  In this case, the DKIM "d=" parameter and the
   RFC5322.From field have identical domains.  This signature MUST meet
   the requirements described in Section 3.1.4.

   The following example meets this case:

   Return-Path: <sender@mailer.example.com>
   From: Awesome Newsletter <newsletter@example.com>
   To: receiver@example.org
   Subject: Super awesome deals for you
   CFBL-Address: fbl@example.com; report=arf
   Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
   Content-Type: text/plain; charset=utf-8
   DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=news;
          h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;

   This is a super awesome newsletter.

3.1.2.  Relaxed

   If the domain in CFBL-Address header field is a child domain of
   RFC5322.From, the RFC5322.From domain MUST be matched by a valid
   [DKIM] signature.  In this case, the DKIM "d=" parameter and the
   RFC5322.From domain have an identical (Example 1) or parent (Example
   2) domain.  This signature MUST meet the requirements described in
   Section 3.1.4.

   Example 1:

   Return-Path: <sender@mailer.example.com>
   From: Awesome Newsletter <newsletter@mailer.example.com>
   To: receiver@example.org
   Subject: Super awesome deals for you
   CFBL-Address: fbl@mailer.example.com; report=arf
   Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
   Content-Type: text/plain; charset=utf-8
   DKIM-Signature: v=1; a=rsa-sha256; d=example.com;
         h=Content-Type:Subject:From:To:Message-ID:
         CFBL-Feedback-ID:CFBL-Address;

   This is a super awesome newsletter.

   Example 2:

   Return-Path: <sender@mailer.example.com>
   From: Awesome Newsletter <newsletter@example.com>
   To: receiver@example.org
   Subject: Super awesome deals for you
   CFBL-Address: fbl@mailer.example.com; report=arf
   Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
   Content-Type: text/plain; charset=utf-8
   DKIM-Signature: v=1; a=rsa-sha256; d=example.com;
         h=Content-Type:Subject:From:To:Message-ID:
         CFBL-Feedback-ID:CFBL-Address;

   This is a super awesome newsletter.

3.1.3.  Third Party Address

   If the domain in RFC5322.From differs from the domain in the CFBL-
   Address header field, an additional valid [DKIM] signature MUST be
   added that matches the domain in the CFBL-Address header field.  The
   other existing valid [DKIM] signature MUST match the domain in the
   RFC5322.From header field.  This double DKIM signature ensures that
   both the domain owner of the RFC5322.From domain and the domain owner
   of the CFBL-Address domain agree on who should receive the Feedback
   Messages.  Both signatures MUST meet the requirements described in
   Section 3.1.4.

   The following example meets this case:

   Return-Path: <sender@saas-mailer.example>
   From: Awesome Newsletter <newsletter@example.com>
   To: receiver@example.org
   Subject: Super awesome deals for you
   CFBL-Address: fbl@saas-mailer.example; report=arf
   Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@example.com>
   Content-Type: text/plain; charset=utf-8
   DKIM-Signature: v=1; a=rsa-sha256; d=saas-mailer.example; s=system;
          h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;
   DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=news;
          h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;

   This is a super awesome newsletter.

   An Email Service Provider may accept pre-signed messages from its
   Message Authors, making it impossible for it to apply the double
   signature described above; in this case, the double signature MUST be
   omitted and the Email Service Provider MUST sign with its domain.
   Therefore, the pre-signed message MUST NOT include "CFBL-Address" and
   "CFBL-Feedback-ID" in its "h=" tag.

   This way, the Email Service Provider has the possibility to accept
   the pre-signed messages and can inject their own CFBL-Address.

   The following example meets this case:

   Return-Path: <newsletter@example.com>
   From: Awesome Newsletter <newsletter@example.com>
   To: receiver@example.org
   Subject: Super awesome deals for you
   CFBL-Address: fbl@saas-mailer.example; report=arf
   Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@example.com>
   Content-Type: text/plain; charset=utf-8
   DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=news;
          h=Subject:From:To:Message-ID;
   DKIM-Signature: v=1; a=rsa-sha256; d=saas-mailer.example; s=system;
          h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;

   This is a super awesome newsletter.

3.1.4.  DKIM Signature

   When present, CFBL-Address and CFBL-Feedback-ID header fields MUST be
   included in the "h=" tag of the aforementioned valid DKIM signature.

   If the domain is not matched by a valid DKIM signature or the header
   field is not covered by the "h=" tag, the Mailbox Provider SHALL NOT
   send a report message.

3.2.  Multiple CFBL-Address Header Fields

   A Message can contain multiple CFBL-Address header fields.  These
   multiple header fields MUST be treated as a list of addresses, each
   of which should receive a report.

3.3.  CFBL-Feedback-ID Header Field

   The Message Originator MAY include a CFBL-Feedback-ID header field in
   its messages for various reasons, e.g., their feedback loop
   processing system can't do anything with the Message-ID header field.

   It is RECOMMENDED that the header field include a hard-to-forge
   protection component, such as an [HMAC] using a secret key, instead
   of a plaintext string.

3.4.  Receiving Report Address

   The receiving report address provided in the CFBL-Address header
   field MUST accept [ARF] reports.

   It is OPTIONAL for the Message Originator to request a [XARF] report,
   as described in Section 3.5.1.

3.5.  Feedback Message

   The Feedback Message (sent by Mailbox Provider to the address
   provided in the CFBL-Address header field) MUST have a valid [DKIM]
   signature.  This signature MUST match the RFC5322.From domain of the
   Feedback Message.

   If the message does not have the required valid [DKIM] signature, the
   Message Originator SHALL NOT process this Feedback Message.

   The Feedback Message MUST be an [ARF] or [XARF] report.  If the
   Message Originator requests it (described in Section 3.5.1) and it is
   technically possible for the Mailbox Provider to do so, the Feedback
   Message MUST be a [XARF] report.  Otherwise, the Feedback Message
   MUST be an [ARF] report.

   The third MIME part of the [ARF] or the "Samples" section of the
   [XARF] report MUST contain the Message-ID [RFC5322] of the received
   message.  If present, the CFBL-Feedback-ID header field of the
   received message MUST be added to the third MIME part of the [ARF] or
   to the "Samples" section of the [XARF] report.

   The Mailbox Provider MAY omit or redact all further header fields
   and/or body to comply with any data regulation laws as described in
   [RFC6590].

3.5.1.  XARF Report

   A Message Originator wishing to receive a [XARF] report MUST append
   "report=xarf" to the CFBL-Address header field (Section 5.1).  The
   report parameter is separated from the report address by a ";".

   The resulting header field would appear as shown below.

   CFBL-Address: fbl@example.com; report=xarf

4.  Implementation

4.1.  Message Originator

   A Message Originator who wishes to use this new mechanism to receive
   Feedback Messages MUST include a CFBL-Address header field in their
   messages.

   It is RECOMMENDED that these Feedback Messages be processed
   automatically.  Each Message Originator must decide for themselves
   what action to take after receiving a Feedback Message.

   The Message Originator MUST take action to address the described
   requirements in Section 3.

4.2.  Mailbox Provider

   A Mailbox Provider who wants to collect user actions that indicate
   the message was not wanted and to send a Feedback Message to the
   Message Originator MAY query the CFBL-Address header field and
   forward the report to the provided CFBL address.

   The Mailbox Provider MUST validate the DKIM requirements of the
   received message described in Section 3.1 and MUST take action to
   address the requirements described in Section 3.5 when sending
   Feedback Messages.

5.  Header Field Syntax

5.1.  CFBL-Address

   The following ABNF imports the rules for fields, CFWS, CRLF, and
   addr-spec from [RFC5322].  Implementations of the CFBL-Address header
   field MUST comply with [RFC6532].

   fields =/ cfbl-address

   cfbl-address = "CFBL-Address:" CFWS addr-spec
                  [";" CFWS report-format] CRLF

   report-format = %s"report=" (%s"arf" / %s"xarf")

5.2.  CFBL-Feedback-ID

   The following ABNF imports the rules for fields, WSP, CRLF, and atext
   from [RFC5322].

   fields =/ cfbl-feedback-id

   cfbl-feedback-id = "CFBL-Feedback-ID:" CFWS fid CRLF

   fid = 1*(atext / ":" / CFWS)

   Empty space is ignored in the fid value and MUST be ignored when
   reassembling the original feedback-id.
   In particular, the Message Originator can safely insert CFWS in the
   fid value in arbitrary places to conform to line length limits when
   adding the header field.

6.  Security Considerations

   This section discusses possible security issues of a CFBL-Address
   header field and their solutions.

6.1.  Attacks on the Feedback Loop Address

   Like any other email address, a CFBL address can be an attack vector
   for malicious messages.  For example, CFBL addresses can be flooded
   with spam.  This is an existing problem with any existing email
   address and is not created by this document.

6.2.  Automatic Suspension of an Account

   Receiving a Feedback Message regarding a Message Author can cause the
   Message Author to be unreachable if an automatic account suspension
   occurs too quickly.  For example, someone sends an invitation to
   their friends, and someone else marks this message as spam for some
   reason.

   If automatic account suspension is too fast, the Message Author's
   account will be blocked and the Message Author will not be able to
   access their emails or send further messages, depending on the
   account suspension the Message Originator has chosen.

   Message Originators must take appropriate measures to prevent account
   suspensions that happen too fast.  Therefore, Message Originators
   have -- mostly proprietary -- ways to assess the trustworthiness of
   an account.  For example, Message Originators may take into account
   the age of the account and/or any previous account suspension before
   suspending an account.

6.3.  Enumeration Attacks / Provoking Unsubscription

   A malicious person may send a series of spoofed Abuse Reporting
   Format (ARF) messages to known CFBL addresses and attempt to guess a
   Message-ID / CFBL-Feedback-ID or any other identifiers.  The
   malicious person may attempt to mass unsubscribe/suspend if such an
   automated system is in place.  This is also an existing problem with
   the current feedback loop implementation and/or One-Click
   Unsubscription [RFC8058].

   The Message Originator must take appropriate measures.  For example,
   the CFBL-Feedback-ID header field (if used) can use a hard-to-forge
   component, such as an [HMAC] with a secret key, instead of a
   plaintext string, to make an enumeration attack impossible.

6.4.  Data Privacy

   The provision of such a header field itself does not pose a data
   privacy issue.  The resulting ARF/XARF report sent by the Mailbox
   Provider to the Message Originator may violate a data privacy law
   because it may contain personal data.

   This document already addresses some parts of this problem and
   describes a way to send a Feedback Message that keeps data privacy
   safe.  As described in Section 3.5, the Mailbox Provider can omit the
   entire body and/or header field and send only the required fields.
   As recommended in [RFC6590], the Mailbox Provider can also redact the
   data in question.  Nevertheless, each Mailbox Provider must consider
   for itself whether this implementation is acceptable and complies
   with existing data privacy laws in their country.

   As described in Sections 3.5 and 3.3, it is also strongly RECOMMENDED
   that the Message-ID and CFBL-Feedback-ID (if used) contain a
   component that is difficult to forge, such as an [HMAC] that uses a
   secret key, rather than a plaintext string.  See Section 8.3 for an
   example.

6.5.  Abusing for Validity and Existence Queries

   This mechanism could be abused to determine the validity and
   existence of an email address, exhibiting another potential data
   privacy issue.  If the Mailbox Provider has an automatic process to
   generate a Feedback Message for a received message, it may not be
   doing the mailbox owner any favors.  As the Mailbox Provider
   generates an automatic Feedback Message for the received message, the
   Mailbox Provider proves to the Message Originator that this mailbox
   exists for sure because it is based on a manual action of the mailbox
   owner.

   The receiving Mailbox Provider must take appropriate measures.  One
   possible countermeasure could be pre-existing reputation data
   (usually proprietary data), for example.  Using this data, the
   Mailbox Provider can assess the trustworthiness of a Message
   Originator and decide whether to send a Feedback Message based on
   this information.

7.  IANA Considerations

7.1.  CFBL-Address

   IANA has registered a new header field, per [RFC3864], in the
   "Provisional Message Header Field Names" registry:

   Header Field Name:  CFBL-Address

   Protocol:  mail

   Status:

   Author/Change controller:  Jan-Philipp Benecke <jpb@cleverreach.com>

   Reference:  RFC 9477

7.2.  CFBL-Feedback-ID

   IANA has registered a new header field, per [RFC3864], in the
   "Provisional Message Header Field Names" registry:

   Header Field Name:  CFBL-Feedback-ID

   Protocol:  mail

   Status:

   Author/Change controller:  Jan-Philipp Benecke <jpb@cleverreach.com>

   Reference:  RFC 9477

8.  Examples

   For simplicity, the DKIM header field has been shortened, and some
   tags have been omitted.

8.1.  Simple

   Email about the report will be generated:

   Return-Path: <sender@mailer.example.com>
   From: Awesome Newsletter <newsletter@example.com>
   To: me@example.net
   Subject: Super awesome deals for you
   CFBL-Address: fbl@example.com; report=arf
   CFBL-Feedback-ID: 111:222:333:4444
   Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
   Content-Type: text/plain; charset=utf-8
   DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=news;
          h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;

   This is a super awesome newsletter.

   Resulting ARF report:

   ------=_Part_240060962_1083385345.1592993161900
   Content-Type: message/feedback-report
   Content-Transfer-Encoding: 7bit

   Feedback-Type: abuse
   User-Agent: FBL/0.1
   Version: 0.1
   Original-Mail-From: sender@mailer.example.com
   Arrival-Date: Tue, 23 Jun 2020 06:31:38 GMT
   Reported-Domain: example.com
   Source-IP: 192.0.2.1

   ------=_Part_240060962_1083385345.1592993161900
   Content-Type: text/rfc822; charset=UTF-8
   Content-Transfer-Encoding: 7bit

   Return-Path: <sender@mailer.example.com>
   From: Awesome Newsletter <newsletter@example.com>
   To: me@example.net
   Subject: Super awesome deals for you
   CFBL-Address: fbl@example.com; report=arf
   CFBL-Feedback-ID: 111:222:333:4444
   Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
   Content-Type: text/plain; charset=utf-8
   DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=news;
          h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;

   This is a super awesome newsletter.
   ------=_Part_240060962_1083385345.1592993161900--

8.2.  Data Privacy Safe Report

   Email about the report will be generated:

   Return-Path: <sender@mailer.example.com>
   From: Awesome Newsletter <newsletter@example.com>
   To: me@example.net
   Subject: Super awesome deals for you
   CFBL-Address: fbl@example.com; report=arf
   CFBL-Feedback-ID: 111:222:333:4444
   Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
   Content-Type: text/plain; charset=utf-8
   DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=news;
          h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;

   This is a super awesome newsletter.

   Resulting ARF report that only contains the CFBL-Feedback-ID:

   ------=_Part_240060962_1083385345.1592993161900
   Content-Type: message/feedback-report
   Content-Transfer-Encoding: 7bit

   Feedback-Type: abuse
   User-Agent: FBL/0.1
   Version: 0.1
   Original-Mail-From: sender@mailer.example.com
   Arrival-Date: Tue, 23 Jun 2020 06:31:38 GMT
   Reported-Domain: example.com
   Source-IP: 2001:DB8::25

   ------=_Part_240060962_1083385345.1592993161900
   Content-Type: text/rfc822-headers; charset=UTF-8
   Content-Transfer-Encoding: 7bit

   CFBL-Feedback-ID: 111:222:333:4444
   ------=_Part_240060962_1083385345.1592993161900--

8.3.  Data Privacy Safe Report with HMAC

   Email about the report will be generated:

   Return-Path: <sender@mailer.example.com>
   From: Awesome Newsletter <newsletter@example.com>
   To: me@example.net
   Subject: Super awesome deals for you
   CFBL-Address: fbl@example.com; report=arf
   CFBL-Feedback-ID: 3789e1ae1938aa2f0dfdfa48b20d8f8bc6c21ac34fc5023d
          63f9e64a43dfedc0
   Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
   Content-Type: text/plain; charset=utf-8
   DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=news;
          h=Subject:From:To:Message-ID:CFBL-Feedback-ID:CFBL-Address;

   This is a super awesome newsletter.

   Resulting ARF report that only contains the CFBL-Feedback-ID:

   ------=_Part_240060962_1083385345.1592993161900
   Content-Type: message/feedback-report
   Content-Transfer-Encoding: 7bit

   Feedback-Type: abuse
   User-Agent: FBL/0.1
   Version: 0.1
   Original-Mail-From: sender@mailer.example.com
   Arrival-Date: Tue, 23 Jun 2020 06:31:38 GMT
   Reported-Domain: example.com
   Source-IP: 2001:DB8::25

   ------=_Part_240060962_1083385345.1592993161900
   Content-Type: text/rfc822-headers; charset=UTF-8
   Content-Transfer-Encoding: 7bit

   CFBL-Feedback-ID: 3789e1ae1938aa2f0dfdfa48b20d8f8bc6c21ac34fc5023d
          63f9e64a43dfedc0
   ------=_Part_240060962_1083385345.1592993161900--

9.  References

9.1.  Normative References

   [ARF]      Shafranovich, Y., Levine, J., and M. Kucherawy, "An
              Extensible Format for Email Feedback Reports", RFC 5965,
              DOI 10.17487/RFC5965, August 2010,
              <https://www.rfc-editor.org/info/rfc5965>.

   [DKIM]     Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,
              <https://www.rfc-editor.org/info/rfc6376>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

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

   [RFC6449]  Falk, J., Ed., "Complaint Feedback Loop Operational
              Recommendations", RFC 6449, DOI 10.17487/RFC6449, November
              2011, <https://www.rfc-editor.org/info/rfc6449>.

   [RFC6532]  Yang, A., Steele, S., and N. Freed, "Internationalized
              Email Headers", RFC 6532, DOI 10.17487/RFC6532, February
              2012, <https://www.rfc-editor.org/info/rfc6532>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [XARF]     "XARF - eXtended Abuse Reporting Format", commit cc1a6e6,
              March 2023, <https://github.com/abusix/xarf>.

9.2.  Informative References

   [HMAC]     Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,
              <https://www.rfc-editor.org/info/rfc3864>.

   [RFC6590]  Falk, J., Ed. and M. Kucherawy, Ed., "Redaction of
              Potentially Sensitive Data from Mail Abuse Reports",
              RFC 6590, DOI 10.17487/RFC6590, April 2012,
              <https://www.rfc-editor.org/info/rfc6590>.

   [RFC8058]  Levine, J. and T. Herkula, "Signaling One-Click
              Functionality for List Email Headers", RFC 8058,
              DOI 10.17487/RFC8058, January 2017,
              <https://www.rfc-editor.org/info/rfc8058>.

Acknowledgments

   Technical and editorial reviews were provided by the colleagues at
   CleverReach, the colleagues at Certified Senders Alliance and eco.de;
   Arne Allisat, Tobias Herkula and Levent Ulucan (1&1 Mail & Media);
   and Sven Krohlas (BFK Edv-consulting).

Author's Address

   Jan-Philipp Benecke
   CleverReach GmbH & Co. KG
   Schafjueckenweg 2
   26180 Rastede
   Germany
   Phone: +49 4402 97390-16
   Email: jpb@cleverreach.com