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Complaint Feedback Loop Operational Recommendations
RFC 6449

Document Type RFC - Informational (November 2011)
Was draft-jdfalk-maawg-cfblbcp (individual in app area)
Author J.D. Falk
Last updated 2018-12-20
RFC stream Internet Engineering Task Force (IETF)
IESG Responsible AD Pete Resnick
Send notices to (None)
RFC 6449
Internet Engineering Task Force (IETF)                      J. Falk, Ed.
Request for Comments: 6449                       Messaging Anti-Abuse WG
Category: Informational                                    November 2011
ISSN: 2070-1721

          Complaint Feedback Loop Operational Recommendations


   Complaint Feedback Loops similar to those described herein have
   existed for more than a decade, resulting in many de facto standards
   and best practices.  This document is an attempt to codify, and thus
   clarify, the ways that both providers and consumers of these feedback
   mechanisms intend to use the feedback, describing some already common
   industry practices.

   This document is the result of cooperative efforts within the
   Messaging Anti-Abuse Working Group, a trade organization separate
   from the IETF.  The original MAAWG document upon which this document
   is based was published in April, 2010.  This document does not
   represent the consensus of the IETF; rather it is being published as
   an Informational RFC to make it widely available to the Internet
   community and simplify reference to this material from IETF work.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It has been approved for publication by the Internet
   Engineering Steering Group (IESG).  Not all documents approved by the
   IESG are a candidate for any level of Internet Standard; see Section
   2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Copyright Notice

   Copyright (c) 2011 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may not be modified, and derivative works of it may not
   be created, except to format it for publication as an RFC or to
   translate it into languages other than English.

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Table of Contents

   1. Overview ........................................................4
   2. Glossary of Standard Terms ......................................5
   3. Mailbox Providers and Feedback Providers ........................9
      3.1. Benefits of Providing Feedback .............................9
      3.2. Collecting Complaints .....................................10
      3.3. Creating Reports ..........................................11
      3.4. Policy Concerns ...........................................11
           3.4.1. Privacy and Regulatory Compliance ..................11
           3.4.2. Terms of Use .......................................12
      3.5. Handling Requests to Receive Feedback .....................12
           3.5.1. Application Web Site ...............................13
           3.5.2. Saying No ..........................................14
           3.5.3. Automation .........................................14
      3.6. Ongoing Maintenance .......................................15
           3.6.1. IP Validation ......................................15
           3.6.2. Email Address Validation ...........................16
           3.6.3. Feedback Production Changes ........................16
   4. Feedback Consumers .............................................16
      4.1. Preparation ...............................................17
      4.2. What You'll Receive .......................................18
           4.2.1. Feedback Reports ...................................18
           4.2.2. Administrative Messages ............................18
           4.2.3. Report Cards .......................................18
      4.3. Handling Feedback Messages ................................19
           4.3.1. Unsubscription or Suppression ......................20
           4.3.2. Trending and Reporting .............................21
      4.4. Automatically Handling an Incoming Feedback Stream ........22
   5. Conclusion .....................................................25
   6. Acknowledgments ................................................26
      6.1. About MAAWG ...............................................26
   7. Security Considerations ........................................26
   8. Informative References .........................................26
   Appendix A. Abuse Reporting Format (ARF) ..........................28
     A.1. A Brief History ............................................28
     A.2. Structure of an ARF Message ................................28
   Appendix B. Using DKIM to Route Feedback ..........................29
   Appendix C. Unsolicited Feedback ..................................30
     C.1. Guidelines .................................................30
     C.2. Pros .......................................................30
     C.3. Cons .......................................................31

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1.  Overview

   The intent of a Complaint Feedback Loop is to provide Feedback
   Consumers with information necessary to mitigate Spam or the
   perception of Spam.  Thus, feedback was originally only offered to
   mailbox, access, and network providers -- in other words, to ISPs --
   who would use the feedback to identify network compromises and
   fraudulent accounts or to notify their downstream customer that there
   may be a problem.

   Senders of bulk, transactional, social, or other types of email can
   also use this feedback to adjust their mailing practices, using Spam
   Complaints as an indicator of whether the Recipient wishes to
   continue receiving email.  Common reactions often include refining
   opt-in practices, mailing frequency, list management, message
   content, and other measures.  Over time, this has become the Feedback
   Consumer use case most often discussed at MAAWG meetings and other
   industry events -- but readers are cautioned that it is not the sole
   use for feedback.

                              [ Feedback Consumer Database ]
   [  User   ]    [ Mailbox  ]         [ Feedback ]
   [ Reports ]--->[ Provider ]--SMTP-->[ Provider ]
   [  Spam   ]         |                    |
                       V                    V               [ Feedback ]
             [Spam Filter Rules]    [ ARF Message ]--SMTP-->[ Consumer ]

                                 Figure 1

   When an End User of a Mailbox Provider issues a Spam Complaint, the
   Feedback Provider sends a report to the Feedback Consumer.  This
   report may include the Full Body of the original email or (less
   commonly) only the full header of the original email.  Some Feedback
   Providers will redact information deemed private, such as the Message
   Recipient's Email Address.

   Ensuring that Feedback Messages are only sent to authorized Feedback
   Consumers is the responsibility of the Feedback Provider, with the
   identity of each message Sender generally determined from the SMTP
   session's connecting IP address or a message's DomainKeys Identified
   Mail (DKIM) signature domain, both of which are hard to forge.  This
   is important because Spammers and other miscreants may also attempt
   to apply for Feedback Loops on networks not belonging to them, in an
   attempt to steal Email Addresses and other private personal or
   corporate information.

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   It is the responsibility of the Feedback Consumer to identify the
   source and nature of the original message in the reports they receive
   and take any appropriate action.  The Feedback Provider does not make
   any claims or judgments about the validity of the complaint, beyond
   whatever technical data the Feedback Provider has themselves
   included.  Every complaint is forwarded to the Feedback Consumer
   without human review, without any additional application of filters;
   thus, some individual reports may prove not to be actionable.

   The Feedback Consumer and the Feedback Provider will each evaluate a
   Spam Complaint for validity and take whatever action deemed necessary
   from their own perspective and, in most cases, will not communicate
   with each other which actions were (or were not) taken.  Similarly,
   it is rare for any party to communicate further with the End User who
   initiated the complaint.

2.  Glossary of Standard Terms

   Wherever possible, these terms are derived from [RFC5598].

   o  Abuse Reporting Format - The standard format for Feedback
      Messages, defined in Appendix A and [MARF].

   o  Access Provider - Any company or organization that provides End
      Users with access to the Internet.  It may or may not be the same
      entity that the End User uses as a Mailbox Provider.

   o  Application for Feedback Loop - the process, manual or online, by
      which a prospective Feedback Consumer requests to receive a
      Feedback Loop from a particular Feedback Provider.

   o  ARF -- See "Abuse Reporting Format".

   o  ARF Report -- See "Feedback Message".

   o  Body - See "Full Body".

   o  Complaint or Complaint Message - See "Feedback Message".

   o  Complaint Feedback Loop - See Overview and Taxonomy section.

   o  Complaint Stream - See "Feedback Stream".

   o  Delivery - See "Message Delivery".

   o  DKIM - DomainKeys Identified Mail, further described in the MAAWG
      email authentication white paper "Trust in Email Begins with
      Authentication" [Trust] and [DKIM].

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   o  End User - A customer of a Mailbox Provider or Access Provider.

   o  Envelope Sender - The Email Address included as the argument to
      the [SMTP] "MAIL" command during transfer of a message.

   o  Email Address - A string of the form user@domain, where the domain
      (after the @ symbol) is used to determine where to transfer an
      email message so that it may be delivered to the mailbox specified
      by the username (before the @ symbol).  The precise technical
      format of an Email Address is defined in [SMTP].  Email delivery
      can be a complex process and is not described further in this

   o  Email Service Provider (ESP) - A provider of email sending
      services; the ESP is often a Message Originator working on behalf
      of a Message Author.  MAAWG uses the term "ESP" solely for this
      definition and does not refer to a Mailbox Provider for End Users
      as ESPs.

   o  FBL - The acronym "FBL" (Feedback Loop) is intentionally not used
      in this document.

   o  Feedback or Feedback Stream - A set (often a continuous stream) of
      Feedback Messages sent from a single Feedback Provider to a single
      Feedback Consumer.

   o  Feedback Consumer - A Recipient of the Feedback Messages, almost
      always on behalf of or otherwise associated with the Message
      Originator.  Often the Message Originator and Feedback Consumer
      are the same entity, but we describe them separately in this
      document because they are each responsible for different parts of
      the Complaint Feedback Loop process, as demonstrated in the
      flowchart in the Overview section.

   o  Feedback Loop - See Complaint Feedback Loop.

   o  Feedback Message - A single message, often using the Abuse
      Reporting Format defined above and outlined in Appendix 1, which
      is part of a Feedback Stream.

   o  Feedback Provider - The Sender of the Feedback Messages, almost
      always on behalf of or associated with the Mailbox Provider.
      Often the Mailbox Provider and Feedback Provider are the same
      entity, but we describe them separately in this document because
      they are each responsible for different parts of the Complaint
      Feedback Loop process.  In some instances, the Feedback Provider
      may be operating solely on behalf of the Message Recipient,
      without any direct participation from their Mailbox Provider.

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   o  Full Body - An email message (the "DATA" portion of the [SMTP]
      conversation) consists of two parts: the header and the body.  The
      "Full Body" is simply the entirety of the body of the message,
      without modification or truncation.  Note that images or other so-
      called "attachments" are actually part of the body, designated in
      accordance with the [MIME] standard.

   o  Full Header Section - An email message (the "DATA" portion of the
      [SMTP] conversation) consists of two parts: the header and the
      body.  The header contains multiple header fields, each formatted
      as "Header-Name: header contents".  Although most Mail User Agents
      (MUAs) only show the basic four header fields (From, To, Date, and
      Subject), every message includes additional header fields that
      primarily contain diagnostic information or data intended to
      assist automatic processing.  Often informally called "Full
      Headers".  These fields are fully defined in [RFC5322]

   o  Header - See "Full Header Section" above.

   o  ISP - Internet Service Provider, usually referred to as either an
      Access Provider or a Mailbox Provider in this paper.

   o  Mail Abuse Reporting Format (MARF) - See "Abuse Reporting Format"

   o  Mailbox Provider - A company or organization that provides email
      mailbox hosting services for End Users and/or organizations.  Many
      Mailbox Providers are also Access Providers.

   o  Mailing List - A set of Email Addresses that will receive specific
      messages in accordance with the policies of that particular list.

   o  Message-ID Header Field - One of the diagnostic header fields
      included in every email message (see "Full Header Section" above)
      is the Message-ID.  Theoretically, it is a unique identifier for
      that individual message.

   o  Message Delivery - The process of transferring a message from one
      mail transfer agent (MTA) to another.  Once the message has been
      accepted by the MTA operating on behalf of the Recipient, it is
      considered to be "delivered" regardless of further processing or
      filtering that may take place after that point.

   o  Message Originator - The Sender, but not necessarily the author or
      creator, of a message.

   o  Message Recipient - The person or mailbox that receives a message
      as final point of delivery.

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   o  MIME - Multipurpose Internet Mail Extensions refers to a set of
      standards permitting non-plaintext data to be embedded in the body
      of a message.  Concepts such as file attachments and formatted or
      "rich" text are all accomplished solely through [MIME].

   o  MUA - Mail User Agent; loosely referring to the software used by
      an End User to access, interact with, or send email messages.

   o  Provider - See "Feedback Provider" above.

   o  Received Header Field - Diagnostic header fields included in an
      email message (see "Full Header Section" above) that start with
      "Received:" and document (from bottom to top) the path a message
      traversed from the originator to its current position.

   o  Recipient - See "Message Recipient" above.

   o  Return-Path - An optional message header field (see "Full Header
      Section" above) that indicates the Envelope Sender of the message.

   o  Reverse DNS - The [DNS] name of an IP address, called "reverse"
      because it is the inverse of the more user-visible query that
      returns the IP address of a DNS name.  Further, a Reverse DNS
      query returns a PTR record rather than an A record.

   o  Sender - see "Message Originator" above.

   o  SMTP - Simple Mail Transfer Protocol, the mechanism and language
      for transferring an email message from one place to another as
      defined in RFC 5321 [SMTP].

   o  Spam - For the purposes of this document (and for most Complaint
      Feedback Loops), "spam" is defined as any message that the
      Recipient chooses to complain about, regardless of the intent of
      the message's author or Sender.

   o  Spam Complaint - See "Complaint" above.

   o  Spammer - An entity that knowingly, intentionally sends Spam
      messages (see "Spam" above).

   o  Terms of Use - A legal document describing how a particular system
      or service is to be used.

   o  VERP - Variable Envelope Return Path [VERP], an informally
      standardized method for encoding information about the Message
      Recipient into the return path while delivering a message in order
      to ensure that any non-delivery notices are processed correctly.

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3.  Mailbox Providers and Feedback Providers

   In practice, a Mailbox Provider receives complaints from their End
   Users, and is often also the Feedback Provider for those complaints
   and is a consumer of feedback from other providers.  In this
   document, we separate the Mailbox Provider and Feedback Provider
   functions to reduce possible confusion over those cases where they
   are separate, and we also urge Mailbox Providers to read the
   "Feedback Consumer" section later in this document.

3.1.  Benefits of Providing Feedback

   The decision to provide a Complaint Feedback Loop service should not
   be taken lightly.  The benefits of a Feedback Loop are great, but
   success depends on a sound plan, organized implementation, and
   dedication to upkeep.

   What are some benefits of providing feedback to fellow Mailbox
   Providers and Access Providers?  Primarily, other industry actors are
   quickly alerted to Spam outbreaks on their networks.

   End Users are becoming more aware of and comfortable with mechanisms
   to report Spam, and a Feedback Loop does just what it implies; it
   closes the loop.  The End User's complaint makes its way back to the
   Message Originator (not necessarily the message Sender, who may be a
   Spammer), allowing the originator to take appropriate action.  In
   this process, the mail system operator is just a messenger, relieved
   of the responsibility of reviewing and forwarding complaints

   Further, because every complaint is sent immediately -- without any
   review or analysis by the Feedback Provider -- the complaint is
   received by the Feedback Consumer in near real time.  If the Feedback
   Consumer is paying attention to their Feedback Stream and taking
   appropriate action on it, the receiving Mailbox Provider receives
   less Spam, blocks less legitimate mail, and does not have to assign
   staff to follow up with the originating network.  If the Mailbox
   Provider does not pay attention to its Feedback Stream, and does not
   take appropriate action, the Feedback Provider may block or otherwise
   filter the email from that Message Originator, considering the
   Feedback Messages to be sufficient notice.

   What are some benefits of providing Feedback Loops to bulk Feedback
   Consumers?  As Message Recipients become more aware of and
   comfortable with Spam reporting mechanisms, they often prefer this
   method over the often-confusing and inconsistent "unsubscribe" or
   "opt-out" mechanisms provided by most legitimate Message Originators
   or Senders.

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   End Users often do not remember what lists they signed up for or are
   otherwise not confident in the established relationship they may have
   with a message Sender.  As such, they often choose to report messages
   as Spam to their Mailbox Providers, considering that to be sufficient
   notification of their desire not to receive such email in the future.

   If the Message Originator is paying attention to and taking
   appropriate action on their Feedback Stream, it will have a happier
   set of Message Recipients and should receive fewer Spam Complaints
   (assuming their opt-in processes are sound).  If the Message
   Originator is not paying attention to Feedback and not taking
   appropriate action, the Mailbox Provider may consider the Feedback
   Stream sufficient notice that messages from that originator may no
   longer be accepted in the future.

3.2.  Collecting Complaints

   To produce Feedback Messages and to ensure they are useful, the
   Feedback Provider needs to obtain near real-time complaints from the
   Mailbox Provider's users.  This is typically done by integrating the
   feedback mechanism with the collection of Spam reports from its

   These reports are typically made using the "Report Spam" buttons
   integrated into Webmail interfaces, or a proprietary desktop client
   provided to users.  Mailbox Providers may also look at deploying a
   toolbar or MUA plug-in that provides a "Report Spam" button in the
   MUA interface.

   Usability studies with average users should be performed on all
   interface changes before implementation.  A "help" interface should
   also be available to educate users about how the Spam button should
   be used and what it does.

   If the Mailbox Provider does not offer its customers a mail client
   with this button, then the Feedback Provider's chances for providing
   an effective Feedback Loop are slim.  While it is possible for the
   Mailbox Provider to instruct its customers to forward unwanted mail
   to a central location and for the Mailbox Provider to explain how to
   ensure the report includes headers and bodies, the success rate of
   customers doing so tends to be low.  Even those complaints that do
   contain all required information might prove difficult to parse, as
   variations in formatting and content types will lead to automated
   tools being consistently updated with new logic blocks for each
   variation that occurs.

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3.3.  Creating Reports

   It is recommended that Feedback Messages be sent using the standard
   Abuse Reporting Format, to facilitate uniformity and ease of
   processing for all consumers of feedback.  This will also enable the
   Feedback Provider to extensively automate the processes of generating
   and sending Feedback Messages and of analyzing complaint statistics.
   This format is described further in Appendix 1.

   Feedback Loops are usually (but not always) keyed to the "last hop"
   IP address (i.e., the IP address that passed the unwanted message to
   the Mailbox Provider's servers).  Consequently, the Feedback Provider
   must be able to process the header from each complaint to determine
   the IP address for the complaint.

   A Feedback Provider may wish to provide, as part of its Feedback
   Loop, other information beyond Spam Complaints that Feedback
   Consumers may find valuable.  It might include summary delivery
   statistics (volume, inbox delivery rate, Spam trap hits, etc.) or
   other data that the Feedback Provider may deem pertinent to Feedback

   Any mature Feedback Loop system will produce situations in which the
   Feedback Consumer may have follow-up questions or have other
   information to provide in regard to the feedback.  Feedback Messages
   should include contact information (typically an Email Address) for
   the Feedback Consumer to use for such questions, and ideally the
   contact Email Address will feed into a ticket system or other
   automated tool used by the Mailbox Provider's postmaster and/or anti-
   abuse staff for handling general email delivery issues.

3.4.  Policy Concerns

3.4.1.  Privacy and Regulatory Compliance

   Feedback Messages provide information relayed by Feedback Providers
   from a Mailbox Provider's End Users to the Feedback Consumer.  There
   might not be any concerns with relaying non-private data to a third
   party.  However, the information provided in the complaints generated
   by the user must be evaluated and any data deemed private may need to
   be removed before distributing to a third party, per local policy.
   For example, the Recipient's or reporter's Email Address and IP
   address may be categorized as private data and removed from the
   feedback report that is provided to the Feedback Consumer.  Privacy
   laws and corporate data classification standards should be consulted
   when determining what information should be considered private.

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   Information provided by the Feedback Consumer to the Feedback
   Provider for the purpose of enrolling in the Feedback Loop should
   also be kept private.  It should only be shared or used for the
   purposes explicitly agreed to during the enrollment process (see the
   "Terms of Use" section below).

   Feedback Loops inevitably span country borders.  Local laws and
   regulations regarding distribution of information domestically and
   internationally need to be considered when implementing a Feedback
   Loop program.  For example, in some European countries, data exchange
   requires permission from governing bodies.  The terms and
   circumstances surrounding the exchange of data need to be clearly
   defined and approved.

3.4.2.  Terms of Use

   A written Terms of Use agreement should be provided by the Feedback
   Provider and agreed to by the Feedback Consumer before any feedback
   is provided.  The following concepts should be considered when
   drafting the terms of use agreement:

   o  Data provided in Feedback Messages are provided to a specific,
      approved entity.  Information should not be transmitted outside of
      the intended, approved Recipient.  Any inappropriate use of the
      information can lead to immediate termination from the feedback

   o  Consumers of Feedback have a responsibility to keep the
      information they provide for Feedback Loop purposes -- such as
      abuse contact information, IP addresses, and other records --
      accurate and up to date.

   o  The providing of Feedback information is a privilege and needs to
      be treated appropriately.  It does not entitle the consumer of the
      feedback to any special sending privileges.

   o  Approval and continued enrollment in the program is a privilege
      that can be denied or revoked for any reason and at any time.

3.5.  Handling Requests to Receive Feedback

   There should be a streamlined application process for receiving
   feedback and the vetting of such applications.  This vetting may be
   stringent in cases where the Mailbox Provider chooses to tie its
   Complaint Feedback Loop program to a whitelist.  Criteria may involve
   the following:

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   o  Cross-checking that the requestor is indeed authorized to receive
      feedback for the IP addresses concerned.

   o  Gathering other information such as whether the IPs are an ISP
      smarthost network, a webhosting farm, an email marketing or
      Mailing List service, or other entity.

   o  Requesting information such as a link to the policies of the
      requestor, contacts to send Feedback Messages, and escalation
      points of contact.

   Ideally, enrollment will be a two-step process, with the applicant
   filling out a form and being required to receive and acknowledge a
   confirmation email (best sent to abuse@ or postmaster@ the domain in
   question) before the applicant's request is even put into the queue
   for the Feedback Provider to process.

   Ownership of IP addresses can and should be cross-checked by means of
   origin Autonomous System Number (ASN), WHOIS/RWHOIS records, Reverse
   DNS of the sending hosts, and other sources.  This can be automated
   to some extent, but it often requires some manual processing.

3.5.1.  Application Web Site

   Applications for Feedback Loops can be accepted on a stand-alone web
   site or can be part of the Mailbox Provider's postmaster site.
   Regardless, the web site for the Complaint Feedback Loop program
   should contain other content specific to the Feedback Loop, including
   FAQs for the Feedback Loop program, the Terms of Service for the
   Feedback Loop, and perhaps a method for enrolled parties to modify
   their existing enrollments.

   The web site should also provide the Feedback Consumer with general
   information on how the feedback will be sent, including:

   o  Report Format (ARF or otherwise)

   o  Sending IP addresses and/or DKIM "d=" string

   o  "From" Email Address

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3.5.2.  Saying No

   Denial of a Feedback Loop application may be appropriate in certain
   cases such as:

   o  Where the Feedback Provider suspects "gaming" of delivery policies
      via the Feedback received, with attempts to pollute Feedback Loop
      metrics by, for example, creating bogus accounts and reporting
      false negatives with these, to offset the negative reputation
      caused by high complaint rates.

   o  Where the Feedback Provider has decided to block the Message
      Originator's IP space for which feedback has been requested on the
      grounds that email from that originator has a sufficiently
      negative reputation that it will not be delivered at all.  This is
      somewhat on the lines of a global unsubscribe of the Message
      Provider's users from the originator's lists, which would make
      rendering additional feedback unnecessary.

   It is recommended that the Feedback Provider send notification if an
   application is denied.  Additionally, they should maintain a
   documented, clear, and transparent appeals process for denial of
   requests.  This process can be as simple as the prospective Feedback
   Consumer replying to the denial email requesting review or escalation
   to a team lead, which also cites reasons the application should be

3.5.3.  Automation

   For a Feedback Loop to be cost-effective and usable for large
   Feedback Consumers and Feedback Providers, it must be possible for
   reports to be generated and processed automatically without any human
   interaction.  On the other hand, it should be possible for small
   Feedback Consumers to handle a low volume of reports manually,
   without requiring any automation.

   In automating the feedback process, the consumer of the Feedback
   Stream must receive enough information about the report that it can
   take appropriate action, typically to remove the Recipient from the
   Mailing List about which it is sending a report.  The Recipient's
   Email Address is not enough, as the Recipient may be on several
   Mailing Lists managed by the Feedback Loop consumer and only need to
   be removed from the particular list reported.

   Also, some producers of Feedback Loops might redact the Recipient's
   Email Address for privacy reasons.  Effective implementation of a
   Complaint Feedback Loop requires that the Feedback Provider put in

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   place as many automated processes and tools as feasible to handle all
   aspects of the process.  Feedback Providers should seek to automate
   or script the following:

   o  Accepting and validating Feedback Loop Applications from
      prospective Feedback Consumers.

   o  Processing requests to determine whether or not they meet the
      Feedback Provider's criteria for enrollment in the program.

   o  Accepting Spam Complaints from End Users; this will form the bulk
      (and perhaps sole) component of the feedback sent by the Feedback

   o  Production of Feedback Messages from Spam Complaints.

   o  Production of other Feedback Loop artifacts as chosen by the
      Feedback Provider.

   o  Optionally, provision of a mechanism for Feedback Consumers to
      further engage a Feedback Provider about a given Feedback Message.

   o  Ongoing validation of Feedback Loop enrollments to determine if a
      currently enrolled IP address or network merits continued
      inclusion in the Feedback Loop.

   o  Optional periodic emails to Feedback Consumers to determine if
      their enrolled Email Addresses are still valid.

3.6.  Ongoing Maintenance

   It is recommended that self-service maintenance be offered to
   Feedback Consumers, to the extent practicable.  The more they can do
   themselves, the less you have to do.

3.6.1.  IP Validation

   The criteria that a Feedback Provider uses to validate a Feedback
   Loop application may change over time.  It is a near certainty at
   least some subset of Feedback Consumers enrolled to receive feedback
   will at some point after enrollment fail to meet those criteria,
   regardless of whether or not the criteria change.

   The Feedback Provider should put in place tools to periodically
   re-validate all Feedback Consumers enrolled in its Feedback Loop
   system against its current criteria.  Additionally, the Feedback

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   Provider will likely have objective criteria for remaining in the
   Feedback Loop for enrolled Feedback Consumers; the enrolled consumers
   should be validated against those criteria as well.

3.6.2.  Email Address Validation

   Just as some Mailing List software has the built-in ability to send
   periodic "probe" emails to subscribed addresses to validate them, so
   too should the Feedback Provider develop tools to send similar emails
   to the addresses receiving Feedback Messages to ensure that they are
   valid.  This is especially true for the addresses that are not the
   abuse@ and postmaster@ addresses originally used as part of the
   enrollment acknowledgment step.  Over time, people may change
   employers, or at least roles, and validating the Email Addresses
   associated with an IP is one way for the Feedback Provider to ensure
   that Feedback Messages are still being accepted and acted upon by the
   Feedback Consumer.

3.6.3.  Feedback Production Changes

   Updating Feedback Consumers when one's own IP addresses are changing
   is an important aspect of Feedback Loop maintenance.  The exact
   format, automation, and other considerations of these updates are
   outside the scope of this document, but are topics worthy of further
   discussion and eventual documentation.

4.  Feedback Consumers

   A Feedback Consumer receives its Feedback Messages after its
   submitted Application for a Complaint Feedback Loop is approved.  A
   Feedback Consumer will usually have Complaint Feedback Loop
   subscriptions set up with multiple Feedback Providers.  Different
   Feedback Streams may be in different formats or include different
   information, and the Feedback Consumer should identify a process to
   organize the data received and take appropriate action.

   A Feedback Consumer, Mailbox Provider, or Access Provider (i.e., a
   hosting company or ISP) will use this Feedback to identify network
   compromises, fraudulent accounts, policy violations, and other
   concerns.  The Feedback Loop provides real-time visibility into Spam
   Complaints from Message Recipients, greatly enabling these Mailbox
   Providers to mitigate Spam propagating from their networks.

   Senders of bulk email should use the complaints to make decisions
   regarding future mailings.  Such decisions may include one or more of
   the following: modification of email frequency, branding, opt-in
   practices, or list management.

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   The authors of this document urge those who are solely Feedback
   Consumers to also read the previous sections for Mailbox Providers
   and Feedback Providers.  This will provide the proper context of the
   recommendations included below.

   Further recommendations for bulk senders may be found in the MAAWG
   Sender Best Communications Practices [MAAWG-BCP].

4.1.  Preparation

   Feedback Consumers need to prepare to process and act on feedback
   before asking to receive it.  At a minimum, make sure to have:

   1.  The "Role" Email Addresses such as abuse@ and postmaster@.  The
       person who applies for the Feedback needs to make sure they have
       access to these Email Addresses.  Feedback Providers often send a
       confirmation link to those accounts to prevent End Users,
       Spammers, or competitors from signing up for Feedback for which
       they are not authorized.

   2.  A dedicated Email Address to receive the Feedback Messages, such
       as or  While not
       required, this will make it easier for to process the reports

   3.  A list of IP addresses for which you want to receive Feedback
       Messages, making sure you can prove the ownership of the IP
       addresses and associated domains.  Feedback Providers often
       require that:

       *  Reverse DNS for each IP shares the domain of either the
          applicant's Email Address or the Email Address that will be
          receiving the Feedback Messages.

       *  WHOIS information for the IPs requested is obviously
          associated with the domain name.

   4.  Contact information such as name, Email Address, phone number,
       and other relevant information.

   5.  The knowledge that if the application form asks for your credit
       card number or other financial information, it is assuredly a

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4.2.  What You'll Receive

   Once a Feedback Consumer has signed up to receive feedback from a
   Feedback Provider, it may also receive several other sorts of
   delivery-related reports.  This includes Feedback Messages,
   administrative messages, and other messages.

4.2.1.  Feedback Reports

   Feedback Messages are the main emails generally associated with a
   Feedback Loop.  Each time a Recipient hits the "This Is Spam" button,
   the Feedback Loop system creates a boilerplate report with a copy of
   the original email attached and sends it to the consumer of the
   Feedback Loop.

   The handling of feedback reports is discussed in the next section.

4.2.2.  Administrative Messages

   Administrative messages will typically be sent to the Email Address
   provided for contacting the person who originally applied for the
   Feedback Loop, rather than to the address provided for handling the
   Feedback Messages.  These messages are likely to be sent infrequently
   and irregularly, but it is important they are seen by the person
   managing the Feedback Stream processor in a timely manner.  It is
   usually a poor idea to have these sent to an individual's Email
   Address since they may be lost if that person is on vacation, changes
   position within the company, or leaves the company.

   Instead, they should be sent to a role account that goes to a
   ticketing system or "exploded" to multiple responsible parties within
   the organization.  If there is not already an appropriate role
   account such as support@ or noc@ that reaches the right team, it may
   be a good idea to set up a dedicated alias such as fblmaster@ to sign
   up for all Feedback Loops.

4.2.3.  Report Cards

   The detail in a report card can vary greatly.  Feedback Providers
   might send a regular summary of traffic levels and complaint rates
   seen, perhaps just an overview or possibly broken down by source IP
   address or some other identifier.  Sometimes these may be sent just
   when some metric (typically a complaint rate) reaches a level that
   causes the Mailbox Provider to notify the Feedback Consumer there may
   be a problem developing that needs to be investigated and addressed.
   At the other extreme, some report cards will contain almost no useful

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   data at all, just a warning that the Message Originator is causing
   complaints -- with the implication that its email will be blocked
   unless it is improved.

   Report cards are human readable, since there are not currently any
   standard machine-readable formats and the information they include,
   both the provided metrics and their semantics, varies widely from one
   Mailbox Provider to another.  They are useful reference overviews for
   a Message Originator to monitor the overall perceived quality of the
   email it sends and, in the case of ESPs, perhaps which customers are
   causing higher than expected rates of complaints.  They can also be
   the only warning of serious problems prior to email being blocked
   altogether by the receiving Mailbox Provider.  It is critical they be
   are seen by someone handling delivery issues for the Message
   Originator, so again, they should be handled by an email alias that
   is always read.

   Report cards also contain useful data to track mechanically and
   perhaps report on trends, though as their content varies, it is hard
   to generalize what use might be made of them.  At the very least, the
   "warning" report cards are something that should be visible on an
   ESP's business intelligence or delivery dashboard.

4.3.  Handling Feedback Messages

   Mailbox Providers sending feedback may have published policies as to
   how they expect a Feedback Consumer to use Feedback Messages or may
   expect the Feedback Consumer to simply "make the problem stop".  In
   practice, this mostly boils down to three things:

   o  First, where the consumer of the feedback has some specific
      control over sending the email, it is expected not to send email
      of the same type to the same Recipient again.

   o  Second, it should identify the underlying problem (if any) and fix
      it so that it receives fewer reports of that type in the future.

   o  Third, it is not necessary to inform the Mailbox Provider or
      Feedback Provider, or their End User(s), of which actions have
      been or will be taken in response to automated complaint feedback.

   If the Feedback Consumer is a separate entity from the Message
   Originator, the two entities are expected to work together to resolve
   any problem.

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4.3.1.  Unsubscription or Suppression

   A Sender (whether author or originator) of commercial email should
   treat the Feedback Message similar to an unsubscribe request,
   ensuring that no further email from that list is sent to that
   Recipient, either by removing the email from that list or adding it
   to the associated suppression list.  It needs to use its best
   judgment, keeping in mind the goal of reducing future complaints, as
   to how broadly to apply that unsubscribe.  Suppressing the address
   across an entire ESP is likely too broad.  However, if a single
   Feedback Consumer (or customer of an ESP) has multiple segmented
   lists, then suppressing them across all those lists is probably a
   good idea.

   It is universally acknowledged that not all complaints are
   intentional; for example, Recipients might accidentally hit the wrong
   button or mark an entire mailbox as Spam.  However, it is best for
   Feedback Consumers to assume the Recipient does not want more email
   and to suppress mail to the Recipient in all but fairly extreme cases
   such as a Mailing List the Recipients pay to receive, email from a
   genuine company to its valid employees, or email from an Access
   Provider or Mailbox Provider to its users.

   This gets more complex in the case of transactional mail -- mail that
   is tied to some other service, such as ticket purchase confirmations
   or billing statements.  In that case, the Feedback Consumer has to,
   again, use its best judgment based on the specific situation.  In
   some cases, the right thing to do may be to communicate with the
   Recipient via another channel, such as a message on a web site used
   for the service; i.e., "You reported your notification mail as Spam
   so we are not going to send you any more messages unless you tell us

   In some cases, the best thing to do may be to ignore the Feedback
   Message.  For example, if your customer has reported as Spam the
   airline tickets he purchased and you emailed him, he probably did not
   mean it and he is going to be very annoyed if you do not send him the
   other tickets he has ordered.  In rare cases, it might be appropriate
   to suppress email to the Recipient, but also to suspend access to a
   service he or she uses until the Recipient confirms a desire to
   receive the associated email.  In all these cases, the important goal
   is to keep the customer happy and reduce future complaints, even in
   the apparently paradoxical situations where the way to do that is to
   ignore their Feedback.  In the real world, however, these are a small
   minority of cases.

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4.3.2.  Trending and Reporting

   Counting the Feedback Messages received over regular time periods can
   provide much useful information to ISPs, ESPs, and other Feedback
   Consumers, especially when broken down appropriately.

   An ISP (Mailbox Provider or Access Provider) might want to count the
   number of Feedback Messages a particular customer or IP address
   causes in a given day.  If there is a sudden increase from a
   particular customer or server, it may be a sign that a Spammer has
   signed up or a system has been compromised.  If there is a high level
   of complaints about a particular customer, it may be worth
   investigating to see if there is a reason for that.  For example, 10
   Feedback Messages a day would be a sign of serious problems in some
   cases, but might be perfectly reasonable "background" levels for a
   Message Originator that sends 300,000 emails a month.  If the count
   shows there may be a problem, the ISP can dig down and look at the
   emails that are being reported to determine the underlying cause.

   An ESP can do similar things but can also break the data down in more
   ways: by customer, by Mailing List, by campaign.  An ESP also has
   access to more information; it knows how many emails were delivered
   to the receiving Mailbox Provider over a given time period.  As a
   result, it can estimate the number of complaints divided by the
   number of emails sent, which is often a more useful metric than the
   absolute number of reports.  This is critical data for ESPs to track
   over time because it can help identify and quantify problem

   An individual Feedback Consumer, whether sending their own email or
   using an ESP, can acquire at least some information from complaint
   rates.  A spike in complaints on an otherwise stable list might be a
   sign there is a problem with address acquisition, if the spike is due
   to reports from new subscribers.  If it came from older subscribers,
   it might be attributable to content of a particular mailing that was
   not well received.  Perhaps the branding was not recognized or the
   content was offensive or inappropriate for the list.

   The complaint rate is determined by the number of Feedback Messages
   received over a given time period divided by the number of emails
   delivered to the associated Mailbox Provider over the same period.
   It is an obvious and useful metric to track, but there are a few
   subtle issues to be aware of.

   One issue is that Feedback Messages tend to be counted on the day the
   complaint was sent, which is the day the original message was read by
   the Recipient.  That may not be the same day that the message was
   sent.  A simple example is the fact that a Message Originator that

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   sends email regularly Monday through Friday will often see a high
   complaint rate on Saturday.  The absolute number of Feedback Messages
   sent by people catching up with the week's email over the weekend may
   not be that high.  However, since hardly any email is sent on
   Saturday, a fairly reasonable number of complaints end up being
   divided by a very small number of total sent emails, possibly even
   zero, which would break the reporting engine.  This can lead to a
   complaint rate that seems to range anywhere from suspicious to
   ridiculous.  Consequently, large Mailing Lists that are virtually
   silent on the weekend could end up receiving more complaints on a
   Saturday than email they sent that day, leading to complaint rates of
   well over 100%.

   Another arithmetic issue to consider is the interaction between the
   inbox, the bulk folder, and the "This Is Spam" button.  If an
   organization sends a high volume of email that has a terrible
   reputation, it may end up with perhaps 500 of its 10,000 mails in the
   inbox and the remaining 9,500 in the bulk folder.  If it gets 10
   Feedback Messages and divides that by the 10,000 emails it sent, it
   will get a very respectable 0.1% complaint rate.  However, the
   Mailbox Provider is probably going to calculate the complaint rate by
   dividing the number of emails delivered to the inbox instead --
   giving a 2% complaint rate, which is probably grounds for immediate
   blocking.  So, if one sees a large difference between a complaint
   rate as reported by a Mailbox Provider or other reputation system and
   the rate calculated from raw delivery numbers, it is important to
   look closely at the data.

4.4.  Automatically Handling an Incoming Feedback Stream

   Even when signing up for a Feedback Loop is partly automated,
   modifications to it tend to be handled manually.  Even something as
   trivial as changing the Email Address that the Feedback Messages are
   sent to can be time-consuming and can cause significant overhead to
   the Feedback Provider.  Multiply that by a dozen Feedback Loops, and
   getting it right the first time can save a lot of time and energy.

   Even the smallest of users should create a unique email alias for
   each Feedback Loop.  There are several advantages to this, even if
   they all deliver to the same person's inbox at first.  Sending each
   Feedback Loop to a unique address makes it immediately clear which
   Feedback Provider was the source of any given report, even if it is
   sent from an inconsistent From address.  It makes it easy to put
   lightweight pre-processing in place for a particular Feedback Stream,
   if needed.  It makes it easy to discard Feedback Messages if needed
   (though only temporarily, as it could be very bad for one's

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   reputation to miss a changing trend).  If a Feedback Consumer needs
   to scale up, it is easy to point the existing aliases at a Feedback
   Loop processing engine.

   If an organization might possibly scale up appreciably in the future
   or consider outsourcing its Feedback Loop processing to a third-party
   Feedback Consumer, it may be even better to create a subdomain for
   handling Feedback Streams.  For example, might use to accept its AOL Feedback Loop, allowing it
   to delegate the whole of to a Feedback Loop handling
   appliance or service, should the need arise.

   Small Feedback Consumers, with lists of no more than a few thousand
   Recipients, or small ISPs with no particular history of problems,
   should be able to handle feedback reports with little or no
   automation, as an ARF message should be readable in most mail
   clients.  It may be worthwhile to add some very lightweight
   processing to the inbound Feedback Messages to make them easier to
   triage from other email client.  For example, arffilter.c [Wise] can
   annotate the Subject line of inbound Feedback Messages with the IP
   address being reported, making it easier to see patterns of problems
   by sorting the messages by Subject line in the mail client.  To
   identify which Recipient is causing the feedback to be sent, small
   Feedback Consumers should add some of the automation mentioned below
   that is intended for larger Feedback Consumers.

   Larger Feedback Consumers need to be able to automate the handling of
   Feedback, as it scales beyond the ability of someone to manage
   manually quite quickly.  The main capability a Feedback Loop
   processor needs is to extract some relevant data from the report,
   reliably.  The most important bits of data tend to be the following:

   o  The Recipient of the original email

   o  The Mailbox Provider originating sending the Feedback Message
      (some Feedback Providers operate on behalf of multiple Mailbox

   o  The customer who sent the original email (in the case of an ESP or
      Mailbox Provider)

   o  The campaign and Mailing List that the original email belonged to,
      if any

   o  (Possibly) the IP address from which the original email was sent
      and any [DKIM] signature domain

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   The last isn't vital, but may be a useful piece of data in diagnosing
   delivery problems.

   It can be very difficult to extract some of this data without some
   upfront work before email is sent.  Some Feedback Providers will
   redact the Email Address in the To: header or Recipient Email
   Addresses anywhere within the message.  Some will delete any
   identifying information they can.  It may be possible to identify the
   End User based on the Message-ID, Subject line, and Received header
   timestamps, if there is access to the mail server logs, but at best
   it is painful and time-consuming, and only worth doing in an
   exceptional case.

   The solution is similar to the one used for automated bounce handling
   using VERP -- embed the information in the email in a way that it is
   unlikely to be removed by Feedback Providers but is easy to recognize
   in the email.  That information may already be there in a form such
   as VERP if the Return-Path header is included in the embedded email,
   or included in one-click unsubscribe links included in the body of
   the email.  If it is not already there, a good place to add the
   information is in the local part of the Message-ID as that is often
   used to track the progress of email through delivery.  It is often
   available from log files as well as in the headers of the original
   message included in the Feedback Message.

   There are several good ways to store the mapping between Recipients
   and identifiers in mail.  For a database-backed ESP or bulk sender, a
   synthesized database primary key can be used.  It is very small, and
   very opaque, and it is not expensive to retrieve the associated data
   from the main database -- but it is impossible to read by hand.
   Therefore, it needs automation with access to the core database to
   map the key onto the actual data.

   Recording the required information directly within the email but
   encrypting it with strong or weak encryption removes the need for
   database access to extract the data.  However, it still does need
   some automation.

   A hybrid approach with the various bits of data stored separately but
   having some pieces either encrypted or obfuscated is possible by just
   including a database ID.  This can provide a good compromise where
   most of the data is not immediately obvious to third parties but
   patterns in it can be recognized by eye.  For example, a Message-ID
   of "" is opaque to a third party, but
   someone familiar with the format can tell that it is a Message-ID
   added by the system.  In this case it starts with "esp" followed by
   three numbers separated by dashes, meaning it is from customer 423,
   campaign 27, and the Recipient has the database key 42460.  Even

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   decoding this manually, while it may not be possible to identify
   customer number 423, it is easy to recognize that 10 Feedback
   Messages in a row relate to the same customer.  From experience, it
   is not unusual for the vast majority of reports at an ESP to be about
   a very small number of customers, and one learns their customer IDs
   very quickly.

   Once a Message Originator embeds Recipient identifiers in an easily
   recognizable format in all its mail, it is quite easy for a Feedback
   Message processor to extract that with a conventional expression
   match and possibly a couple of database queries.  It can then
   suppress that Email Address and record the customer and campaign for
   future reporting.  In the case where the Feedback Messages are
   recorded in a ticketing system, it can also annotate the tickets with
   that data (again, for reporting and trending analysis).

   A Feedback Message processor is often bolted onto the side of an
   already complex bulk mail generator, making it difficult to reliably
   suppress mail to the Recipient.  If the delivery data is stored in a
   way that makes it easy to convert into the same format as the VERP
   string used for bounce processing then the Feedback processor can
   create a "fake" hard bounce and send it to the existing bounce
   processor, suppressing mail to that address.

   Mailbox Providers and Access Providers also need to automate Feedback
   processing.  They are usually less interested in the details about
   the message and more interested in the IP address and which customer
   sent it.  In most cases, the IP address can be extracted easily from
   ARF metadata; whereas, in other cases, it may need to be extracted
   from the Received headers embedded in the included original message.
   That data can then be used both for automated forwarding of Feedback
   Messages to the originating customer, if the ISP feels that is
   appropriate, and also for reporting on complaint levels across the
   ISP's customer base.

5.  Conclusion

   Whether you are acting as a Mailbox Provider or a Feedback Consumer,
   Complaint Feedback processing can be complex and scary -- or, with
   some intelligence and automation, simple and easy.  In either case,
   it is an important and necessary tool for detecting messaging abuse
   and ensuring End User satisfaction.

   MAAWG encourages all Mailbox Providers to offer Feedback of whatever
   form is appropriate for their local user base and legal framework,
   and it encourages all Senders of email to consume and act upon any
   Feedback available.  An actively maintained list of known Feedback
   Loops can be found at [Wise].

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6.  Acknowledgments

   This document was written within the MAAWG Collaboration Committee.
   The project was led by John Feaver and Kate Nowrouzi.  The primary
   authors were Steve Atkins, Christine Murphy Borgia, J.D. Falk, John
   Feaver, Todd Herr, John Levine, Heather Lord, Kate Nowrouzi, and
   Suresh Ramasubramanian.

   The document was edited by John Levine, J.D. Falk, and Linda Marcus.
   Further editing and formatting required for this version to be
   published by the IETF was performed by J.D. Falk, with advice from
   Barry Leiba and Murray Kucherawy.

6.1.  About MAAWG

   [MAAWG] is the largest global industry association working against
   Spam, viruses, denial-of-service attacks, and other online
   exploitation.  Its members include ISPs, network and mobile
   operators, key technology providers, and volume sender organizations.
   It represents over one billion mailboxes worldwide, and its
   membership contributed their expertise in developing this description
   of current Feedback Loop practices.

7.  Security Considerations

   Security and privacy considerations are discussed in many sections of
   this document, most notably Sections 1, 3.4, and 3.5.

8.  Informative References

   [DKIM]        Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
                 Identified Mail (DKIM) Signatures", RFC 6376,
                 September 2011.

   [DNS]         Mockapetris, P., "Domain names - concepts and
                 facilities", STD 13, RFC 1034, November 1987.

   [DomainKeys]  Delany, M., "Domain-Based Email Authentication Using
                 Public Keys Advertised in the DNS (DomainKeys)",
                 RFC 4870, May 2007.

   [MAAWG]       Messaging Anit-Abuse Working Group,

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RFC 6449                  CFBL Recommendations             November 2011

   [MAAWG-BCP]   MAAWG, "MAAWG Sender Best Communications Practices
                 Executive Summary and MAAWG Sender Best Communications
                 Practices Version 2.0a-Updated", September 2011,

   [MARF]        Shafranovich, Y., Levine, J., and M. Kucherawy, "An
                 Extensible Format for Email Feedback Reports",
                 RFC 5965, August 2010.

   [MIME]        Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part One: Format of Internet
                 Message Bodies", RFC 2045, November 1996.

   [RFC5322]     Resnick, P., Ed., "Internet Message Format", RFC 5322,
                 October 2008.

   [RFC5598]     Crocker, D., "Internet Mail Architecture", RFC 5598,
                 July 2009.

   [SMTP]        Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
                 October 2008.

   [Trust]       Crocker, D., Ed., "Trust in Email Begins with
                 Authentication", Issued by the Messaging Anti-Abuse
                 Working Group (MAAWG), June 2008,

   [VERP]        Wikipedia, "Variable Envelope Return Path",

   [Wise]        "arffilter - rewrite ARF reports",

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Appendix A.  Abuse Reporting Format (ARF)

A.1.  A Brief History

   The approach used by the first Feedback Loop to be deployed -- the
   "scomp" system at AOL -- was to send an entire copy of the message to
   the consumer of the Feedback Loop.  It expected that large Feedback
   Consumers would embed sufficient information in the email so they
   could identify which Message Recipient had complained.

   That worked well enough when there was only a single entity providing
   feedback, but as other Mailbox Providers started to offer Feedback,
   it became clear that it would be useful for the Feedback Provider to
   be able to add some additional information, both machine readable and
   human readable, to the report.  This led to ARF, the Abuse Reporting
   Format, which quickly became the de facto standard for Feedback

   Today, ARF is used by nearly all Feedback Providers, both within
   MAAWG and without, constituting the vast majority of all Feedback
   Messages generated worldwide.  ARF is recognized by all MAAWG members
   that have developed software or services that consume and process
   Feedback Messages.  There are no competing standards for reporting
   individual messages.

   ARF has now been published by the IETF as RFC 5965 [MARF].

A.2.  Structure of an ARF Message

   An ARF report (Feedback Message) is sent by email, with one message
   sent for each Spam report made.  It consists of three sections, in a
   standard [MIME] message format called multipart/report.

   The first section contains human-readable plaintext, primarily for
   the benefit of small Feedback Consumers who are handling reports
   manually.  It typically contains boilerplate text explaining that
   this is a Feedback Message and providing URLs to other data such as
   contact information for the Feedback Provider or Mailbox Provider
   that originated the Feedback Message.

   The second section contains some machine-readable information,
   including the version of the ARF protocol used and the type of report
   it is ("abuse," "fraud," or other label).  It also might include some
   optional information about the email being reported, such as the
   original Envelope Sender or the time the mail was received.  In
   theory, the information in this section can be used to mechanically
   route and triage the report, though in current practice most Feedback

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   Messages are treated identically.  As a result, this section is often
   ignored entirely by Feedback Consumers who prefer to process the
   third section themselves.

   The third section of the report consists of a copy of the original
   email that the report is about, as a standard [MIME] message/rfc822
   attachment.  While ideally this would be an unmodified copy of the
   original email, it is likely that many producers of reports will
   modify or "redact" some elements of the report, especially the Email
   Address of the Recipient, due to privacy or other legal concerns.

   The strict technical specifications of ARF, as well as some example
   reports and tools to handle the format, can be found at
   <>, [Wise], and in [MARF]

Appendix B.  Using DKIM to Route Feedback

   Historically, the IP address of the "last hop" -- the MTA that
   transferred a message into the receiving Mailbox Provider's
   administrative domain -- was the sole reliable identifier used to
   denote the source of a message.  With the emergence of authentication
   technologies such as [DKIM], another identifier can now be used;
   specifically, the authenticated domain associated with a message.
   This domain is the "d=" value in a DKIM-Signature header field.

   In a social or policy context, applying a DKIM signature to a message
   is tantamount to stating, "I take responsibility for this message".
   The DKIM signature is most often applied by the author or originator
   of a message, which may be far upstream of the "last hop" MTA.  This
   is true particularly in cases where the originator's intended
   Recipient Email Address is configured to forward to another Recipient
   Email Address.  Stories of users who have strung together multiple
   forwarding accounts are not uncommon, and these users are unable to
   complain effectively about Spam because their Mailbox Providers
   cannot easily or reliably follow the path of a message back to the
   initial originator.

   A single DKIM "d=" value may be used across multiple servers with
   multiple IP addresses.  Servers may be added or removed at any time
   without changing the dynamics of the DKIM signature.  When a Feedback
   Loop is based on the IP address, the Feedback Consumer must contact
   the Feedback Provider to change its subscription options every time
   an IP address needs to be added or removed.  However, when a Feedback
   Loop uses DKIM, no reconfiguration is necessary because the signing
   domain does not change.

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   One recurring concern with DKIM, however, is that ESPs often send
   messages addressed with hundreds or thousands of customer domains,
   yet they want to receive Feedback Messages for all of these domains.
   This was particularly difficult with [DomainKeys] (the predecessor to
   DKIM), which tied the "d=" to the "From" header field.  DKIM removed
   this tie, so it is simple for an ESP to use a domain of its own to
   sign the message and sign up for Feedback regarding all messages
   signed with that domain.  Such a signature may be in addition to, or
   instead of, signatures from the various client domains.  While there
   are still many unknowns related to reputation (which will be
   addressed in a future MAAWG document), this is clearly an appropriate
   use of DKIM to take responsibility (and receive Feedback) for a

Appendix C.  Unsolicited Feedback

   Is it always necessary for a Feedback Consumer to apply for a
   Feedback Loop or is it permissible for a Feedback Provider to
   configure a Feedback Loop for a Feedback Consumer without an explicit
   request?  There is continuing debate about whether this is an
   acceptable practice, and MAAWG is neither endorsing nor condemning
   such activity at this time.

   That said, if a Feedback Provider chooses to send Feedback without
   being asked first, certain guidelines should be followed.  In
   general, it should make prudent decisions to minimize the negative
   impact on Mailbox Providers and Access Providers.

C.1.  Guidelines

   This should only be done for Mailbox and Access Providers.

   This should only be done after attempting to contact the provider to
   ask if it is possible to set up a Feedback Loop via the normal

   These Feedback Loops should only be set up to send to the published
   abuse address from the provider's WHOIS record.

C.2.  Pros

   Feedback Consumers may not realize they have abuse problems until
   they begin receiving the spam complaints.

   Feedback Consumers may not be aware of Feedback Loops and may
   appreciate the additional data feed.

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   Upstream providers have an additional information stream to help them
   identify problem customers.

   Spam coming from a network is abuse; therefore it is appropriate to
   send reports of the abuse back to the Mailbox Provider or Access
   Provider.  Setting up a Feedback Loop automates the process.

C.3.  Cons

   It creates confusion for Feedback Consumers if they did not apply and
   do not understand why they are suddenly receiving complaints.

   It can conflict with existing Terms of Service because a new feed of
   information is available.  For example, if a provider has a policy to
   terminate service after a certain number of abuse complaints, and it
   starts receiving unexpected Feedback Loop complaints, it may either
   be forced to terminate customers that did not have a previous issue
   or be required to update its Terms of Service and Acceptable Use
   Policy agreements.

   Upstream providers do not have access to the mail being sent by their
   customers, so they cannot tell whether bulk mail complaints
   constitute a problem.

   The listed abuse address may not be the correct place for automated
   spam complaints to be sent.

   The listed abuse address may feed into a ticketing system that is not
   capable of correctly handling ARF messages.

   Feedback Consumers may not be equipped to handle the volume or format
   of complaints without some warning and preparation.

Author's Address

   J.D. Falk (editor)
   Messaging Anti-Abuse Working Group
   Presidio of San Francisco
   P.O. Box 29920
   572 B Ruger Street
   San Francisco, CA  94129-0920


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