Internet Engineering Task Force (IETF) J. Falk
Request for Comments: 6650 Return Path
Updates: 5965 M. Kucherawy, Ed.
Category: Standards Track Cloudmark
ISSN: 2070-1721 June 2012 Creation and Use of Email Feedback Reports:
An Applicability Statement for the Abuse Reporting Format (ARF)
RFC 5965 defines an extensible, machine-readable format intended for
mail operators to report feedback about received email to other
parties. This applicability statement describes common methods for
utilizing this format for reporting both abuse and authentication
failure events. Mailbox Providers of any size, mail-sending
entities, and end users can use these methods as a basis to create
procedures that best suit them. Some related optional mechanisms are
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................32. Definitions .....................................................43. Solicited and Unsolicited Reports ...............................44. Generating and Handling Solicited Abuse Reports .................44.1. General Considerations for Feedback Providers ..............44.2. Where to Send Reports ......................................54.3. What to Put in Reports .....................................54.4. General Considerations for Feedback Consumers ..............54.5. What to Expect .............................................64.6. What to Do with Reports ....................................65. Generating and Handling Unsolicited Abuse Reports ...............65.1. General Considerations .....................................65.2. When to Generate Reports ...................................75.3. Where to Send Reports ......................................75.4. What to Put in Reports .....................................85.5. What to Do with Reports ....................................96. Generating Automatic Authentication Failure Reports ............107. Security Considerations ........................................117.1. Security Considerations in Other Documents ................117.2. Forgeries .................................................117.3. Amplification Attacks .....................................117.4. Automatic Generation ......................................117.5. Reporting Multiple Incidents ..............................128. Acknowledgements ...............................................139. References .....................................................139.1. Normative References ......................................139.2. Informative References ....................................14
The Abuse Reporting Format (ARF) was initially developed for two very
specific use cases. Initially, it was intended to be used for
reporting feedback between large email operators, or from large email
operators to end user network access operators, any of whom could be
presumed to have automated abuse-handling systems. Secondarily, it
is used by those same large mail operators to send those same reports
to other entities, including those involved in sending bulk email for
commercial purposes. In either case, the reports would be triggered
by direct end user action such as clicking on a "report spam" button
in their email client.
Though other uses for ARF as defined in [RFC5965] have been discussed
(and may be documented similarly in the future), abuse reporting
remains the primary application, with a small amount of adoption of
extensions that enable authentication failure reporting.
This applicability statement provides direction for using ARF in both
contexts. It also includes some statements about the use of ARF in
conjunction with other email technologies.
The purpose for reporting abusive messages is to stop recurrences.
The methods described in this document focus on automating abuse
reporting as much as practical, so as to minimize the work of a
site's abuse team. There are further reasons why abuse feedback
generation is worthwhile, such as instruction of mail filters or
reputation trackers, or initiation of investigations of particularly
egregious abuses. These other applications are not discussed in
Further introduction to this topic may be found in [RFC6449], which
has more information about the general topic of abuse reporting.
Many of the specific ARF guidelines in this document were taken from
the principles presented in [RFC6449].
At the time of publication of this document, five feedback types are
registered. This document only discusses two of them ("abuse"
[RFC5965] and "auth-failure" [RFC6591]), as they are seeing
sufficient use in practice that applicability statements can be made
about them. The others, i.e., "fraud" [RFC5965], "other" [RFC5965],
and "not-spam" [RFC6430], are either too new or too seldom used to be
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119] and are
intended to replace the Requirement Levels described in Section 3.3
Some of the terminology used in this document is taken from
"Mailbox Provider" refers to an organization that accepts, stores,
and offers access to [RFC5322] messages ("email messages") for end
users. Such an organization has typically implemented SMTP [RFC5321]
and might provide access to messages through IMAP [RFC3501], the Post
Office Protocol (POP) [RFC1939], a proprietary interface designed for
HTTP [RFC2616], or a proprietary protocol.
3. Solicited and Unsolicited Reports
The original, and still by far the most common, application of
[RFC5965] is when two mail systems make a private agreement to
exchange abuse reports -- usually reports due to recipients manually
reporting messages as spam. We refer to these as solicited reports.
Other uses for ARF involve such reports sent between parties that
don't know each other. These unsolicited reports are sent without
prior arrangement between the parties as to the context and meaning
of the reports. Therefore, the constraints on how these unsolicited
reports need to be structured such that they are likely to be useful
to the recipient -- e.g., to what address(es) they can usefully be
sent, what issues they can be used to report, and how they can be
handled by the receiver of the report -- are very different.
The two cases are covered separately in the sections that follow.
4. Generating and Handling Solicited Abuse Reports
4.1. General Considerations for Feedback Providers
A Mailbox Provider receives reports of abusive or unwanted mail from
its users, most often by providing a "report spam" button (or similar
nomenclature) in the MUA (Mail User Agent). The method of
transferring this message and any associated metadata from the MUA to
the Mailbox Provider's ARF processing system is not defined by any
standards document but is discussed further in Section 3.2 of
[RFC6449]. Policy concerns related to the collection of this data
are discussed in Section 3.4 of [RFC6449].
To implement the recommendations of this memo, the reports are
formatted per [RFC5965] and transmitted as an email message
[RFC5322], typically using SMTP [RFC5321].
Ongoing maintenance of an ARF processing system is discussed in
Section 3.6 of [RFC6449].
4.2. Where to Send Reports
The Mailbox Provider SHOULD NOT send reports to addresses that have
not explicitly requested them. A valid deviation might be the result
of local policy instructions. The process whereby such parties may
request the reports is discussed in Section 3.5 of [RFC6449].
4.3. What to Put in Reports
The reports SHOULD use "Feedback-Type: abuse" for the report type.
Although a Mailbox Provider generating the reports can use other
types appropriate to the nature of the abuse being reported, the
operator receiving the reports might not treat different feedback
The following fields are optional in [RFC5965] but SHOULD be used in
this context when their corresponding values are available:
Original-Mail-From, Arrival-Date, Source-IP, and Original-Rcpt-To.
Other optional fields can be included as deemed appropriate by the
User-identifiable data MAY be obscured as described in [RFC6590].
4.4. General Considerations for Feedback Consumers
ARF report streams are established proactively between Feedback
Providers and Feedback Consumers. Recommendations for preparing to
request feedback are discussed in Section 4.1 of [RFC6449].
Operators MUST be able to accept ARF [RFC5965] reports as email
messages [RFC5322] over SMTP [RFC5321]. These messages, and other
types of email messages that can be received, are discussed in
Section 4.2 of [RFC6449].
Recipients of feedback reports that are part of formal feedback
arrangements have to be capable of handling large volumes of reports.
This could require automation of report processing as discussed in
Section 4.4 of [RFC6449].
4.5. What to Expect
The list of valid Feedback-Types is defined in [RFC5965], which
created an IANA registry for valid values to allow for extensions.
However, to allow for handling of new types that are not yet
supported, an automated report processing system MUST NOT reject (in
the SMTP sense) a report based solely on an unknown Feedback-Type.
The automated system can simply set reports of unknown types aside
for manual handling. However, Mailbox Providers might only make use
of the "abuse" Feedback-Type. Therefore, report receivers might be
required to do additional analysis to separate different types of
abuse reports after receipt if they do not have prior specific
knowledge of the sender of the report.
Report receivers MUST accept reports that have obscured their user-
identifiable data as described in [RFC6590]. That document also
discusses the handling of such reports. This technique is also
discussed in Section 4.4 of [RFC6449].
4.6. What to Do with Reports
Section 4.3 of [RFC6449] discusses actions that mail operators might
take upon receiving a report (or multiple reports).
5. Generating and Handling Unsolicited Abuse Reports
5.1. General Considerations
It is essential for report recipients to be capable of throttling
reports being sent to avoid damage to their own installations.
Therefore, Feedback Providers MUST provide a way for report
recipients to request that no further reports be sent.
Unfortunately, no standardized mechanism for such requests exists to
date, and all existing mechanisms for meeting this requirement are
Message authentication is generally a good idea, but it is especially
important to encourage credibility of, and thus response to,
unsolicited reports. Therefore, as with any other message, Feedback
Providers sending unsolicited reports SHOULD send reports that they
expect will pass the Sender Policy Framework (SPF) [RFC4408] and/or
DomainKeys Identified Mail (DKIM) [RFC6376] checks.
5.2. When to Generate Reports
Handling of unsolicited reports has a significant cost to the report
receiver. Senders of unsolicited reports, especially those sending
large volumes of them automatically, SHOULD NOT send reports that
cannot be used as a basis for action by the recipient, whether this
is due to the report being sent about an incident that is not abuse-
related, the report being sent to an email address that won't result
in action, or the content or format of the report being hard for the
recipient to read or use.
Feedback Providers SHOULD NOT report all mail sent from a particular
sender merely because some of it is determined to be abusive.
Mechanical reports of mail that "looks like" spam, based solely on
the results of inline content analysis tools, SHOULD NOT be sent
since, because of their subjective nature, they are unlikely to
provide a basis for the recipient to take action. Complaints
generated by end users about mail that is determined by them to be
abusive, or mail delivered to "spam trap" or "honeypot" addresses,
are far more likely to be accurate and MAY be sent.
If a Feedback Provider applies SPF [RFC4408] to arriving messages, a
report SHOULD NOT be generated to the RFC5321.MailFrom domain if the
SPF evaluation produced a "Fail", "SoftFail", "TempError", or
"PermError" report, as no reliable assertion or assumption can be
made that use of the domain was authorized. A valid exception would
be specific knowledge that the SPF result is not definitive for that
domain under those circumstances (for example, a message that is also
signed using DKIM [RFC6376] by the same domain, and that signature
5.3. Where to Send Reports
Rather than generating feedback reports themselves, MUAs SHOULD
create abuse reports and send these reports back to their Mailbox
Providers so that they can generate and send ARF messages on behalf
of end users (see Section 3.2 of [RFC6449]). This allows centralized
processing and tracking of reports, and provides training input to
filtering systems. There is, however, no standard mechanism for this
signaling between MUAs and Mailbox Providers to trigger abuse
Feedback Providers SHOULD NOT send reports to recipients that are
uninvolved or only peripherally involved. For example, they SHOULD
NOT send reports to the operator of every Autonomous System in the
path between the apparent originating system and the operator
generating the report. Instead, they need to send reports to
recipients that are both responsible for the messages and able to do
something about them.
Deciding where to send an unsolicited report will typically rely on
heuristics. Abuse addresses in WHOIS [RFC3912] records of the IP
address relaying the subject message and/or of the domain name found
in the results of a PTR ("reverse lookup") query on that address are
likely reasonable candidates, as is the abuse@domain role address
(see [RFC2142]) of related domains. Unsolicited reports SHOULD NOT
be sent to email addresses that are not clearly intended to handle
abuse reports. Legitimate candidates include those found in WHOIS
records or on a web site that either are explicitly described as an
abuse contact or are of the form "abuse@domain".
Where an abusive message is authenticated using a domain-level
authentication technology such as DKIM [RFC6376] or SPF [RFC4408],
the domain that has been verified by the authentication mechanism is
often a reasonable candidate for receiving feedback about the
message. For DKIM, though, while the authenticated domain has some
responsibility for the mail sent, it can be a poor contact point for
abuse issues (for example, it could represent the message's author
but not its sender, it could identify the bad actor responsible for
the message, or it could refer to a domain that cannot receive mail
Often, unsolicited reports will have no meaning if sent to abuse
reporting addresses belonging to the abusive parties themselves. In
fact, it is possible that such reports might reveal information about
complainants. Reports SHOULD NOT be sent to such addresses if they
can be identified beforehand, except where the abusive party is known
to be responsive to such reports.
5.4. What to Put in Reports
Reports SHOULD use "Feedback-Type: abuse" but can use other types as
appropriate. However, the Mailbox Provider generating the reports
cannot assume that the operator receiving the reports will treat
different Feedback-Types differently.
Reports SHOULD include the following optional fields whenever their
corresponding values are available and applicable to the report:
Original-Mail-From, Arrival-Date, Source-IP, and Original-Rcpt-To.
Other optional fields can be included as deemed appropriate by the
Experience suggests that the use of ARF is advisable in most
contexts. Automated recipient systems can handle abuse reports sent
in ARF at least as well as any other format such as plain text, with
or without a copy of the message attached. That holds even for
systems that did not request ARF reports, assuming such reports are
generated considering the possibility of recipients that don't use
automated ARF parsing. Anyone sending unsolicited reports in ARF can
legitimately presume that some recipients will only be able to access
the human-readable (first, text/plain) part of it and SHOULD include
all information needed also in this part. Further, they SHOULD
ensure that the report is readable when viewed as plain text, to give
low-end ticketing systems as much assistance as possible. In extreme
cases, failure to take these steps may result in the report being
discarded or ignored.
5.5. What to Do with Reports
Receivers of unsolicited reports can take advantage of the
standardized parts of ARF to automate processing. Independent of the
sender of the report, they can improve processing by separating valid
reports from invalid reports by, for example, looking for references
to IP address ranges, domains, and mailboxes for which the recipient
organization is responsible in the copy of the reported message, and
by correlating multiple reports of similar messages to identify bulk
Per Section 4.4 of [RFC6449], a network service provider MAY use ARF
data for automated forwarding of feedback messages to the originating
Published abuse mailbox addresses SHOULD NOT reject non-ARF messages
based solely on the format, as generation of ARF messages can
occasionally be unavailable or not applicable. Deviation from this
requirement could be done due to local policy decisions regarding
other message criteria.
Although [RFC6449] suggests that replying to feedback is not useful,
in the case of receipt of ARF reports where no feedback arrangement
has been established, a non-automated reply might be desirable to
indicate what action resulted from the complaint, heading off more
severe filtering by the Feedback Provider. In addition, using an
address that cannot receive replies precludes any requests for
additional information and increases the likelihood that further
reports will be discarded or blocked. Thus, a Feedback Provider
sending unsolicited reports SHOULD NOT generate reports for which a
reply cannot be received. Where an unsolicited report results in the
establishment of contact with a responsible and responsive party,
this data can be saved for future complaint handling and possible
establishment of a formal (solicited) feedback arrangement. See
Section 3.5 of [RFC6449] for a discussion of establishment of
6. Generating Automatic Authentication Failure Reports
There are some cases where report generation is caused by automation
rather than user requests. A specific example of this is reporting,
using ARF (or extensions to it), of messages that fail particular
message authentication checks. Examples of this include [RFC6651]
and [RFC6652]. The considerations presented below apply in those
The applicability statement for this use case is somewhat smaller, as
many of the issues associated with abuse reports are not relevant to
reports about authentication failures.
Automatic feedback generators MUST select actual message recipients
based on data provided by willing report receivers. In particular,
recipients MUST NOT be selected using heuristics.
If the message under evaluation by the Verifier is an ARF [RFC5965]
message, a report MUST NOT be automatically generated.
The message for a new report sent via SMTP MUST be constructed so as
to avoid amplification attacks, deliberate or otherwise. The
envelope sender address of the report MUST be chosen so that these
reports will not generate mail loops. Similar to Section 2 of
[RFC3464], the envelope sender address of the report MUST be chosen
to ensure that no feedback reports will be issued in response to the
report itself. Therefore, when an SMTP transaction is used to send a
report, the MAIL FROM command SHOULD use the NULL reverse-path, i.e.,
"MAIL FROM:<>". An exception to this would be the use of a reverse-
path selected such that SPF checks on the report will pass; in such
cases, the operator will need to make provisions to avoid the
amplification attack or mail loop via other means.
Reports SHOULD use "Feedback-Type: auth-failure" but MAY use other
types as appropriate. However, the Mailbox Provider generating the
reports cannot assume that the operator receiving the reports will
treat different Feedback-Types differently.
These reports SHOULD include the following fields, although they are
optional in [RFC5965], whenever their corresponding values are
available: Original-Mail-From, Arrival-Date, Source-IP, and
Original-Rcpt-To. Other optional fields can be included as deemed
appropriate by the implementer.
7. Security Considerations
7.1. Security Considerations in Other Documents
Implementers are strongly urged to review, at a minimum, the Security
Considerations sections of [RFC5965] and [RFC6449].
Feedback Providers that relay user complaints directly, rather than
by reference to a stored message (e.g., IMAP or POP), could be duped
into sending a complaint about a message that the complaining user
never actually received, as an attack on the purported originator of
the falsified message. Feedback Providers need to be resilient to
such attack methods.
Also, these reports may be forged as easily as ordinary Internet
electronic mail. User agents and automatic mail handling facilities
(such as mail distribution list exploders) that wish to make
automatic use of reports of any kind should take appropriate
precautions to minimize the potential damage from denial-of-service
Perhaps the simplest means of mitigating this threat is to assert
that these reports should themselves be signed with something like
DKIM and/or authorized by something like SPF. Note, however, that if
there is a problem with the email infrastructure at either end, DKIM
and/or SPF may result in reports that aren't trusted or even accepted
by their intended recipients, so it is important to make sure those
components are properly configured. The use of both technologies in
tandem can resolve this concern to a degree, since they generally
have disjoint failure modes.
7.3. Amplification Attacks
Failure to comply with the recommendations regarding selection of the
envelope sender can lead to amplification denial-of-service attacks.
This is discussed in Section 6 as well as in [RFC3464].
7.4. Automatic Generation
ARF [RFC5965] reports have historically been generated individually
as a result of some kind of human request, such as someone clicking a
"Report Abuse" button in a mail reader. In contrast, the mechanisms
described in some extension documents (i.e., [RFC6651] and [RFC6652])
are focused around automated reporting. This obviously implies the
potential for much larger volumes or higher frequency of messages,
and thus greater mail system load (both for Feedback Providers and
Those mechanisms are primarily intended for use in generating reports
to aid implementers of DKIM [RFC6376], Author Domain Signing
Practices (ADSP) [RFC5617], and SPF [RFC4408], and other related
protocols during development and debugging. They are not generally
intended for prolonged forensic use, specifically because of these
load concerns. However, extended use is possible by ADministrative
Management Domains (ADMDs) that want to keep a close watch for fraud
or infrastructure problems. It is important to consider the impact
of doing so on both Feedback Providers and the requesting ADMDs.
A sender requesting these reports can cause its mail servers to be
overwhelmed if it sends out signed messages whose signatures fail to
verify for some reason, provoking a large number of reports from
Feedback Providers. Similarly, a Feedback Provider could be
overwhelmed by a large volume of messages requesting reports whose
signatures fail to validate, as the Feedback Provider now needs to
send reports back to the Signer.
Limiting the rate of generation of these messages may be appropriate
but threatens to inhibit the distribution of important and possibly
In general ARF feedback loop terms, it is often suggested that
Feedback Providers only create these (or any) ARF reports after an
out-of-band arrangement has been made between two parties. These
extension mechanisms provide ways to adjust parameters of an
authorized abuse report feedback loop that is configured and
activated by private agreement. The alternative (sending reports
automatically based solely on data found in the messages) may have
7.5. Reporting Multiple Incidents
If it is known that a particular host generates abuse reports upon
certain incidents, an attacker could forge a high volume of messages
that will trigger such a report. The recipient of the report could
then be inundated with reports. This could easily be extended to a
distributed denial-of-service attack by finding a number of report-
The incident count referenced in ARF [RFC5965] provides a limited
form of mitigation. The host that generates reports can elect to
send reports only periodically, with each report representing a
number of identical or nearly identical incidents. One might even do
something inverse-exponentially, sending reports for each of the
first ten incidents, then every tenth incident up to 100, then every
100th incident up to 1000, etc., until some period of relative quiet
after which the limitation resets.
The use of this technique for "nearly identical" incidents in
particular causes a degradation in reporting quality, however. If
for example a large number of pieces of spam arrive from one
attacker, a reporting agent could decide only to send a report about
a fraction of those messages. While this averts a flood of reports
to a system administrator, the precise details of each incident are
similarly not sent.
Other rate-limiting provisions might be considered, such as detecting
a temporary failure response from the report destination and thus
halting report generation to that destination for some period, or
simply imposing or negotiating a hard limit on the number of reports
to be sent to a particular receiver in a given time frame.
The author and editor wish to thank Steve Atkins, John Levine, Shmuel
Metz, S. Moonesamy, and Alessandro Vesely for their contributions to
All of the best practices referenced by this document are found in
[RFC6449], written within the Collaboration Committee of the
Messaging Anti-Abuse Working Group (MAAWG).
Finally, the original author wishes to thank the doctors and staff
at the University of Texas MD Anderson Cancer Center for doing what
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
[RFC5965] Shafranovich, Y., Levine, J., and M. Kucherawy, "An
Extensible Format for Email Feedback Reports", RFC 5965,
[RFC6591] Fontana, H., "Authentication Failure Reporting Using the
Abuse Reporting Format", RFC 6591, April 2012.
9.2. Informative References
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[RFC2026] Bradner, S., "The Internet Standards Process --
Revision 3", BCP 9, RFC 2026, October 1996.
[RFC2142] Crocker, D., "Mailbox Names for Common Services, Roles and
Functions", RFC 2142, May 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464,
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
VERSION 4rev1", RFC 3501, March 2003.
[RFC3912] Daigle, L., "WHOIS Protocol Specification", RFC 3912,
[RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1",
RFC 4408, April 2006.
[RFC5617] Allman, E., Fenton, J., Delany, M., and J. Levine,
"DomainKeys Identified Mail (DKIM) Author Domain Signing
Practices (ADSP)", RFC 5617, August 2009.
[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", RFC 6376,
[RFC6430] Li, K. and B. Leiba, "Email Feedback Report Type Value:
not-spam", RFC 6430, November 2011.
[RFC6449] Falk, J., Ed., "Complaint Feedback Loop Operational
Recommendations", RFC 6449, November 2011.
[RFC6590] Falk, J., Ed., and M. Kucherawy, Ed., "Redaction of
Potentially Sensitive Data from Mail Abuse Reports",
RFC 6590, April 2012.
[RFC6651] Kucherawy, M., "Extensions to DomainKeys Identified Mail
(DKIM) for Failure Reporting", RFC 6651, June 2012.
[RFC6652] Kitterman, S., "Sender Policy Framework (SPF)
Authentication Failure Reporting Using the Abuse Reporting
Format", RFC 6652, June 2012.
100 Mathilda Place, Suite 100
Sunnyvale, CA 94086
Murray S. Kucherawy (editor)
128 King St., 2nd Floor
San Francisco, CA 94107