| Internet-Draft | CFBL Address Header | July 2021 |
| Benecke | Expires 9 January 2022 | [Page] |
This document describes a method to allow a mail sender to specify a complaint feedback loop address as an email header and how a mail receiver can use it. This document also defines the rules for processing and forwarding such a complaint. The motivation for this arises out of the absence of a standardized and automated way to provide a complaint feedback loop address to mailbox providers. Currently, providing and maintaining such an address is a manual to a time-consuming process for mail senders and mailbox providers.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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This Internet-Draft will expire on 9 January 2022.¶
Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. 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.¶
For a long time there is a way for a mailbox provider to forward manual complaints back to the mail sender, which can be a mailbox provider, or an operator of a broadcast marketing list. The mailbox provider provides a so-called feedback loop [RFC6449]. This feedback loop is being used to give e.g., operators of broadcast marketing lists feedback about resulting complaints from their marketing mailings. Those complaints are based on manual user interaction e.g., IMAP movement to "Junk".¶
As described in [RFC6449] the registration for such a feedback loop needs to be done manually by a human at any mailbox provider who provides an FBL, and he wants to receive complaints from. This can be quite time-consuming if there are new feedback loops rising up, or the mail sender wants to add new ip addresses or DKIM domains. Besides, a manual process isn't well suitable and/or doable for smaller mailbox providers.¶
The change of such a complaint address e.g., due to an infrastructure change is another problem. Due to this manual process the mail sender needs to go through all providers again and delete his existing subscriptions and re-signup with the new complaint address.¶
This document addresses this problem with a new email header. It extends the described complaint feedback loop recommendations in [RFC6449] with an automated way to provide the complaint feedback loop address to mailbox providers.¶
Mail senders can add this header and willing mailbox provider can use this header to forward the generated report to the provided complaint address. The mail sender just needs to add an email header and isn't required to signup manually at every feedback loop provider. Another benefit would be the mailbox provider doesn't need to develop a manual registration process and verification process.¶
A new email header has been chosen over a new DNS record in favour to be able to easily distinguish between multiple broadcast marketing list operators / mail senders, without the intervention of its users or administrators. For example, if a company uses multiple sending systems, each system can set this header on their own, without the need of a change that has to be done by its users or administrators. On the side of the mailbox provider, there is no need to do an additional DNS query to get the complaint address.¶
This document has been created with GDPR and other data-regulation laws in mind and to address the resulting problems in providing an automated complaint feedback loop address, as the email may contain personal data.¶
Summarised this document has following goals:¶
For good reasons there is already the One-Click-Unsubscribe [RFC8058] signaling, which may have several interests in common with this document. However, this header requires the List-Unsubscribe header, which intention is to provide the unsubscription link of a list. For this reason, this header is only used by broadcast marketing list operator or mailing list operators, but not in normal email traffic.¶
The main interest of this document is now to provide an automated way to signal a complaint feedback loop address to mailbox providers. It is the mail sender's obligation to consider on their own which measure they want to take after receiving a report, this is out of scope of this document.¶
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
The keyword "FBL" in this document is the abbreviation for "feedback loop" and will hereafter be used.¶
The keyword "CFBL" in this document is the abbreviation for "complaint feedback loop" and will hereafter be used.¶
The keyword "MBP" in this document is the abbreviation for "mailbox provider", it is the party who receives an email, and will be used hereafter.¶
The keyword "mail sender" in this document is used to describe the party who sends an email, this can be an MBP, a broadcast marketing list operator or any other email sending party. It will be used hereafter.¶
The email (sent by mail sender to MBP) about which a complaint should be sent MUST have at least a valid [DKIM] signature. The aforementioned valid DKIM signature MUST cover at least the CFBL-Address header domain. It is RECOMMENDED that the DKIM signature aligns with the CFBL-Address header domain and the From header [MAIL] domain where possible. The CFBL-Address header MUST be included in the "h=" tag of the aforementioned valid DKIM-Signature.¶
If the message isn't properly aligned, nor it does have the required header coverage by the "h=" tag of a valid DKIM-Signature, the MBP SHALL NOT send a report email.¶
This ensures that only reports are sent to the complaint address that are based on an authenticated email.¶
The report email (sent by MBP to mail sender) MUST have a valid [DKIM] signature and MUST cover the From header [MAIL] domain.¶
If the message does not have the required valid [DKIM], the mail sender SHALL NOT process this email.¶
It is highly RECOMMENDED that the mail sender does further plausibility checks.¶
A mail sender that wishes to receive complaints about their emails MUST place a CFBL-Address header in the message. The mail sender MAY place a CFBL-Feedback-ID header in the message out of various reasons.¶
The receiving complaint FBL address, placed in the message, MUST accept by default [ARF] compatible reports.¶
The mail sender can OPTIONAL request, as described in Section 5.1, a [XARF] compatible report. The MBP MAY send a [XARF] compatible report, if it is technical possible for him, otherwise a [ARF] compatible reports will be sent.¶
It is highly RECOMMENDED processing these reports automatically. Each mail sender must consider on their own which measure they take after receiving a report.¶
The mail sender MUST take action to address the described requirements in Section 3.¶
An MBP MAY process the complaint and forward it to the complaint FBL address. If the MBP wants to process the complaints and forwards it, he MUST query the CFBL-Address header.¶
By default, an [ARF] compatible report MUST be sent when a manual action has been taken e.g., when a receiver marks a mail as spam, by clicking the "This is spam"-button in any web portal or by moving a mail to junk folder, this includes also [IMAP] and [POP3] movements. The MBP SHALL NOT send any report when an automatic decisions has been made e.g., spam filtering.¶
The MBP SHOULD send a [XARF] compatible report, if the mail sender requests it as described in Section 5.1. If this is not possible a [ARF] compatible report MUST be sent.¶
The MBP MUST validate and take action to address the described requirements in Section 3.¶
The complaint report (sent by MBP to mail sender) MUST be an [ARF] report by default. The complaint report MAY be an [XARF] report, if the mail sender requests it, and the MBP can send it.¶
The report MUST contain at least the Message-ID [MAIL]. If present, the header "CFBL-Feedback-ID" of the complaining email MUST be added additionally.¶
The MBP MAY omit all further headers and/or body to comply with any data-regulation laws.¶
It is highly RECOMMENDED that, if used, the CFBL-Feedback-ID includes a hard to forge component such as an [HMAC] using a secret key, instead of a plain-text string.¶
A mail sender that wishes to receive a [XARF] compatible report, MUST append "report=xarf" to the CFBL-Address header (Section 6.1). The resulting header would be the following:¶
CFBL-Address: fbl@example.com; report=xarf¶
The following ABNF imports fields, WSP, CRLF and addr-spec from [MAIL].¶
fields /= cfbl-address
cfbl-address = "CFBL-Address:" 0*1WSP addr-spec
[";" 0*1WSP report-format] CRLF
report-format = "report=" ("arf" / "xarf")
¶
The following ABNF imports fields, WSP, CRLF and atext from [MAIL].¶
fields /= cfbl-feedback-id cfbl-feedback-id = "CFBL-Feedback-ID:" 0*1WSP fid CRLF fid = 1*(atext / ":")¶
This section discusses possible security issues, and their possible solutions, of a complaint FBL address header.¶
As any other email address, a complaint FBL addresses can be an attack vector for malicious emails. The complaint FBL addresses can be for example flooded with spam. This is an existing problem with any existing email address and isn't newly created by this document.¶
The mail sender must take appropriated measures. One possible countermeasure would be a rate limit on the delivering IP. However, this should be done with caution, the normal FBL email traffic must not be impaired.¶
Sending a FBL report against a mailbox may lead into an inaccessibility for the account owner, if there is a too quick automatic account suspension. For example, someone sends an invitation to his friends. For somewhat reason someone marks this mail as spam. Now, if there is a too quick automatic account suspension in place, the senders account will be suspended, and the sender can't access his mails anymore.¶
MBPs and mail senders MUST take appropriate measures to prevent this. MBPs and mail senders have therefore, mostly proprietary, ways to evaluate the trustworthiness of an account. For example MBPs and mail senders can consider the account age and/or any other account suspension that happened beforehand, before an account gets suspended.¶
A malicious person can send a bunch of forged ARF reports to a known complaint FBL addresses and try to guess a Message-ID/CFBL-Feedback-ID. He might try to do a mass-unsubscription/suspension, if there is such an automatic in place. This is also an already existing problem with the current FBL implementation and/or the One-Click-Unsubscription [RFC8058].¶
The receiving mail sender MUST take appropriated measures.¶
As a countermeasure it is recommended that the Message-ID and, if used, CFBL-Feedback-ID uses a hard to forge component such as an [HMAC] using a secret key, instead of a plain-text string, to make an enumeration attack impossible.¶
If it is impossible for the mail sender to use a hard to forge component, the mail sender should take measures to avoid enumeration attacks.¶
Providing such a header itself doesn't produce a data-regulation law problem. The resulting ARF report, that is sent to the mail sender by the MBP, may conflict with a data-regulation law, as it may contain personal data.¶
This document already addresses some parts of this problem and describes a data-regulation law safe way to send a FBL report. As described in Section 5, the MBP may omit the complete body and/or headers and just sends the required fields. Nevertheless, each MBP must consider on their own, if this implementation is acceptable and complies with the existing data-regulation laws.¶
As described in Section 5, it is also highly RECOMMENDED that the Message-ID and, if used, the CFBL-Feedback-ID includes a hard to forge component such as an [HMAC] using a secret key, instead of a plain-text string. See Section 9.3 for an example.¶
Using HMAC, or any other hard to forge component, ensures that only the mail sender has knowledge about the data.¶
The IANA is requested to register a new header field, per [RFC3864], into the "Permanent Message Header Field Names" registry:¶
Header field name: CFBL-Address Applicable protocol: mail Status: experimental Author/Change controller: Jan-Philipp Benecke <jpb@cleverreach.com> Specification document: this document¶
The IANA is requested to register a new header field, per [RFC3864], into the "Permanent Message Header Field Names" registry:¶
Header field name: CFBL-Feedback-ID Applicable protocol: mail Status: experimental Author/Change controller: Jan-Philipp Benecke <jpb@cleverreach.com> Specification document: this document¶
For simplicity the DKIM header has been shortened, and some tags has been omitted.¶
Email about the report will be generated:¶
Return-Path: <sender@mailer.example.com>
From: Awesome Newsletter <newsletter@example.com>
To: me@example.net
Subject: Super awesome deals for you
CFBL-Address: fbl@example.com; report=arf
Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
Content-Type: text/plain; charset=utf-8
DKIM-Signature: v=1; a=rsa-sha256; d=example.com;
h=Content-Type:Subject:From:To:Message-ID:
CFBL-Feedback-ID:CFBL-Address;
This is a super awesome newsletter.
¶
Resulting ARF report:¶
------=_Part_240060962_1083385345.1592993161900
Content-Type: message/feedback-report
Content-Transfer-Encoding: 7bit
Feedback-Type: abuse
User-Agent: FBL/0.1
Version: 0.1
Original-Mail-From: sender@mailer.example.com
Arrival-Date: Tue, 23 Jun 2020 06:31:38 GMT
Reported-Domain: example.com
Source-Ip: 192.0.2.1
------=_Part_240060962_1083385345.1592993161900
Content-Type: text/rfc822; charset=UTF-8
Content-Transfer-Encoding: 7bit
Return-Path: <sender@mailer.example.com>
From: Awesome Newsletter <newsletter@example.com>
To: me@example.net
Subject: Super awesome deals for you
CFBL-Address: fbl@example.com; report=arf
Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
Content-Type: text/plain; charset=utf-8
DKIM-Signature: v=1; a=rsa-sha256; d=example.com;
h=Content-Type:Subject:From:To:Message-ID:
CFBL-Feedback-ID:CFBL-Address;
This is a super awesome newsletter.
------=_Part_240060962_1083385345.1592993161900--
¶
Email about the report will be generated:¶
Return-Path: <sender@mailer.example.com>
From: Awesome Newsletter <newsletter@example.com>
To: me@example.net
Subject: Super awesome deals for you
CFBL-Address: fbl@example.com; report=arf
Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
CFBL-Feedback-ID: 111:222:333:4444
Content-Type: text/plain; charset=utf-8
DKIM-Signature: v=1; a=rsa-sha256; d=example.com;
h=Content-Type:Subject:From:To:Message-ID:
CFBL-Feedback-ID:CFBL-Address;
This is a super awesome newsletter.
¶
Resulting ARF report contains only the CFBL-Feedback-ID:¶
------=_Part_240060962_1083385345.1592993161900 Content-Type: message/feedback-report Content-Transfer-Encoding: 7bit Feedback-Type: abuse User-Agent: FBL/0.1 Version: 0.1 Original-Mail-From: sender@mailer.example.com Arrival-Date: Tue, 23 Jun 2020 06:31:38 GMT Reported-Domain: example.com Source-Ip: 2001:db8:deaf:beef::25 ------=_Part_240060962_1083385345.1592993161900 Content-Type: text/rfc822-headers; charset=UTF-8 Content-Transfer-Encoding: 7bit CFBL-Feedback-ID: 111:222:333:4444 ------=_Part_240060962_1083385345.1592993161900--¶
Email about the report will be generated:¶
Return-Path: <sender@mailer.example.com>
From: Awesome Newsletter <newsletter@example.com>
To: me@example.net
Subject: Super awesome deals for you
CFBL-Address: fbl@example.com; report=arf
Message-ID: <a37e51bf-3050-2aab-1234-543a0828d14a@mailer.example.com>
CFBL-Feedback-ID: 3789e1ae1938aa2f0dfdfa48b20d8f8bc6c21ac34fc5023d
63f9e64a43dfedc0
Content-Type: text/plain; charset=utf-8
DKIM-Signature: v=1; a=rsa-sha256; d=example.com;
h=Content-Type:Subject:From:To:Message-ID:
CFBL-Feedback-ID:CFBL-Address;
This is a super awesome newsletter.
¶
Resulting ARF report contains only the CFBL-Feedback-ID:¶
------=_Part_240060962_1083385345.1592993161900
Content-Type: message/feedback-report
Content-Transfer-Encoding: 7bit
Feedback-Type: abuse
User-Agent: FBL/0.1
Version: 0.1
Original-Mail-From: sender@mailer.example.com
Arrival-Date: Tue, 23 Jun 2020 06:31:38 GMT
Reported-Domain: example.com
Source-Ip: 192.0.2.1
------=_Part_240060962_1083385345.1592993161900
Content-Type: text/rfc822-headers; charset=UTF-8
Content-Transfer-Encoding: 7bit
CFBL-Feedback-ID: 3789e1ae1938aa2f0dfdfa48b20d8f8bc6c21ac34fc5023d
63f9e64a43dfedc0
------=_Part_240060962_1083385345.1592993161900--
¶
Technical and editorial reviews and comments were provided by the colleagues at CleverReach, the colleagues at Certified Senders Alliance and eco.de, Arne Allisat and Tobias Herkula (1&1 Mail & Media) and Sven Krohlas (BFK Edv-consulting).¶