Network Working Group J. Levine
Internet-Draft Taughannock Networks
Intended status: Standards Track T. Herkula
Expires: March 19, 2017 optivo GmbH
September 15, 2016

Signalling one-click functionality for list email headers


This document describes a method for signaling a one-click function for the list-unsubscribe email header. The need for this arises out of the actuality that mail software sometimes fetches URLs in mail headers, and thereby accidentally triggers unsubscriptions in the case of the list-unsubscribe header.

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Table of Contents

1. Introduction and Motivation

An [RFC2369] email header can contain HTTPS [RFC7230] URIs. In a List-Unsubscribe Header the HTTPS URI is intended to unsubscribe the recipient of the email from the list. But anti-spam software often fetches all resources in mail headers automatically, without any action by the user. To prevent accidental unsubscriptions, senders return landing pages with a confirmation step to finish the unsubscribe request. This has undesirable consequences for mailers who wish for the unsubscription process to be as simple as possible.

Different types of mailing lists are managed in different ways. Non-commercial discussion lists that exchange messages among the list's subscribers typically try to ensure that requests to subscribe and unsubscribe are valid, but don't worry too much about message delivery, since all the messages are typically delivered to the recipients. Commercial broadcast lists are much more concerned about deliverability, whether the mail is delivered to the recipients and how the messages are presented, e.g., whether in the primary inbox or in a junk folder. Many mail systems allow recipients to report mail as spam or junk, and mail from senders with a lot of junk reports tends to have poor deliverability. Hence the mailers want to make it as easy as possible for recipients to unsubscribe, since the recipient's alternative to a difficult unsubscription process is to report mail from the sender as junk until it goes away.

The recipient mail systems are aware that their users do not make a clear distinction between unsubscription and junk, so in many cases they allow trustworthy mailers to request notification when their mail is reported as junk, so they can unsubscribe the recipient. Since the process of identifying trustworthy mailers and notifying them does not scale well to large numbers of small mailers, this specification provides a way for recipient systems to notify the mailer automatically, using only information within the mail message. Some recipient systems might wish to send an unsubscription notice to mailers whenever a user reports a message as junk, or they might give the user the option to report and unsubscribe.

If a mail recipient is unsubscribing manually and the unsubscription process requires confirmation, the resulting web page is presented to the recipient who can then click the appropriate button. But when the unusubscribe action is combined with a MUA junk report, there is no direct user interaction with the mailer's web site. Similarly, there can be no interaction when the action is performed automatically on mail sent to an abandoned or closed mailbox. In those cases, the unsubscription process has to work without manual intervention, and in particular without requiring that software attempt to interpret the contents of a confirmation page.

This document addresses this part of the problem, with a POST action for receivers that senders can distinguish from other requests and handle as a one-click unsubscription without manual intervention by the mail recipient.

A List-Unsubscribe header can also contain a mailto: URI with an address to which an unsubscription request is sent. While these URIs can be for a one-click unsubscribe, experience has shown that they do not work well in high volume environments, because the recipient mail systems (typically e-mail service providers that are optimized to send large volumes of mail) cannot keep up with the required number of mailed removal requests. Hence this document considers only HTTPS URIs.

This document has several goals.

2. Definitions

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] when written in all capital letters.

"One-click" describes an action that directly triggers a change in a system's state, intended to be applied only with a user's intent.

3. Implementation

3.1. Mail senders

An entity which is responsible for sending an email that wishes to add an HTTPS URI for one-click unsubscriptions places both a List-Unsubscribe and a List-Unsubscribe-Post header in the message. The List-Unsubscribe-Post header may contain multiple key value pairs needed by the sending entity. It also MUST contain the key value pair "List-Unsubscribe=One-Click".

The combination of the URI in the List-Unsubscribe header and the POST arguments in the List-Unsubscribe-Post header MUST contain enough information to identify the mail recipient and the list from which the recipient is to be removed, so that the unsubscription process can complete automatically. In particular, One-click has no way to ask the user what address he or she wishes to unsubscribe.

The URI and POST arguments SHOULD include a hard to forge component such as a hash in addition to or instead of the plain-text names of the list and the subscriber. This will deter attacks in which a malicious party sends fraudulent mail purporting to be from the list, with the intention of getting the user to unsubscribe from the actual list.

The sending entity needs to provide the infrastructure to handle POST requests to the specified URI in the List-Unsubscribe header, and to handle the reasonably foreseeable volume of unsubscribe requests that its mail will provoke.

The One-Click action triggered by this URI SHOULD complete promptly and not burden the requester in an inappropriate way. The sending entity cannot expect that HTTPS redirects are followed by the requester, since redirected POST actions have historically not worked reliably.

3.2. Mail receivers

A receiving entity which wants to use a List-Unsubscribe HTTPS URI from an email that also contains a List-Unsubscribe-Post header performs an HTTPS POST to the first HTTPS URI in the List-Unsubscribe header and sends the content of the List-Unsubscribe-Post header as the request body.

The POST content SHOULD be sent as "multipart/form-data" [RFC7578] and MAY be sent as "application/x-www-form-urlencoded". These encodings are the ones used by web browsers when sending forms. The target of the POST action will typically be the same as or similar to the one in the manual confirmation page when doing a two-click unsubscribe, so this is intended to allow the same server code to handle both.

The receiving entity MUST NOT perform a POST on the the HTTPS URI without user consent. When and how the user consent is obtained is not part of this specification.

The Request uses the HTTPS verb POST. The HEAD and GET requests are not intended to be used to trigger a state change. PUT and DELETE would offer similar functionality but are often unavailable.

4. Additional Requirements

The email needs at least one valid authentication identifier. In this version of the specification the only supported identifier type is DKIM [RFC6376], that provides a domain-level identifier in the content of the "d=" tag of a validated DKIM-Signature header field.

The List-Unsubscribe and List-Unsubscribe-Post headers need to be covered by the signature, and hence must be included in the "h=" tag of a valid DKIM-Signature header field.

5. Header Syntax

The following ABNF imports fields, WSP, and CRLF from [RFC5322]. It imports ALPHA and DIGIT from [RFC5234].

fields /= l-u-post

ldh = ALPHA 0*(ALPHA | DIGIT | "-")

l-u-post = "List-Unsubscribe-Post:" 0*1WSP postarg 0*("&" postarg) CRLF

postarg = ALPHA 0*ldh "=" freetext

freetext = 1*(%x20-%xfe)
   ; ampersand, percent, and equal sign must be percent encoded

6. IANA Considerations

IANA is requested to add a new entry to the Permanent Message Header Field Names registry.

Header field name: List-Unsubscribe-Post

Applicable protocol: mail

Status: standard

Author/Change controller: IETF

Specification document: this document

7. Examples

7.1. Simple

Header in Email

List-Unsubscribe: <>
List-Unsubscribe-Post: List-Unsubscribe=One-Click&

Resulting POST request

POST /unsubscribe.html HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Content-Length: 49


7.2. Complex

Header in Email

List-Unsubscribe: <>,
List-Unsubscribe-Post: List-Unsubscribe=One-Click&

Resulting POST request

POST /unsubscribe.html?campaign=123456789 HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Content-Length: 49


8. Security Considerations

The List-Unsubscribe-Post header will typically contain the recipient address, but that address is usually also in the To: header. This specification allows anyone with access to a message to unsubscribe the recipient of the message, but that's typically the case with existing List-Unsubscribe, just with more steps.

A creative mailer could send spam with content intended to provoke large numbers of unsubscriptions, with suitably crafted headers to send POST requests with arbitrary contents to servers that perhaps don't want them. But it's been possible to provoke GET requests in a similar way for a long time (and much easier, due to spam filter auto-fetches) so the chances of significantly increased annoyance seem low.

Since the mailer's server that receives the POST request cannot in general tell where it is coming from, the URI or POST arguments SHOULD contain a hash or other hard to forge component to verify the list and recipient address to ensure that the request originated from List-Unsubscribe and List-Unsubscribe-Post headers in a message the mailer sent.

9. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC2369] Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax for Core Mail List Commands and their Transport through Message Header Fields", RFC 2369, DOI 10.17487/RFC2369, July 1998.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[RFC5322] Resnick, P., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008.
[RFC6376] Crocker, D., Hansen, T. and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014.
[RFC7578] Masinter, L., "Returning Values from Forms: multipart/form-data", RFC 7578, DOI 10.17487/RFC7578, July 2015.

Appendix A. Change Log

Remove this section before publication, please.

A.1. Changes from -04 to -05

Reorganize first sections and add more background. Add ABNF. Add more security advice.

A.2. Changes from -03 to -04

Require HTTPS. More motivation.

A.3. Changes from -02 to -03

Describe motivation in intro. Clarify required DKIM. More paranoid scenarios.

Authors' Addresses

John Levine Taughannock Networks PO Box 727 Trumansburg, NY 14886 Phone: +1 831 480 2300 EMail: URI:
Tobias Herkula optivo GmbH Wallstrasse 16 Berlin, 10179 DE Phone: +49 30 768078 129 EMail: URI: