What about mailing lists?

When you deploy DMARC on your domain, you might find that messages relayed by mailing lists are failing DMARC, most likely because the mailing list is spoofing your from address, and modifying the subject, footer, or other part of the message, thereby breaking the DKIM signature.

Mailing list best practices

Ideally, a mailing list should forward messages without altering the headers or body content at all. Joe Nelson does a fantastic job of explaining exactly what mailing lists should and shouldn’t do to be fully DMARC compliant. Rather than repeat his fine work, here’s a summary:

Do

Retain headers from the original message
Add RFC 2369 List-Unsubscribe headers to outgoing messages, instead of adding unsubscribe links to the body

List-Unsubscribe: https://list.example.com/unsubscribe-link

Add RFC 2919 List-Id headers instead of modifying the subject

List-Id: Example Mailing List

Modern mail clients and webmail services generate unsubscribe buttons based on these headers.

Do not

Remove or modify any existing headers from the original message, including From, Date, Subject, etc.
Add to or remove content from the message body, including traditional disclaimers and unsubscribe footers

In addition to complying with DMARC, this configuration ensures that Reply and Reply All actions work like they would with any email message. Reply replies to the message sender, and Reply All replies to the sender and the list.

Even without a subject prefix or body footer, mailing list users can still tell that a message came from the mailing list, because the message was sent to the mailing list post address, and not their email address.

Configuration steps for common mailing list platforms are listed below.

Mailman 2

Navigate to General Settings, and configure the settings below

============================ ==========
**Setting**                  **Value**
**subject_prefix**
**from_is_list**             No
**first_strip_reply_to**     No
**reply_goes_to_list**       Poster
**include_rfc2369_headers**  Yes
**include_list_post_header** Yes
**include_sender_header**    No
============================ ==========

Navigate to Non-digest options, and configure the settings below

=================== ==========
**Setting**         **Value**
**msg_header**
**msg_footer**
**scrub_nondigest**  No
=================== ==========

Navigate to Privacy Options> Sending Filters, and configure the settings below

====================================== ==========
**Setting**                            **Value**
**dmarc_moderation_action**            Accept
**dmarc_quarantine_moderation_action** Yes
**dmarc_none_moderation_action**       Yes
====================================== ==========

Mailman 3

Navigate to Settings> List Identity

Make Subject prefix blank.

Navigate to Settings> Alter Messages

Configure the settings below

====================================== ==========
**Setting**                            **Value**
**Convert html to plaintext**          No
**Include RFC2369 headers**            Yes
**Include the list post header**       Yes
**Explicit reply-to address**
**First strip replyto**                 No
**Reply goes to list**                 No munging
====================================== ==========

Navigate to Settings> DMARC Mitigation

Configure the settings below

================================== ===============================
**Setting**                            **Value**
**DMARC mitigation action**            No DMARC mitigations
**DMARC mitigate unconditionally** No
================================== ===============================

Create a blank footer template for your mailing list to remove the message footer. Unfortunately, the Postorius mailing list admin UI will not allow you to create an empty template, so you’ll have to create one using the system’s command line instead, for example:

touch var/templates/lists/list.example.com/en/list:member:regular:footer

Where list.example.com the list ID, and en is the language.

Then restart mailman core.

LISTSERV

LISTSERV 16.0-2017a and higher will rewrite the From header for domains that enforce with a DMARC quarantine or reject policy.

Some additional steps are needed for Linux hosts.

Workarounds

If a mailing list must go against best practices and modify the message (e.g. to add a required legal footer), the mailing list administrator must configure the list to replace the From address of the message (also known as munging) with the address of the mailing list, so they no longer spoof email addresses with domains protected by DMARC.

Configuration steps for common mailing list platforms are listed below.

Mailman 2

Navigate to Privacy Options> Sending Filters, and configure the settings below

====================================== ==========
**Setting**                            **Value**
**dmarc_moderation_action**            Munge From
**dmarc_quarantine_moderation_action** Yes
**dmarc_none_moderation_action**       Yes
====================================== ==========

:::{note} Message wrapping could be used as the DMARC mitigation action instead. In that case, the original message is added as an attachment to the mailing list message, but that could interfere with inbox searching, or mobile clients.

On the other hand, replacing the From address might cause users to accidentally reply to the entire list, when they only intended to reply to the original sender.

Choose the option that best fits your community. :::

Mailman 3

In the DMARC Mitigations tab of the Settings page, configure the settings below

================================== ===============================
**Setting**                            **Value**
**DMARC mitigation action**            Replace From: with list address
**DMARC mitigate unconditionally** No
================================== ===============================

On the other hand, replacing the From address might cause users to accidentally reply to the entire list, when they only intended to reply to the original sender. :::