DMARC Mitigations

In order to mitigate the effects of DMARC on mailing list traffic, list administrators have the ability to apply transformations to messages delivered to list members. These transformations are applied only to individual messages sent to list members and not to messages in digests, archives, or gated via NNTP.

The messages can be transformed by either munging the From: header and putting original From: in Cc: or Reply-To:, or by wrapping the original message in an outer message From: the list.

Exactly which transformations are applied depends on a number of list settings.

The settings and their effects are:

anonymous_list: If True, no mitigations are ever applied because the message is already From: the list.
dmarc_mitigate_action: The action to apply to messages From: a domain publishing a DMARC policy of reject or quarantine, or to all messages depending on the setting of dmarc_mitigate_unconditionally.
dmarc_mitigate_unconditionally: If True, apply dmarc_mitigate_action to all messages, but only if dmarc_mitigate_action is neither reject or discard.
dmarc_moderation_notice: Text to include in any rejection notice to be sent when dmarc_policy_mitigation of reject applies. This overrides the built-in default text.
dmarc_wrapped_message_text: Text to be added as a separate text/plain MIME part preceding the original message part in the wrapped message when a wrap_message mitigation applies. If this is not provided the separate text/plain MIME part is not added.
reply_goes_to_list: If this is set to other than no-munging of Reply-To:, the original From: goes in Cc: rather than Reply-To:. This is intended to make MUA functions of reply and reply-all have the same effect with messages to which mitigations have been applied as they do with other messages.

The possible actions for dmarc_mitigate_action are:

no_mitigation: Make no transformation to the message.
munge_from: Change the From: header and put the original From: in Reply-To: or in some cases Cc:.
wrap_message: Wrap the message in an outer message with headers from the original message as in munge_from.
reject: Bounce the message back to the sender with a default reason or one supplied in dmarc_moderation_notice.
discard: Silently discard the message.

Here’s what happens when we munge the From:.

>>> from mailman.interfaces.mailinglist import (
...     DMARCMitigateAction, ReplyToMunging)

>>> from mailman.app.lifecycle import create_list
>>> mlist = create_list('ant@example.com')
>>> mlist.dmarc_mitigate_action = DMARCMitigateAction.munge_from
>>> mlist.reply_goes_to_list = ReplyToMunging.no_munging

>>> from mailman.testing.helpers import (specialized_message_from_string
...   as message_from_string)
>>> msg = message_from_string("""\
... From: Anne Person <aperson@example.com>
... To: ant@example.com
...
... A message of great import.
... """)
>>> msgdata = dict(dmarc=True, original_sender='aperson@example.com')

>>> from mailman.handlers.dmarc import process
>>> process(mlist, msg, msgdata)
>>> print(msg.as_string())
To: ant@example.com
From: Anne Person via Ant <ant@example.com>
Reply-To: Anne Person <aperson@example.com>

A message of great import.

Here we wrap the message without adding a text part.

>>> mlist.dmarc_mitigate_action = DMARCMitigateAction.wrap_message
>>> mlist.dmarc_wrapped_message_text = ''

>>> msg = message_from_string("""\
... From: Anne Person <aperson@example.com>
... To: ant@example.com
...
... A message of great import.
... """)
>>> msgdata = dict(dmarc=True, original_sender='aperson@example.com')

>>> process(mlist, msg, msgdata)
>>> print(msg.as_string())
To: ant@example.com
MIME-Version: 1.0
Message-ID: <...>
From: Anne Person via Ant <ant@example.com>
Reply-To: Anne Person <aperson@example.com>
Content-Type: message/rfc822
Content-Disposition: inline

From: Anne Person <aperson@example.com>
To: ant@example.com

A message of great import.

And here’s a wrapped message with an added text part.

>>> mlist.dmarc_wrapped_message_text = 'The original message is attached.'

>>> msg = message_from_string("""\
... From: Anne Person <aperson@example.com>
... To: ant@example.com
...
... A message of great import.
... """)
>>> msgdata = dict(dmarc=True, original_sender='aperson@example.com')

>>> process(mlist, msg, msgdata)
>>> print(msg.as_string())
To: ant@example.com
MIME-Version: 1.0
Message-ID: <...>
From: Anne Person via Ant <ant@example.com>
Reply-To: Anne Person <aperson@example.com>
Content-Type: multipart/mixed; boundary="..."

--...
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Disposition: inline

The original message is attached.
--...
Content-Type: message/rfc822
MIME-Version: 1.0
Content-Disposition: inline

From: Anne Person <aperson@example.com>
To: ant@example.com

A message of great import.

--...--