[libvirt] FYI: intention to remove mail subject prefix & footer text

[Daniel P. Berrangé]

Hi List Subscribers,

In recent months we have been seeing an increasing number of bounced
deliveries from libvirt mailing lists[1] due to DMARC policies on list
subscriber's mail servers. IOW, many subscribers are only receiving
a subset of mails sent to the libvirt mailing lists.

We believe the root cause of many of the problems is that mailman is
modifying the mail subject to add the "[libvirt]" / "[libvirt-users]"
prefix, and modifying the mail body to add the footer with links to
the listinfo page.

These modifications invalidate the DKIM signatures on mails sent to
the list by some of our subscribers. This in turn causes DMARC policy
rejections by the destination SMTP servers when mailman delivers
messages.

The solution is to disable any feature in mailman which modifies
parts of the mail validated by the DKIM signature. This means removing
the subject prefix and the mail body header. Further information on
this approach can be seen here:

   https://begriffs.com/posts/2018-09-18-dmarc-mailing-list.html

QEMU has made the same change on their mailing list last year:

   https://lists.gnu.org/archive/html/qemu-devel/2019-09/msg00416.html

If you are currently doing mail filtering / sorting based on the subject
prefix, you will need to change to use the List-Id header instead.

I will wait until the latter part of next week before making this change
to allow people time to adapt any filters.

Regards,
Daniel

[1] technically moderators have only been seeing bounces for messages on
    libvirt-users-list, but that's because we've got mailman configured
    to send libvir-list bounces to /dev/null.
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