docs: document workaround for DMARC countermeasures

If a contributor's email domain has a DMARC policy of 'p=quarantine'
or 'p=reject', mailman will apply DMARC countermeasures on all mails
sent to lists.libvirt.org rewriting the "From" header to remove the
sender's email address. e.g.

  From: Your Name via <lists.libvirt.org>

If these countermeasures were not applied, affected mail would either
have gone directly to SPAM, or have been entirely rejected. Mailman3
is unable to be configured to guarantee no mangling of the mail body
so these countermeasures are unavoidable for lists.libvirt.org.

Amongst the various downsides, the From address rewriting has the
bad effect of mangling git commit author attribution.

To avoid this it is required to add two additional git config
settings:

  $ git config --global format.from "Your Name <your@email.com>"
  $ git config --global format.forceInBodyFrom true

Note, *both* are required, even if your ``format.from`` matches
your existing git identity, because the latter only takes effect
once the former is set.

Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

docs/submitting-patches.rst

@@ -43,21 +43,7 @@ series of two or more patches needs a cover letter.
 Note that the ``git send-email`` subcommand may not be in the
 main git package and using it may require installation of a
 separate package, for example the "git-email" package in Fedora
-and Debian. If this is your first time using
-``git send-email``, you might need to configure it to point it
-to your SMTP server with something like:
-
-::
-
-  $ git config --global sendemail.smtpServer stmp.youremailprovider.net
-
-If you get tired of typing ``--to=devel@lists.libvirt.org`` all
-the time, you can configure that to be automatically handled as
-well:
-
-::
-
-  $ git config sendemail.to devel@lists.libvirt.org
+and Debian.
 
 Avoid using mail clients for sending patches, as most of them
 will mangle the messages in some way, making them unusable for
@@ -88,6 +74,56 @@ Moreover, such patch needs to be prefixed correctly with
 ``git send-email`` (substitute ``v2`` with the
 correct version if needed though).
 
+Git Configuration
+-----------------
+
+If this is your first time using ``git send-email``, you will probably
+need to setup your global git configuration, to point to your outgoing
+SMTP server with something like:
+
+::
+
+  $ git config --global sendemail.smtpServer stmp.youremailprovider.net
+
+If your email provider (often your employer) has configured a DMARC
+policy for their domain, there are some additional settings that will
+be required. Before doing this, check the DMARC policy with
+
+::
+
+  $ host -t txt _dmarc.$YOURDOMAIN.COM
+
+If this returns no output, or contains ``p=none`` then no configuration
+is required. If it reports ``p=quarantine`` or ``p=reject``, then the
+libvirt lists will apply DMARC countermeasures to your email. To ensure
+that git authorship is preserved add
+
+::
+
+  $ git config --global format.from "Your Name <your@email.com>"
+  $ git config --global format.forceInBodyFrom true
+
+This will force git to always add an additional line
+
+::
+
+   From: Your Name <your@email.com>
+
+in the body of the patch, guaranteeing correct author records even
+when the main ``From`` header is rewritten by mailman.
+
+If you get tired of typing ``--to=devel@lists.libvirt.org`` all
+the time, you can configure that to be automatically handled by
+adding a local repository setting:
+
+::
+
+  $ git config sendemail.to devel@lists.libvirt.org
+
+This last setting is not required if using ``git-publish`` to send
+patches, as that auto-identifies the mailing list address from its
+config file stored in git.
+
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