social/mail_tracking/views/assets.xml

<?xml version="1.0" encoding="utf-8" ?>
<!-- Copyright 2016 Antonio Espinosa - <antonio.espinosa@tecnativa.com>
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html). -->
<odoo>
    <template
        id="assets_backend"
        name="mail_tracking assets"
        inherit_id="web.assets_backend"
    >
        <xpath expr="." position="inside">
            <link
                rel="stylesheet"
                href="/mail_tracking/static/src/css/mail_tracking.scss"
            />
            <link
                rel="stylesheet"
                href="/mail_tracking/static/src/css/failed_message.scss"
            />
            <script
                type="text/javascript"
                src="/mail_tracking/static/src/js/mail_tracking.js"
            />
            <script
                type="text/javascript"
                src="/mail_tracking/static/src/js/failed_message/discuss.js"
            />
            <script
                type="text/javascript"
                src="/mail_tracking/static/src/js/failed_message/thread.js"
            />
        </xpath>
    </template>
</odoo>