app-odoo/app_mail_superbar/views/mail_mail_views.xml

<odoo>
  <data>
    <record id="app_view_mail_search" model="ir.ui.view">
        <field name="name">app.mail.mail.search</field>
        <field name="model">mail.mail</field>
        <field name="inherit_id" ref="mail.view_mail_search"/>
        <field name="arch" type="xml">
            <xpath expr="//search">
                <superbar view_mode="kanban,tree">
                    <field name="state"/>
                    <field name="author_id"/>
                </superbar>
            </xpath>
        </field>
    </record>
  </data>
</odoo>