mail-addons/mail_private/full_composer_wizard.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

        <record model="ir.ui.view" id="email_compose_message_wizard_form_private">
            <field name="name">mail.compose.message.form.private</field>
            <field name="model">mail.compose.message</field>
            <field name="groups_id" eval="[(4,ref('base.group_user'))]"/>
            <field name="inherit_id" ref="mail.email_compose_message_wizard_form"/>
            <field name="arch" type="xml">
                <data>

                    <xpath expr="//field[@name='active_domain']" position="after">
                        <field name="is_private" invisible="1" />
                    </xpath>

                    <xpath expr="//div[@groups='base.group_user']/span[2]" position="attributes">
                        <attribute name="attrs">{'invisible': [('is_private', '=', True)]}
                        </attribute>
                    </xpath>

                </data>
            </field>
        </record>

</odoo>