mail-addons/mail_private/static/src/xml/mail_private.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!--Copyright 2016-2017 Ivan Yelizariev <https://it-projects.info/team/yelizariev>
    Copyright 2019 Artem Rafailov <https://it-projects.info/team/Ommo73/>
    License MIT (https://opensource.org/licenses/MIT).-->
<template>
    <t t-extend="mail.chatter.Buttons">
        <t t-jquery="button[title='Send a message']" t-operation="after">
            <button
                class="btn btn-link oe_compose_post_private"
                title="Send a message to specified recipients only"
            >
                Send internal message
            </button>
        </t>
    </t>
    <t t-extend="mail.chatter.Composer">
        <t t-jquery="small[class='o_chatter_composer_info']" t-operation="replace">
            <small class="o_chatter_composer_info" t-if="!widget.options.is_private">
                To: Followers of
                <t t-if="widget.options.record_name">
                        &quot;<t t-esc="widget.options.record_name" />&quot;
                    </t>
                <t t-if="!widget.options.record_name">this document</t>
            </small>
        </t>
        <t t-jquery="div[class='o_composer_suggested_partners']" t-operation="after">
            <button
                class="btn btn-link oe_composer_uncheck"
                t-if="widget.options.is_private"
            >
                Uncheck all
            </button>
        </t>
    </t>
</template>