mail-addons/mail_all/static/src/xml/menu.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!--# Copyright 2018 Artyom Losev <https://it-projects.info/team/ArtyomLosev>
    # Copyright 2018 Kolushov Alexandr <https://it-projects.info/team/KolushovAlexandr>
    # License MIT (https://opensource.org/licenses/MIT). -->
<template>
    <!--Inherit Sidebar and add All messages menu item after Starred -->
    <t t-extend="mail.discuss.Sidebar">
        <t t-jquery="div[data-thread-id=mailbox_starred]" t-operation="after">
            <div
                t-attf-class="o_mail_discuss_title_main o_mail_mailbox_title_all o_mail_discuss_item #{(activeThreadID == 'channel_all') ? 'o_active': ''}"
                data-thread-id="mailbox_channel_all"
            >
                <span class="o_channel_name mail_all"> <i
                        class="fa fa-database"
                    /> All messages </span>
            </div>
        </t>
    </t>
    <!--Add message about empty all messages page-->
    <t t-extend="mail.widget.Thread.Empty">
        <t t-jquery="t:last" t-operation="after">
            <t t-if="thread.getID() === 'mailbox_channel_all'">
                <div class="o_thread_title">No messages</div>
            </t>
        </t>
    </t>
</template>