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

<?xml version="1.0" encoding="UTF-8" ?>
<template>
    <!--Inherit Sidebar and add Sent menu item after Starred -->
    <t t-extend="mail.chat.Sidebar">
        <t t-jquery="div[data-channel-id=channel_inbox]" t-operation="after">
            <div
                t-attf-class="o_mail_chat_title_main o_mail_chat_channel_item #{(active_channel_id == 'channel_sent') ? 'o_active': ''}"
                data-channel-id="channel_sent"
            >
                <span class="o_channel_name mail_sent"> <i
                        class="fa fa-send-o"
                    /> Sent </span>
            </div>
        </t>
    </t>
    <!--Add message about empty sent page-->
    <t t-extend="mail.EmptyChannel">
        <t t-jquery="t:last-child" t-operation="after">
            <t t-if="options.channel_id==='channel_sent'">
                <div class="o_thread_title">No sent messages</div>
                <div
                >You can send messages and then these messages will appear here.</div>
            </t>
        </t>
    </t>
</template>