How do you display buttons in the Outlook Quick Access Toolbar using XML

I've created a new Outlook 2013 addin C# VSTO project in VS 2012. I've added an XML file to customise the ribbon to add a new tab with 2 existing buttons. But I can't make buttons appear in the Quick Access Toolbar (QAT).

Below is the XML I've written. It displays the new Tab and buttons. But doesn't display the specified buttons in the Quick Access Toolbar.

<?xml version="1.0" encoding="UTF-8"?>
<customUI xmlns="http://ift.tt/1gKfCPl" onLoad="Ribbon_Load">
    <ribbon>
        <qat>
            <sharedControls>
                <button idMso="NewMailMessage" visible="true" />
                <button idMso="NewTask" visible="true" />
            </sharedControls>
        </qat>
    <tabs>
        <tab id="newTab" label="New Tab" insertBeforeMso="TabCalendarTableView">
            <group id="newGroup" label="New">
                <button idMso="NewMailMessage" size="large"/>
                <button idMso="NewAppointment" size="large"/>
                <button idMso="NewContact" size="large"/>
                <button idMso="NewDistributionList" size="large"/>
                <button idMso="NewTask" size="large"/>
                <button idMso="MailNewItemMenu" size="large"/>
            </group>
         </tab>
    </tabs>
</ribbon>

What do I need to change to make it display the buttons in the Quick Access Toolbar?