How to create a checkbox for Outlook-Addin near "New-Mail" Button

I want to create a CheckBox for "New Mails" near the Send-Button.

Here is my CustomUI.xml Code.

<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui" onLoad="Ribbon_Load">
  <ribbon>
    <tabs>
      <tab idMso="TabAddIns">
        <group id="grpTabAddIns"
               label="E-Mail2">
          <button id="btnEinstellungenOeffnen"
                  label="Einstellungen"
                  getImage="GetImage"
                  showImage="true"
                  size="large"
                  supertip="Abrufen und Ändern der Einstellungen des E-Mail2 Add-Ins"
                  onAction="btnEinstellungenOeffnen_Click"/>
        </group>
      </tab>
    </tabs>
  </ribbon>
  <contextMenus>
    <contextMenu idMso="ContextMenuMailItem">
      <button id="btnZuordnungContextMenu"
              label=" Zuordnung erstellen/ändern"
              getImage="GetImage"
              showImage="true"
              supertip=" Zuordnung erstellen/ändern"
              onAction="btnZuordnung_Click"/>
    </contextMenu>
  </contextMenus>
</customUI>

Is it possible to set a CheckBox in yellow place?