Ribbon1.xml

<?xml version="1.0" encoding="UTF-8"?>
<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui" onLoad="Ribbon_Load" >
    <ribbon startFromScratch="false">
    <tabs>
      <tab idMso="TabMail" visible="true">
          <group  id="grousp1"  getLabel="GetGroupLabel" visible="true">
            <button id="button1D"
             size="large" 
                        getLabel="GetButtonLabel"
                        getSupertip="GetSupertipLabel"
                        getImage="GetImage"
                        onAction="ShowMessageClick"></button>
          </group>
      </tab>
    </tabs>
  </ribbon>
  
  <contextMenus>
    <contextMenu idMso="ContextMenuMailItem">
      <button id="SynchronizeMailItem" 
              onAction="ShowMessageClick" 
              showImage="true"              
              getImage="GetImage"
              getLabel="GetContextMenuLabel"/>
    </contextMenu>
  </contextMenus>
</customUI>