I am fixed this issue by removing the TinyMCE preview , and now using fancy box for preview.
$("body").delegate("#previewMail", "click", function(event) {
$.fancybox({
'transitionIn': 'none',
'transitionOut': 'none',
'titlePosition' : 'inside',
'transitionIn' : 'none',
'transitionOut' : 'none',
'content':tinyMCE.activeEditor.getContent(),
});
});
$(document).on('click','.fancybox-inner',function(e){ e.preventDefault(); });