Así funciona:
Código PHP:
<script>
function preview(){
prewin=window.open('','prewin','width=500,height=440,top=0,left=0, toolbar=no,location=no,directories=no,status=no,me nubar=no,scrollbars=yes,resizable=yes');
prewin.document.open();
prewin.document.write("<title>Preview window</title><body bgcolor=#CAE4FF><br>");
prewin.document.write("<table border=0 width=100% cellspacing=0 cellpadding=8 bgcolor=gray style=\"border: 1 dotted gray;\"><tr><td bgcolor=#FFFFFF width=100%><font face=Verdana size=1>");
prewin.document.write(document.forms[0].real_email_message.value);
prewin.document.write("</td></tr></table>");
prewin.document.close();
}
</script>