Would this give you any hint ?
Code:
<script language="JavaScript">
<!--
function deleteAllCookies()
{
if (document.cookie)
{
var today=new Date;
today.setYear(today.getYear()-10);
var cookies=document.cookie.split(";");
var i=0;
for (i=0; i<cookies.length; i++)
{
document.cookie=cookies[i]+" ;expires="+today.toGMTString();
}
}
}
// -->
</script>
<body onUnload="javascript:deleteAllCookies()">
Suitable placed in your main window, which opens the mail-popup.