I had this problem a couple of years ago, but I've managed to find the web page I created. (I found this workaround somewhere on the web, sorry don't remeber).
var isOpera, isIE = false;
if(typeof(window.opera) != 'undefined'){isOpera = true;}
if(!isOpera && navigator.userAgent.indexOf('Internet Explorer')){isIE = true;}
function goto(url){
location.href = url;
}
if(isIE){
function goto(url){
var referLink = document.createElement('a');
referLink.href = url;
document.body.appendChild(referLink);
referLink.click();
}
}
This forces IE to recreate the header when clicking on a link as, sometimes, explorer fails to send headers for some unknown reason. You'll have to put this fix on your www.example1.com website.
Can't you pass some crypted variabiles by querystring?