XDomainRequest open not working on clients IE8 machine

Question

Can anyone tell me why XDomainRequest is not working on my clients login computers? It works on my IE9 and my servers. It works on their servers but will not work on their IE8 logins

        function CreateCrossDomainMessage(ExecuteExternalWebServiceClass, EndpointAddress, PostData) {
        alert("Testing XDomainRequest");
        var xdr = new XDomainRequest();
        xdr.open("POST", EndpointAddress); // Does not open on clients computer, works on their servers ????
        xdr.timeout = 2000;
        xdr.ontimeout = function () {
            alert("ontimeout");
        };
        xdr.onerror = function () {
            alert("onerror");
        };
        xdr.onload = function () {
            alert("onload");
        };
        xdr.send(PostData);

// new CrossDomainMessage(ExecuteExternalWebServiceClass, EndpointAddress, PostData); }