XMLHTTPRequest works only using Alert - xmlhttprequest

Question

I have an ASP.NET application which uses XMLHTTPRequest to call a server side method when the user closes the browser(the requirement is I need to send a email when the user closes the browser). The code works perfectly when I have an Javascript alert. But it doesnt work when I remove the alert button. I read another article in StackOverflow here which has similar issues. Can someone tell me a work around for this which guarantees that my server side code is executed when the user closes the browser. Please find the code snippet below.
var ajaxRequest;
window.onbeforeunload = function(e) {
//function ConfirmClose(e) {
var evtobj = window.event ? event : e;
if (evtobj == e) {
//firefox
if (!evtobj.clientY) {
AjaxRequest();
//evtobj.returnValue = message;
}
}
else {
//IE
if (evtobj.clientY < 0) {
AjaxRequest();
}
}
}
function AjaxRequest() {
var url = "closing.aspx?function=EmailUnsubmittedApplication";
if (window.XMLHttpRequest) {
AJAX = new XMLHttpRequest();
}
else {
AJAX = new ActiveXObject("Microsoft.XMLHTTP");
}
if (AJAX) {
AJAX.open("POST", url, false);
AJAX.send(null);
return AJAX.responseText;
}
else {
return false;
}
}