var http = false;
            if(navigator.appName == "Microsoft Internet Explorer") {
                http = new ActiveXObject("Microsoft.XMLHTTP");
            }else {
                http = new XMLHttpRequest();
            }
   
            function checkform(email) {
                http.abort();
                http.open("GET", "boletines.php?email=" + email, true);
   
                http.onreadystatechange=function(){
                    if(http.readyState == 4){
                        //document.getElementById('ajaxresponse').innerHTML = http.responseText;
                        alert(http.responseText)
                    }
                    if(http.readyState == 1){
                        //document.getElementById('ajaxresponse').innerHTML = <img src="images/favicon.ico">
                        
                        }
                }
   
                http.send(null);
            }