ajax 检测邮箱是否被注册

if (document.register.email.value.charAt(0)=="." ||       
         document.register.email.value.charAt(0)=="@"||      
         document.register.email.value.indexOf('@', 0) == -1 ||
         document.register.email.value.indexOf('.', 0) == -1 ||
         document.register.email.value.lastIndexOf("@")==document.register.email.value.length-1 ||
         document.register.email.value.lastIndexOf(".")==document.register.email.value.length-1)
     {
      alert("电子邮箱格式不正确！");
      document.register.email.focus();
      return false;
      }

sendRequest("checkemail.asp?email=" + document.register.email.value);
 if (return_str=="0"){
 alert("该邮箱已经存在，请另选一个");
 document.register.email.focus();
    document.register.email.select();
    return false
 }

var return_str=""

xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
function sendRequest(sndStr)
{
var xmlDom = new ActiveXObject("Msxml2.DOMDocument");
xmlhttp.open("GET", sndStr, false);
xmlhttp.onreadystatechange = sendRequest_check;
xmlhttp.Send(xmlDom);
}
//--------------------------------------------------------
function sendRequest_check()
{
 var state = xmlhttp.readyState;
 
 var xmlDom = new ActiveXObject("Msxml2.DOMDocument");

 if (state == 4)
 {
 xmlDom.loadXML(xmlhttp.responseXML.xml);
 return_str = xmlDom.documentElement.selectSingleNode("//objXML").text;
 }
}