function isAGoodEmail(email) {
  if (email.indexOf(" ") != -1) return false;
  var atPos = email.indexOf("@");
  var dotPos = email.indexOf(".");
  if ((atPos == -1) || (dotPos == -1)) return false;
  return true;
}

function precheck(myform) {
  var emailaddress = myform.emailaddress.value;
  if (emailaddress == "") {
    alert("您忘了输入电子邮件地址。");
    myform.emailaddress.focus();
    return false;
  }
  if (!isAGoodEmail(emailaddress)) {
    alert("请输入您真实有效的电子邮件地址。");
    myform.emailaddress.value="";
    myform.emailaddress.focus();
    return false;
  }
  return true;
}