function FormValidateNewsletter() {

  if (emValidate(document.FormNewsletter.email) != "OK")  {
  	alert (emValidate(document.FormNewsletter.email));
        document.FormNewsletter.email.focus();
	return (false);
	
  }

 return true;
}

function emValidate(tfName){
  var GoodChars = "@_-.:/ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
  var UpperEmail = tfName.value.toUpperCase()
  var ValidChars = true;
  for (tfCharNum = 0; tfCharNum < tfName.value.length; tfCharNum++)
  {
     Char = UpperEmail.charAt(tfCharNum);
     for (gcCharNum = 0;  gcCharNum < GoodChars.length;  gcCharNum++)
       if (Char == GoodChars.charAt(gcCharNum))
           break;
     
    if (gcCharNum == GoodChars.length)
         {
           ValidChars = false;
           break;
         }
  }
  if (!ValidChars)
    return ("Please enter only digit, whitespace and \"-@.:/\" characters in the \"email address\" field.\n");

  if (tfName.value.length < 7)
    return ("Please enter a valid email address.");

  if (tfName.value.indexOf("@") == "-1")
    return ("Please enter a valid email address.");

  if (tfName.value.indexOf(".") == "-1")
    return ("Please enter a valid email address.");

  if (tfName.value.indexOf("@") != tfName.value.lastIndexOf("@"))
    return ("Please enter a valid email address.");

  return ("OK");
}

<!-- Script open popup window  -->

win = null;

function popup(mypage,myname,w,h,features) {
  if(screen.width){
  var winl = (screen.width-w)/2;
  var wint = (screen.height-h)/2;
  }else{winl = 0;wint =0;}
  if (winl < 0) winl = 0;
  if (wint < 0) wint = 0;
  var settings = 'height=' + h + ',';
  settings += 'width=' + w + ',';
  settings += 'top=' + wint + ',';
  settings += 'left=' + winl + ',';
  settings += 'resizable'+ ',' + features;
  win = window.open(mypage,myname,settings);
  win.window.focus();
}

function clearField(myField){
  myField.value="";
}