Questo funziona...

codice:
<script>
function testa() {
  if (document.MYform.Email.value.indexOf('@', 0) == -1){ 
    alert("Please your E-mail!"); 
    document.MYform.Email.focus(); 
    return (false); 
  } 
  return (true); 
} 
//--> 
</script> 
<body>
<form name="MYform" onsubmit="return testa();">
<input type="text" name="Email">
<input type="submit">
</form>
</body>
ciao