a me funziona
codice:
<html>
<head>
<script>
function emailvalidation(entered, alertbox){
  with (entered){
    apos=value.indexOf("@"); 
    dotpos=value.lastIndexOf(".");
    lastpos=value.length-1;
    if (apos<1 || dotpos-apos<2 || lastpos-dotpos>3 || lastpos-dotpos<2){
      if (alertbox) {
        alert(alertbox);
      } 
      return false;
    }
    else {return true;}
  }
} 
</script>
</head>
<body>
<form onSubmit="return emailvalidation(this.requiredmail,'Indirizzo E-mail non Corretto!');">
<input type=text name=requiredmail onChange="emailvalidation(this,'Indirizzo E-mail non Corretto!');">
<input type="submit">
</form>
</body>
</html>