function checkEmail() {
	var email = document.getElementById('txtEmail');
	var filter = /^([a-zA-Z0-9_.-])+@(([a-zA-Z0-9-])+.)+([a-zA-Z0-9]{2,4})+$/;
	if (!filter.test(email.value)) {
		alert('Inserire un indirizzo email valido');
		email.focus();
		return false;
	}else{
		document.getElementById("formGuestbook").submit();
	}
}