function checkMail(email)
{
	if (email.length==0) {
		alert('Please, introduce an email address');
		return false;
	}
	var filter  = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;
	if (filter.test(email)) return true;
	else {
		alert('Please, introduce a valid email address');
		return false;
	};
}