function validar() {
if (document.contact_us.email.value=="") {
    alert("Please enter your email address.");
    document.contact_us.email.focus();
    return false;
}

parte1 = document.contact_us.email.value.indexOf("@");
parte2 = document.contact_us.email.value.indexOf(".");
parte3 = document.contact_us.email.value.length;
if (!(parte1 >= 3 && parte2 >= 6 && parte3 >= 9)) {
    alert ("Not a valid e-mail address!");
    document.contact_us.email.focus();
    return false;
}

if (document.contact_us.name.value=="") {
    alert("Please enter your name.");
    document.contact_us.name.focus();
    return false;
}

if (document.contact_us.message.value=="") {
    alert("Please enter your message.");
    document.contact_us.message.focus();
    return false;
}

}