function objetoAjax(){
var xmlhttp=false;

try {
xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
} catch (e) {
try {
xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
} catch (E) {
xmlhttp = false;
}
}

if (!xmlhttp && typeof XMLHttpRequest!='undefined') {
xmlhttp = new XMLHttpRequest();
}

return xmlhttp;
}

//funcion enviar
function enviar(){
c = document.getElementById('enviar-x-mail');

id=document.enviar_mail.id.value;
des=document.enviar_mail.email.value;
tit=document.enviar_mail.titulo.value;

ajax=objetoAjax();
c.innerHTML = '<p style="text-align:center;"><img src="img/icons/waiting.gif"/></p>'; 
ajax.open("POST", "includes/mail.php",true);
ajax.onreadystatechange=function() {
if (ajax.readyState==4) {
c.innerHTML = ajax.responseText
}
}
ajax.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
ajax.send("destinatario="+des+"&titulo="+tit+"&id="+id)
}