Mouseout once

I would like to know how I can make this script work once, because I can not do it:

// intenção de sair
function addEvent(obj, evt, fn) {
    if (obj.addEventListener) {
        obj.addEventListener(evt, fn, false);
    }
    else if (obj.attachEvent) {
        obj.attachEvent("on" + evt, fn);
    }
}

// gatilho 
addEvent(document, 'mouseout', function(evt) {

    if (evt.toElement == null && evt.relatedTarget == null ) {
        $('#lightbox_overlay').slideDown();
    };

});

// fechando o popup           
$('#lightbox_overlay, #close').on('click', function(event) { 
    $('.lightbox').slideUp();                            
});