Indirizzo email errato, riprovare:
            <div id="form_newsletter_box">
                <form id="form_newsletter">
                    <input type="text" id="form_newsletter_email" value="indirizzo email" onclick="this.value=''">
                    <input onclick="newsletter_form_submit()" type="button" value="Iscriviti">
                </form>
            </div>