<?php if (!$_POST['newsletter-enviar']){ ?>
            <form id="newsletter-cadastro" method="post" action="">
                <input type="text" name="newsletter-email" class="newsletter-campo-texto" />
                <input type="submit" name="newsletter-enviar" class="newsletter-campo-enviar" value="Cadastrar" />
            </form>
        <?php }

        else {
            $email_inscrito = $_POST['newsletter-email'];
            echo $email_inscrito;
            $wpMail = new wpMail();
            if (class_exists('wpMail')) {
                if ($wpMail = new wpMail()) {
                    global $Subscriber;

                    $subscriber = array(
                        'email'             =>  $email_inscrito,        //the email of the subscriber
                        'list_type'         =>  "specific",             //always leave this "specific"
                        'list_id'           =>  "2",                    //the ID of the mailing list to subscribe to
                    );

                    if ($Subscriber -> optin($subscriber, true)) {
                        echo "Usuario cadastrado";
                    }
                }
            }
        }
        ?>