[Contact Form 7] dynamically added checkboxes return only one value in email

I created dynamic checkboxes from my custom taxonomy terms to the contact form by adding this snippet to my functions.php:

wpcf7_add_shortcode('choosecat', 'createbox', true);
    function createbox(){
        global $post;
        $args = array('hide_empty' => false, 'orderby' => 'name');
        $terms = get_terms( 'branchen', $args );
        $output .= '<ul class="checkboxes">';
        foreach ( $terms as $term ) : setup_postdata($cat);
            $title = $term->name;
            $output .= "<li><input name='branches' type='checkbox' value='$title'> $title</input></li>";
        endforeach;
        $output .= '</ul>';
        return $output;
    }

So far so good, the terms are being displayed.

But after selecting multiple chechboxes, still only one value is being output in the email – why is that?

Please advise, much appreciated!