This hook allows you to alter or remove the email headers subscribe2 uses at the time of sending. Headers include ‘From’, ‘Reply-To’, ‘Return-path’, ‘Precedence’ and ‘Content-Type’. If your hosting provider does not allow use of the ‘Reply-To’ header you an use code like this:

function my_s2_headers($header) {
    unset($header['Reply-To']);
    return $header;
}
add_filter('s2_email_headers', 'my_s2_headers');