roundcubemail/plugins/additional_message_headers/additional_message_headers.php

<?php

/**
 * Additional Message Headers
 *
 * Very simple plugin which will add additional headers
 * to or remove them from outgoing messages.
 *
 * Enable the plugin in config.inc.php and add your desired headers:
 * $config['additional_message_headers'] = array('User-Agent' => 'My-Very-Own-Webmail');
 *
 * @version @package_version@
 * @author Ziba Scott
 * @website http://roundcube.net
 */
class additional_message_headers extends rcube_plugin
{
    function init()
    {
        $this->add_hook('message_before_send', array($this, 'message_headers'));
    }

    function message_headers($args)
    {
        $this->load_config();

        $headers = $args['message']->headers();
        $rcube   = rcube::get_instance();

        // additional email headers
        $additional_headers = $rcube->config->get('additional_message_headers', array());
        foreach ((array)$additional_headers as $header => $value) {
            if (null === $value) {
                unset($headers[$header]);
            }
            else {
                $headers[$header] = $value;
            }
        }

        $args['message']->_headers = array();
        $args['message']->headers($headers);

        return $args;
    }
}