roundcubemail/plugins/show_additional_headers/show_additional_headers.php

<?php

/**
 * Show additional message headers
 *
 * Proof-of-concept plugin which will fetch additional headers
 * and display them in the message view.
 *
 * Enable the plugin in config.inc.php and add your desired headers:
 *   $config['show_additional_headers'] = ['User-Agent'];
 *
 * @author Thomas Bruederli
 * @license GNU GPLv3+
 */
class show_additional_headers extends rcube_plugin
{
    public $task = 'mail';

    function init()
    {
        $rcmail = rcmail::get_instance();

        if ($rcmail->action == 'show' || $rcmail->action == 'preview') {
            $this->add_hook('storage_init', [$this, 'storage_init']);
            $this->add_hook('message_headers_output', [$this, 'message_headers']);
        }
        else if ($rcmail->action == '') {
            // with enabled_caching we're fetching additional headers before show/preview
            $this->add_hook('storage_init', [$this, 'storage_init']);
        }
    }

    function storage_init($p)
    {
        $rcmail = rcmail::get_instance();

        if ($add_headers = (array) $rcmail->config->get('show_additional_headers', [])) {
            $p['fetch_headers'] = trim($p['fetch_headers']. ' ' . strtoupper(join(' ', $add_headers)));
        }

        return $p;
    }

    function message_headers($p)
    {
        $rcmail = rcmail::get_instance();

        foreach ((array) $rcmail->config->get('show_additional_headers', []) as $header) {
            if ($value = $p['headers']->get($header)) {
                $p['output'][$header] = ['title' => $header, 'value' => $value];
            }
        }

        return $p;
    }
}