--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/plugins/show_additional_headers/show_additional_headers.php	Thu Jan 04 15:50:29 2018 -0500
@@ -0,0 +1,50 @@
+<?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'] = array('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') {
+      $this->add_hook('storage_init', array($this, 'storage_init'));
+      $this->add_hook('message_headers_output', array($this, 'message_headers'));
+    } else if ($rcmail->action == '') {
+      // with enabled_caching we're fetching additional headers before show/preview
+      $this->add_hook('storage_init', array($this, 'storage_init'));
+    }
+  }
+
+  function storage_init($p)
+  {
+    $rcmail = rcmail::get_instance();
+    if ($add_headers = (array)$rcmail->config->get('show_additional_headers', array()))
+      $p['fetch_headers'] = trim($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', array()) as $header) {
+      if ($value = $p['headers']->get($header))
+        $p['output'][$header] = array('title' => $header, 'value' => $value);
+    }
+
+    return $p;
+  }
+}