diff --git a/plugins/libkolab/lib/kolab_storage.php b/plugins/libkolab/lib/kolab_storage.php
index 3a70bd52..17609393 100644
--- a/plugins/libkolab/lib/kolab_storage.php
+++ b/plugins/libkolab/lib/kolab_storage.php
@@ -154,7 +154,7 @@ class kolab_storage
 
         if (self::setup()) {
             foreach ((array)self::list_folders('', '*', $type, $subscribed, $folderdata) as $foldername) {
-                $folders[$foldername] = new kolab_storage_folder($foldername, $folderdata[$foldername]);
+                $folders[$foldername] = new kolab_storage_folder($foldername, $type, $folderdata[$foldername]);
             }
         }
 
@@ -171,7 +171,7 @@ class kolab_storage
     {
         if (self::setup()) {
             foreach ((array)self::list_folders('', '*', $type . '.default', false, $folderdata) as $foldername) {
-                return new kolab_storage_folder($foldername, $folderdata[$foldername]);
+                return new kolab_storage_folder($foldername, $type, $folderdata[$foldername]);
             }
         }
 
@@ -203,11 +203,11 @@ class kolab_storage
     {
         self::setup();
         $folder = null;
-        foreach ((array)self::list_folders('', '*', $type) as $foldername) {
+        foreach ((array)self::list_folders('', '*', $type, null, $folderdata) as $foldername) {
             if (!$folder)
-                $folder = new kolab_storage_folder($foldername);
+                $folder = new kolab_storage_folder($foldername, $type, $folderdata[$foldername]);
             else
-                $folder->set_folder($foldername);
+                $folder->set_folder($foldername, $type, $folderdata[$foldername]);
 
             if ($object = $folder->get_object($uid, '*'))
                 return $object;
@@ -230,11 +230,11 @@ class kolab_storage
         $folder = null;
         $result = array();
 
-        foreach ((array)self::list_folders('', '*', $type) as $foldername) {
+        foreach ((array)self::list_folders('', '*', $type, null, $folderdata) as $foldername) {
             if (!$folder)
-                $folder = new kolab_storage_folder($foldername);
+                $folder = new kolab_storage_folder($foldername, $type, $folderdata[$foldername]);
             else
-                $folder->set_folder($foldername);
+                $folder->set_folder($foldername, $type, $folderdata[$foldername]);
 
             foreach ($folder->select($query, '*') as $object) {
                 $result[] = $object;
@@ -901,7 +901,7 @@ class kolab_storage
                 !self::folder_is_subscribed($foldername, true) &&
                 !in_array(self::$imap->folder_namespace($foldername), (array)$exclude_ns)
               ) {
-                $folders[] = new kolab_storage_folder($foldername, $folderdata[$foldername]);
+                $folders[] = new kolab_storage_folder($foldername, $type, $folderdata[$foldername]);
             }
         }
 
@@ -974,7 +974,7 @@ class kolab_storage
                     $parent_parent = join($delim, $path);
                     if (!$refs[$parent]) {
                         if ($folder->type && self::folder_type($parent) == $folder->type) {
-                            $refs[$parent] = new kolab_storage_folder($parent, $folder->type);
+                            $refs[$parent] = new kolab_storage_folder($parent, $folder->type, $folder->type);
                             $refs[$parent]->parent = $parent_parent;
                         }
                         else if ($parent_parent == $other_ns) {
@@ -1542,7 +1542,7 @@ class kolab_storage
             foreach ($folders as $userfolder) {
                 foreach ((array)self::list_folders($userfolder->name . $delimiter, '*', $type, false, $folderdata) as $foldername) {
                     if (!$folders[$foldername]) {
-                        $folders[$foldername] = new kolab_storage_folder($foldername, $folderdata[$foldername]);
+                        $folders[$foldername] = new kolab_storage_folder($foldername, $type, $folderdata[$foldername]);
                         $userfolder->children[] = $folders[$foldername];
                     }
                 }
diff --git a/plugins/libkolab/lib/kolab_storage_cache.php b/plugins/libkolab/lib/kolab_storage_cache.php
index 524a03a2..4f12df71 100644
--- a/plugins/libkolab/lib/kolab_storage_cache.php
+++ b/plugins/libkolab/lib/kolab_storage_cache.php
@@ -80,6 +80,7 @@ class kolab_storage_cache
         $this->db = $rcmail->get_dbh();
         $this->imap = $rcmail->get_storage();
         $this->enabled = $rcmail->config->get('kolab_cache', false);
+        $this->folders_table = $this->db->table_name('kolab_folders');
 
         if ($this->enabled) {
             // always read folder cache and lock state from DB master
@@ -98,8 +99,7 @@ class kolab_storage_cache
      */
     public function select_by_id($folder_id)
     {
-        $folders_table = $this->db->table_name('kolab_folders', true);
-        $sql_arr = $this->db->fetch_assoc($this->db->query("SELECT * FROM $folders_table WHERE `folder_id` = ?", $folder_id));
+        $sql_arr = $this->db->fetch_assoc($this->db->query("SELECT * FROM `{$this->cache_table}` WHERE `folder_id` = ?", $folder_id));
         if ($sql_arr) {
             $this->metadata = $sql_arr;
             $this->folder_id = $sql_arr['folder_id'];
@@ -120,14 +120,13 @@ class kolab_storage_cache
     {
         $this->folder = $storage_folder;
 
-        if (empty($this->folder->name)) {
+        if (empty($this->folder->name) || !$this->folder->valid) {
             $this->ready = false;
             return;
         }
 
         // compose fully qualified ressource uri for this instance
         $this->resource_uri = $this->folder->get_resource_uri();
-        $this->folders_table = $this->db->table_name('kolab_folders');
         $this->cache_table = $this->db->table_name('kolab_cache_' . $this->folder->type);
         $this->ready = $this->enabled && !empty($this->folder->type);
         $this->folder_id = null;
diff --git a/plugins/libkolab/lib/kolab_storage_folder.php b/plugins/libkolab/lib/kolab_storage_folder.php
index d6d17c82..d9c79d5f 100644
--- a/plugins/libkolab/lib/kolab_storage_folder.php
+++ b/plugins/libkolab/lib/kolab_storage_folder.php
@@ -30,14 +30,22 @@ class kolab_storage_folder extends kolab_storage_folder_api
      */
     public $cache;
 
-    private $type_annotation;
+    /**
+     * Indicate validity status
+     * @var boolean
+     */
+    public $valid = false;
+
     private $resource_uri;
 
 
     /**
      * Default constructor
+     *
+     * @param string The folder name/path
+     * @param string Expected folder type
      */
-    function __construct($name, $type = null)
+    function __construct($name, $type = null, $type_annotation = null)
     {
         parent::__construct($name);
         $this->imap->set_options(array('skip_deleted' => true));
@@ -49,28 +57,31 @@ class kolab_storage_folder extends kolab_storage_folder_api
      * Set the IMAP folder this instance connects to
      *
      * @param string The folder name/path
+     * @param string Expected folder type
      * @param string Optional folder type if known
      */
-    public function set_folder($name, $type = null)
+    public function set_folder($name, $type = null, $type_annotation = null)
     {
-        $this->type_annotation = $type ? $type : kolab_storage::folder_type($name);
+        if (empty($type_annotation)) {
+            $type_annotation = kolab_storage::folder_type($name);
+        }
 
         $oldtype = $this->type;
-        list($this->type, $suffix) = explode('.', $this->type_annotation);
+        list($this->type, $suffix) = explode('.', $type_annotation);
         $this->default      = $suffix == 'default';
         $this->subtype      = $this->default ? '' : $suffix;
         $this->name         = $name;
         $this->id           = kolab_storage::folder_id($name);
+        $this->valid        = !empty($this->type) && $this->type != 'mail' && (!$type || $this->type == $type);
 
         // reset cached object properties
         $this->owner = $this->namespace = $this->resource_uri = $this->info = $this->idata = null;
 
-        // get a new cache instance of folder type changed
-        if (!$this->cache || $type != $oldtype)
+        // get a new cache instance if folder type changed
+        if (!$this->cache || $this->type != $oldtype)
             $this->cache = kolab_storage_cache::factory($this);
 
         $this->imap->set_folder($this->name);
-        $this->cache->set_folder($this);
     }
 
 
@@ -204,6 +215,10 @@ class kolab_storage_folder extends kolab_storage_folder_api
      */
     public function count($query = null)
     {
+        if (!$this->valid) {
+            return 0;
+        }
+
         // synchronize cache first
         $this->cache->synchronize();
 
@@ -221,6 +236,10 @@ class kolab_storage_folder extends kolab_storage_folder_api
     {
         if (!$type) $type = $this->type;
 
+        if (!$this->valid) {
+            return array();
+        }
+
         // synchronize caches
         $this->cache->synchronize();
 
@@ -238,9 +257,14 @@ class kolab_storage_folder extends kolab_storage_folder_api
      */
     public function select($query = array())
     {
+        if (!$this->valid) {
+            return array();
+        }
+
         // check query argument
-        if (empty($query))
+        if (empty($query)) {
             return $this->get_objects();
+        }
 
         // synchronize caches
         $this->cache->synchronize();
@@ -258,6 +282,10 @@ class kolab_storage_folder extends kolab_storage_folder_api
      */
     public function get_uids($query = array())
     {
+        if (!$this->valid) {
+            return array();
+        }
+
         // synchronize caches
         $this->cache->synchronize();
 
@@ -319,6 +347,10 @@ class kolab_storage_folder extends kolab_storage_folder_api
      */
     public function get_object($uid, $type = null)
     {
+        if (!$this->valid) {
+            return false;
+        }
+
         // synchronize caches
         $this->cache->synchronize();
 
@@ -348,7 +380,7 @@ class kolab_storage_folder extends kolab_storage_folder_api
      */
     public function get_attachment($uid, $part, $mailbox = null, $print = false, $fp = null, $skip_charset_conv = false)
     {
-        if ($msguid = ($mailbox ? $uid : $this->cache->uid2msguid($uid))) {
+        if ($this->valid && ($msguid = ($mailbox ? $uid : $this->cache->uid2msguid($uid)))) {
             $this->imap->set_folder($mailbox ? $mailbox : $this->name);
 
             if (substr($part, 0, 2) == 'i:') {
@@ -392,6 +424,10 @@ class kolab_storage_folder extends kolab_storage_folder_api
      */
     public function read_object($msguid, $type = null, $folder = null)
     {
+        if (!$this->valid) {
+            return false;
+        }
+
         if (!$type) $type = $this->type;
         if (!$folder) $folder = $this->name;
 
@@ -537,6 +573,10 @@ class kolab_storage_folder extends kolab_storage_folder_api
      */
     public function save(&$object, $type = null, $uid = null)
     {
+        if (!$this->valid) {
+            return false;
+        }
+
         if (!$type)
             $type = $this->type;
 
@@ -732,6 +772,10 @@ class kolab_storage_folder extends kolab_storage_folder_api
      */
     public function delete($object, $expunge = true)
     {
+        if (!$this->valid) {
+            return false;
+        }
+
         $msguid = is_array($object) ? $object['_msguid'] : $this->cache->uid2msguid($object);
         $success = false;
 
@@ -759,6 +803,10 @@ class kolab_storage_folder extends kolab_storage_folder_api
      */
     public function delete_all()
     {
+        if (!$this->valid) {
+            return false;
+        }
+
         $this->cache->purge();
         $this->cache->bypass(true);
         $result = $this->imap->clear_folder($this->name);
@@ -776,6 +824,10 @@ class kolab_storage_folder extends kolab_storage_folder_api
      */
     public function undelete($uid)
     {
+        if (!$this->valid) {
+            return false;
+        }
+
         if ($msguid = $this->cache->uid2msguid($uid, true)) {
             $this->cache->bypass(true);
             $result = $this->imap->set_flag($msguid, 'UNDELETED', $this->name);
@@ -799,6 +851,10 @@ class kolab_storage_folder extends kolab_storage_folder_api
      */
     public function move($uid, $target_folder)
     {
+        if (!$this->valid) {
+            return false;
+        }
+
         if (is_string($target_folder))
             $target_folder = kolab_storage::get_folder($target_folder);
 
@@ -1054,8 +1110,6 @@ class kolab_storage_folder extends kolab_storage_folder_api
      */
     private function trigger_url($url, $auth_user = null, $auth_passwd = null)
     {
-        require_once('HTTP/Request2.php');
-
         try {
             $request = libkolab::http_request($url);