diff --git a/plugins/calendar/calendar.php b/plugins/calendar/calendar.php
--- a/plugins/calendar/calendar.php
+++ b/plugins/calendar/calendar.php
@@ -2505,17 +2505,37 @@
   /****  Event invitation plugin hooks ****/
 
   /**
+   * Find an event in user calendars
+   */
+  protected function find_event($event)
+  {
+    $this->load_driver();
+
+    // We search for writeable calendars in personal namespace by default
+    $result = $this->driver->get_event($event, calendar_driver::FILTER_WRITEABLE | calendar_driver::FILTER_PERSONAL);
+
+    // Some plugins may search in other users calendars, e.g. where delegation is involved
+    $plugin = $this->rc->plugins->exec_hook('calendar_event_find', array(
+            'search'   => $event,
+            'result'   => $result,
+            'calendar' => $this,
+    ));
+
+    return $plugin['result'];
+  }
+
+  /**
    * Handler for calendar/itip-status requests
    */
   function event_itip_status()
   {
     $data = rcube_utils::get_input_value('data', rcube_utils::INPUT_POST, true);
 
-    // find local copy of the referenced event
     $this->load_driver();
-    $existing = $this->driver->get_event($data, calendar_driver::FILTER_WRITEABLE | calendar_driver::FILTER_PERSONAL);
 
-    $itip = $this->load_itip();
+    // find local copy of the referenced event
+    $existing = $this->find_event($data);
+    $itip     = $this->load_itip();
     $response = $itip->get_itip_status($data, $existing);
 
     // get a list of writeable calendars to save new events to
@@ -2914,7 +2934,7 @@
       // save to calendar
       if ($calendar && $calendar['editable']) {
         // check for existing event with the same UID
-        $existing = $this->driver->get_event($event, calendar_driver::FILTER_WRITEABLE | calendar_driver::FILTER_PERSONAL);
+        $existing = $this->find_event($event);
 
         if ($existing) {
           // forward savemode for correct updates of recurring events
diff --git a/plugins/kolab_delegation/kolab_delegation.php b/plugins/kolab_delegation/kolab_delegation.php
--- a/plugins/kolab_delegation/kolab_delegation.php
+++ b/plugins/kolab_delegation/kolab_delegation.php
@@ -54,6 +54,7 @@
         $this->add_hook('calendar_user_emails', array($this, 'calendar_user_emails'));
         $this->add_hook('calendar_list_filter', array($this, 'calendar_list_filter'));
         $this->add_hook('calendar_load_itip', array($this, 'calendar_load_itip'));
+        $this->add_hook('calendar_event_find', array($this, 'calendar_event_find'));
 
         // delegation support in kolab_auth plugin
         $this->add_hook('kolab_auth_emails', array($this, 'kolab_auth_emails'));
@@ -260,6 +261,22 @@
     }
 
     /**
+     * calendar::find_event() handler
+     */
+    public function calendar_event_find($args)
+    {
+        // If the event can't be found in user personal folders, we'll
+        // look in delegators' folders (T1264)
+
+        if (!empty($_SESSION['delegators']) && empty($args['result'])) {
+            $engine = $this->engine();
+            $engine->delegator_find_event($args);
+        }
+
+        return $args;
+    }
+
+    /**
      * Delegation support in Calendar plugin UI
      */
     public function calendar_ui()
diff --git a/plugins/kolab_delegation/kolab_delegation_engine.php b/plugins/kolab_delegation/kolab_delegation_engine.php
--- a/plugins/kolab_delegation/kolab_delegation_engine.php
+++ b/plugins/kolab_delegation/kolab_delegation_engine.php
@@ -864,6 +864,39 @@
     }
 
     /**
+     * Finds an event in delegators' folders. Calendar looks only in
+     * personal namespace, we "extend" this to delegators' folders.
+     *
+     * @param array $args Reference to plugin hook arguments
+     */
+    public function delegator_find_event(&$args)
+    {
+        // The event wasn't found and current user has delegators
+        if (!empty($_SESSION['delegators']) && empty($args['result'])) {
+            $event     = $args['search'];
+            $ns_root   = kolab_storage::namespace_root('other');
+            $storage   = $this->rc->get_storage();
+            $delimiter = $storage->get_hierarchy_delimiter();
+            $folders   = $storage->list_folders_subscribed($ns_root, '*', 'event', 'w');
+
+            // search in all delegators' calendars
+            foreach ($folders as $folder) {
+                list($uid, $path) = explode($delimiter, substr($folder, strlen($ns_root)), 2);
+
+                if (!empty($_SESSION['delegators'][$uid])) {
+                    $event['calendar'] = kolab_storage::folder_id($folder, true);
+                    $result = $args['calendar']->driver->get_event($event, calendar_driver::FILTER_WRITEABLE);
+
+                    if ($result) {
+                        $args['result'] = $result;
+                        return;
+                    }
+                }
+            }
+        }
+    }
+
+    /**
      * Compares two ACLs (according to supported rights)
      *
      * @param array $acl1 ACL rights array (or string)