diff --git a/lib/kolab_sync_data_calendar.php b/lib/kolab_sync_data_calendar.php
--- a/lib/kolab_sync_data_calendar.php
+++ b/lib/kolab_sync_data_calendar.php
@@ -116,8 +116,7 @@
     const SENSITIVITY_PRIVATE      = 2;
     const SENSITIVITY_CONFIDENTIAL = 3;
 
-    const KEY_DTSTAMP          = 'x-custom.X-ACTIVESYNC-DTSTAMP';
-    const KEY_RESPONSE_DTSTAMP = 'x-custom.X-ACTIVESYNC-RESPONSE-DTSTAMP';
+    const KEY_DTSTAMP = 'x-custom.X-ACTIVESYNC-DTSTAMP';
 
     /**
      * Mapping of attendee status
@@ -507,10 +506,10 @@
         }
 
         // Attendees
-        // Outlook 2013 sends a dummy update just after MeetingResponse has been processed,
-        // this update resets attendee status set in the MeetingResponse request.
-        // We ignore changes to attendees data on such updates
-        if ($is_outlook && $this->isDummyOutlookUpdate($data, $entry, $event)) {
+        // Whenever Outlook sends dummy timezone it is an event where the user is an attendee.
+        // In these cases Attendees element is bogus: contains invalid status and does not
+        // contain all attendees. We have to ignore it.
+        if ($is_outlook && $data->timezone === $dummy_tz) {
             $this->logger->debug('Dummy outlook update detected, ignoring attendee changes.');
             $attendees = $entry['attendees'];
         }
@@ -593,7 +592,6 @@
         }
 
         $event['attendees']  = $attendees;
-
         $event['categories'] = $categories;
         $event['exceptions'] = isset($event['recurrence']['EXCEPTIONS']) ? $event['recurrence']['EXCEPTIONS'] : array();
 
@@ -664,10 +662,6 @@
 
             // Update/Save the event
             if (empty($existing)) {
-                if ($dtstamp) {
-                    $this->setKolabDataItem($event, self::KEY_RESPONSE_DTSTAMP, $dtstamp);
-                }
-
                 $folder = $this->save_event($event, $status);
 
                 // Create SyncState for the new event, so it is not synced twice
@@ -695,10 +689,6 @@
                 }
             }
             else {
-                if ($dtstamp) {
-                    $this->setKolabDataItem($existing, self::KEY_RESPONSE_DTSTAMP, $dtstamp);
-                }
-
                 $folder = $this->update_event($event, $existing, $status, $request->instanceId);
             }
 
@@ -1127,39 +1117,4 @@
 
         return $rec;
     }
-
-    /**
-     * Check if the event update request is a fake (for Outlook)
-     */
-    protected function isDummyOutlookUpdate($data, $entry, &$result)
-    {
-        $is_dummy = false;
-
-        // Outlook 2013 sends a dummy update just after MeetingResponse has been processed,
-        // this update resets attendee status set in the MeetingResponse request.
-        // We ignore attendees data in such updates, they should not happen according to
-        // https://msdn.microsoft.com/en-us/library/office/hh428685(v=exchg.140).aspx
-        // but they will contain some data as alarms and free/busy status so we don't
-        // ignore them completely
-        if (!empty($entry) && !empty($data->attendees) && stripos($this->device->devicetype, 'outlook') !== false) {
-            // Some of these requests use just dummy Timezone
-            $dummy_tz = str_repeat('A', 230) . '==';
-
-            if ($data->timezone == $dummy_tz) {
-                $is_dummy = true;
-            }
-
-            // But some of them do not, so we have check if that is a first
-            // update immediately (up to 5 seconds) after MeetingResponse request
-            if (!$is_dummy && ($dtstamp = $this->getKolabDataItem($entry, self::KEY_RESPONSE_DTSTAMP))) {
-                $dtstamp  = new DateTime($dtstamp);
-                $now      = new DateTime('now', new DateTimeZone('UTC'));
-                $is_dummy = $now->getTimestamp() - $dtstamp->getTimestamp() <= 5;
-            }
-
-            $this->unsetKolabDataItem($result, self::KEY_RESPONSE_DTSTAMP);
-        }
-
-        return $is_dummy;
-    }
 }