diff --git a/lib/kolab_sync_data.php b/lib/kolab_sync_data.php
--- a/lib/kolab_sync_data.php
+++ b/lib/kolab_sync_data.php
@@ -1265,10 +1265,12 @@
         if ($count == 2 && $name_items[0] == 'x-custom') {
             $value = null;
 
-            foreach ((array) $data['x-custom'] as $val) {
-                if (is_array($val) && $val[0] == $name_items[1]) {
-                    $value = $val[1];
-                    break;
+            if (!empty($data['x-custom']) && is_array($data['x-custom'])) {
+                foreach ($data['x-custom'] as $val) {
+                    if (is_array($val) && $val[0] == $name_items[1]) {
+                        $value = $val[1];
+                        break;
+                    }
                 }
             }
 
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
@@ -116,7 +116,8 @@
     const SENSITIVITY_PRIVATE      = 2;
     const SENSITIVITY_CONFIDENTIAL = 3;
 
-    const KEY_DTSTAMP = 'x-custom.X-ACTIVESYNC-DTSTAMP';
+    const KEY_DTSTAMP   = 'x-custom.X-ACTIVESYNC-DTSTAMP';
+    const KEY_REPLYTIME = 'x-custom.X-ACTIVESYNC-REPLYTIME';
 
     /**
      * Mapping of attendee status
@@ -231,7 +232,7 @@
                 break;
 
             case 'free_busy':
-                if (!$is_outlook && !empty($value)) {
+                if (!empty($value)) {
                     $value = $this->busyStatusMap[$value];
                 }
                 break;
@@ -287,17 +288,17 @@
             $user_emails = $this->user_emails();
 
             foreach ($event['attendees'] as $idx => $attendee) {
-                $att = array();
-
-                if ($email = $attendee['email']) {
-                    $att['email'] = $email;
-                }
-                else {
+                if (empty($attendee['email'])) {
                     // In Activesync email is required
                     continue;
                 }
 
-                $att['name'] = $attendee['name'] ?: $email;
+                $email = $attendee['email'];
+
+                $att = [
+                    'email' => $email,
+                    'name' => !empty($attendee['name']) ? $attendee['name'] : $email,
+                ];
 
                 $type   = isset($attendee['role'])   ? $this->attendeeTypeMap[$attendee['role']] : null;
                 $status = isset($attendee['status']) ? $this->attendeeStatusMap[$attendee['status']] : null;
@@ -306,7 +307,7 @@
                     if (isset($attendee['cutype']) && strtolower($attendee['cutype']) == 'resource') {
                         $att['attendeeType'] = self::ATTENDEE_TYPE_RESOURCE;
                     } else {
-                        $att['attendeeType']   = $type ?: self::ATTENDEE_TYPE_REQUIRED;
+                        $att['attendeeType'] = $type ?: self::ATTENDEE_TYPE_REQUIRED;
                     }
                     $att['attendeeStatus'] = $status ?: self::ATTENDEE_STATUS_UNKNOWN;
                 }
@@ -316,7 +317,7 @@
                     $resp_type = $status ?: self::ATTENDEE_STATUS_UNKNOWN;
 
                     // Synchronize the attendee status to the event status to get the same behaviour as outlook.
-                    if ($is_outlook) {
+                    if ($is_outlook && isset($attendee['status'])) {
                         if ($attendee['status'] == 'ACCEPTED') {
                             $result['busyStatus'] = self::BUSY_STATUS_BUSY;
                         }
@@ -324,7 +325,6 @@
                             $result['busyStatus'] = self::BUSY_STATUS_TENTATIVE;
                         }
                     }
-
                 }
 
                 $result['attendees'][] = new Syncroton_Model_EventAttendee($att);
@@ -341,6 +341,17 @@
         $result['responseRequested'] = $result['meetingStatus'] == 3 && $user_rsvp ? 1 : 0;
         $result['responseType']      = $result['meetingStatus'] == 3 ? $resp_type : null;
 
+        // Appointment Reply Time (without it Outlook displays e.g. "Accepted on None")
+        if ($resp_type != self::ATTENDEE_STATUS_UNKNOWN) {
+            if ($reply_time = $this->getKolabDataItem($event, self::KEY_REPLYTIME)) {
+                $result['appointmentReplyTime'] = new DateTime($reply_time, new DateTimeZone('UTC'));
+            } elseif (!empty($event['changed'])) {
+                $reply_time = clone $event['changed'];
+                $reply_time->setTimezone(new DateTimeZone('UTC'));
+                $result['appointmentReplyTime'] = $reply_time;
+            }
+        }
+
         return $as_array ? $result : new Syncroton_Model_Event($result);
     }
 
@@ -654,11 +665,9 @@
                 case 'DECLINED':  $event['free_busy'] = 'free';      break;
             }
 */
-            // Store Outlook response timestamp for further use
-            if (stripos($this->device->devicetype, 'outlook') !== false) {
-                $dtstamp = new DateTime('now', new DateTimeZone('UTC'));
-                $dtstamp = $dtstamp->format(DateTime::ATOM);
-            }
+            // Store response timestamp for further use
+            $reply_time = new DateTime('now', new DateTimeZone('UTC'));
+            $this->setKolabDataItem($event, self::KEY_REPLYTIME, $reply_time->format('Ymd\THis\Z'));
 
             // Update/Save the event
             if (empty($existing)) {
@@ -778,6 +787,13 @@
         // to an iTip request with bumped SEQUENCE
         $old['sequence'] += 1;
 
+        // Copy new custom properties
+        if (!empty($event['x-custom'])) {
+            foreach ($event['x-custom'] as $key => $val) {
+                $old['x-custom'][$key] = $val;
+            }
+        }
+
         // Update the event
         return $this->save_event($old, $status);
     }