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
@@ -803,21 +803,88 @@
             throw new Syncroton_Exception_Status_MeetingResponse(Syncroton_Exception_Status_MeetingResponse::INVALID_REQUEST);
         }
 
-        if ($event['free_busy']) {
-            $old['free_busy'] = $event['free_busy'];
+        $canceled = ($event['_method'] ?? '') == 'CANCEL' || ($event['status'] ?? '') == 'CANCELLED';
+
+        // A single recurring event occurrence
+        if (!empty($event['recurrence_date'])) {
+            $event['recurrence'] = [];
+
+            // A cancellation (it should not happen)
+            if ($canceled) {
+                // remove the matching RDATE entry
+                if (!empty($old['recurrence']['RDATE'])) {
+                    foreach ($old['recurrence']['RDATE'] as $j => $rdate) {
+                        if ($rdate->format('Ymd') == $event['recurrence_date']->format('Ymd')) {
+                            unset($old['recurrence']['RDATE'][$j]);
+                            break;
+                        }
+                    }
+                }
+
+                // add exception to master event
+                $old['recurrence']['EXDATE'][] = $event['recurrence_date'];
+            }
+            else {
+                if ($status) {
+                    $this->update_attendee_status($event, $status);
+                    $status = null;
+                }
+
+                if (!isset($old['exceptions'])) {
+                    if (isset($old['recurrence']['EXCEPTIONS'])) {
+                        $old['exceptions'] = &$old['recurrence']['EXCEPTIONS'];
+                    }
+                    else {
+                        $old['exceptions'] = [];
+                    }
+                }
+
+                $existing = false;
+                foreach ($old['exceptions'] as $i => $exception) {
+                    if (libcalendaring::is_recurrence_exception($event, $exception)) {
+                        $old['exceptions'][$i] = $event;
+                        $existing = true;
+                    }
+                }
+
+                if (!$existing) {
+                    $old['exceptions'][] = $event;
+                }
+            }
+        }
+        // User accepted an event cancellation (it should not happen)
+        else if ($canceled) {
+            // FIXME: What should happen when user declined a cancellation?
+            $old['status'] = 'CANCELLED';
+            // FIXME: Should we still update the attendee status at all? Maybe force CANCELLED?
+            $status = null;
+            // FIXME: We probably should just delete the event. Some clients do not handle cancelled status
+            //        properly (maybe it's our fault?).
+        }
+        // A main event update
+        else if (isset($event['sequence']) && $event['sequence'] > $old['sequence']) {
+            // FIXME: Can we be smarter here? Should we update everything? What about e.g. new attendees?
+            //        And do we need to check the sequence?
+            $props = ['start', 'end', 'title', 'description', 'location', 'free_busy'];
+
+            foreach ($props as $prop) {
+                if (isset($event[$prop])) {
+                    $old[$prop] = $event[$prop];
+                }
+            }
+
+            // Copy new custom properties
+            if (!empty($event['x-custom'])) {
+                foreach ($event['x-custom'] as $key => $val) {
+                    $old['x-custom'][$key] = $val;
+                }
+            }
         }
 
         // Updating an existing event is most-likely a response
         // 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);
     }
diff --git a/lib/kolab_sync_data_email.php b/lib/kolab_sync_data_email.php
--- a/lib/kolab_sync_data_email.php
+++ b/lib/kolab_sync_data_email.php
@@ -458,6 +458,10 @@
                 $meeting['responseRequested'] = 1;
             } else {
                 $meeting['meetingMessageType'] = Syncroton_Model_EmailMeetingRequest::MESSAGE_TYPE_NORMAL;
+
+                if ($event['_method'] == 'CANCEL') {
+                    $result['messageClass'] = 'IPM.Schedule.Meeting.Canceled';
+                }
             }
 
             // New time proposals aren't supported by Kolab.