diff --git a/apps/dav/lib/CalDAV/Schedule/IMipService.php b/apps/dav/lib/CalDAV/Schedule/IMipService.php
--- a/apps/dav/lib/CalDAV/Schedule/IMipService.php
+++ b/apps/dav/lib/CalDAV/Schedule/IMipService.php
@@ -950,6 +950,52 @@

         } elseif ($method === IMipPlugin::METHOD_REPLY) {

+            // ===== OUTLOOK RSVP FIX (SAFE OVERRIDE) =====
+            $this->logger->debug('RSVP FIX ACTIVE');
+
+            $uid = isset($vevent->UID) ? (string)$vevent->UID : '';
+            if (!empty($uid)) {
+
+                $objects = $this->calDavBackend->getCalendarObjectsByUID(
+                    $calendar['id'],
+                    $uid
+                );
+
+                if (!empty($objects)) {
+
+                    $object = $objects[0];
+                    $existing = \Sabre\VObject\Reader::read($object['calendardata']);
+
+                    foreach ($vevent->ATTENDEE as $incomingAttendee) {
+
+                        $incomingEmail = strtolower(str_replace('mailto:', '', (string)$incomingAttendee));
+                        $incomingEmail = trim($incomingEmail);
+
+                        if (empty($incomingEmail) && isset($incomingAttendee['EMAIL'])) {
+                            $incomingEmail = strtolower((string)$incomingAttendee['EMAIL']);
+                        }
+
+                        $partstat = strtoupper((string)$incomingAttendee['PARTSTAT']);
+                        if (empty($partstat)) {
+                            $partstat = 'ACCEPTED';
+                        }
+
+                        foreach ($existing->VEVENT->ATTENDEE as $existingAttendee) {
+
+                            $existingEmail = strtolower(str_replace('mailto:', '', (string)$existingAttendee));
+                            $existingEmail = trim($existingEmail);
+
+                            if (strcasecmp($incomingEmail, $existingEmail) === 0) {
+
+                                $existingAttendee['PARTSTAT'] = $partstat;
+
+                                $this->calDavBackend->updateCalendarObject(
+                                    $calendar['id'],
+                                    $object['uri'],
+                                    $existing->serialize()
+                                );
+
+                                return;
+                            }
+                        }
+                    }
+                }
+            }