diff --git a/apps/dav/lib/AppInfo/Application.php b/apps/dav/lib/AppInfo/Application.php
index 69094f6da8470..6b0f22a5022a5 100644
--- a/apps/dav/lib/AppInfo/Application.php
+++ b/apps/dav/lib/AppInfo/Application.php
@@ -19,6 +19,7 @@
 use OCA\DAV\CalDAV\Reminder\NotificationProvider\PushProvider;
 use OCA\DAV\CalDAV\Reminder\NotificationProviderManager;
 use OCA\DAV\CalDAV\Reminder\Notifier as NotifierCalDAV;
+use OCA\DAV\CalDAV\TipBroker;
 use OCA\DAV\Capabilities;
 use OCA\DAV\CardDAV\ContactsManager;
 use OCA\DAV\CardDAV\Notification\Notifier as NotifierCardDAV;
@@ -108,6 +109,7 @@
 use OCP\User\Events\UserIdUnassignedEvent;
 use Psr\Container\ContainerInterface;
 use Psr\Log\LoggerInterface;
+use Sabre\VObject;
 use Throwable;
 use function is_null;
 
@@ -238,6 +240,8 @@ public function register(IRegistrationContext $context): void {
 
 	#[\Override]
 	public function boot(IBootContext $context): void {
+		VObject\Component\VCalendar::$propertyMap[TipBroker::INVITATION_FORWARDING_PROPERTY] = VObject\Property\Boolean::class;
+
 		// Load all dav apps
 		$context->getServerContainer()->get(IAppManager::class)->loadApps(['dav']);
 
diff --git a/apps/dav/lib/CalDAV/TipBroker.php b/apps/dav/lib/CalDAV/TipBroker.php
index 0bc48c2922ae7..fad529b0bc87a 100644
--- a/apps/dav/lib/CalDAV/TipBroker.php
+++ b/apps/dav/lib/CalDAV/TipBroker.php
@@ -11,10 +11,18 @@
 
 use Sabre\VObject\Component;
 use Sabre\VObject\Component\VCalendar;
+use Sabre\VObject\Component\VEvent;
 use Sabre\VObject\ITip\Broker;
 use Sabre\VObject\ITip\Message;
+use Sabre\VObject\Parameter;
+use Sabre\VObject\Property;
+use Sabre\VObject\Property\Boolean;
+use Sabre\VObject\Property\ICalendar\CalAddress;
+use Sabre\VObject\Property\ICalendar\DateTime;
+use Sabre\VObject\Recur\EventIterator;
 
 class TipBroker extends Broker {
+	public const INVITATION_FORWARDING_PROPERTY = 'X-NC-INVITATION-FORWARDING';
 
 	public $significantChangeProperties = [
 		'DTSTART',
@@ -79,6 +87,191 @@ protected function processMessageCancel(Message $itipMessage, ?VCalendar $existi
 		return $existingObject;
 	}
 
+	#[\Override]
+	protected function processMessageReply(Message $itipMessage, ?VCalendar $existingObject = null) {
+		// A reply can only be processed based on an existing object.
+		// If the object is not available, the reply is ignored.
+		if ($existingObject === null) {
+			return null;
+		}
+		$instances = [];
+		$requestStatus = '2.0';
+
+		/** @var list<VEvent> $vevents */
+		$vevents = $itipMessage->message->select('VEVENT');
+
+		// Finding all the instances the attendee replied to.
+		foreach ($vevents as $vevent) {
+			// Use the Unix timestamp returned by getTimestamp as a unique identifier for the recurrence.
+			// The Unix timestamp will be the same for an event, even if the reply from the attendee
+			// used a different format/timezone to express the event date-time.
+			$recurId = $this->getRecurrenceKey($vevent);
+			$attendee = $this->getFirstAttendee($vevent);
+			if ($attendee === null) {
+				continue;
+			}
+			$partstat = $attendee->offsetGet('PARTSTAT');
+			if (!$partstat instanceof Parameter) {
+				continue;
+			}
+			$instances[$recurId] = $partstat->getValue();
+			if (isset($vevent->{'REQUEST-STATUS'})) {
+				$requestStatus = $vevent->{'REQUEST-STATUS'}->getValue();
+				[$requestStatus] = explode(';', $requestStatus);
+			}
+		}
+
+		// Now we need to loop through the original organizer event, to find
+		// all the instances where we have a reply for.
+		$masterObject = $this->getMasterEvent($existingObject);
+		$masterAllowInvitationForwarding = $masterObject === null || $this->allowInvitationForwarding($masterObject);
+
+		/** @var list<VEvent> $vevents */
+		$vevents = $existingObject->select('VEVENT');
+
+		foreach ($vevents as $vevent) {
+			// Use the Unix timestamp returned by getTimestamp as a unique identifier for the recurrence.
+			$recurId = $this->getRecurrenceKey($vevent);
+			if (isset($instances[$recurId])) {
+				$allowInvitationForwarding = $this->allowInvitationForwarding($vevent);
+				$attendeeFound = false;
+				if (isset($vevent->ATTENDEE)) {
+					foreach ($vevent->ATTENDEE as $attendee) {
+						if ($attendee->getValue() === $itipMessage->sender) {
+							$attendeeFound = true;
+							$attendee['PARTSTAT'] = $instances[$recurId];
+							$attendee['SCHEDULE-STATUS'] = $requestStatus;
+							// Un-setting the RSVP status, because we now know
+							// that the attendee already replied.
+							unset($attendee['RSVP']);
+							break;
+						}
+					}
+				}
+				if (!$attendeeFound && $allowInvitationForwarding) {
+					// Adding a new attendee. The iTip documentation calls this
+					// a party crasher.
+					$parameters = [
+						'PARTSTAT' => $instances[$recurId],
+					];
+					if ($itipMessage->senderName) {
+						$parameters['CN'] = $itipMessage->senderName;
+					}
+					$vevent->add('ATTENDEE', $itipMessage->sender, $parameters);
+				}
+				unset($instances[$recurId]);
+			}
+		}
+
+		if ($masterObject === null) {
+			// No master object, we can't add new instances.
+			return null;
+		}
+		// If we got replies to instances that did not exist in the
+		// original list, it means that new exceptions must be created.
+		foreach ($instances as $recurId => $partstat) {
+			$recurrenceIterator = new EventIterator($existingObject, $itipMessage->uid);
+			$found = false;
+			$iterations = 1000;
+			do {
+				$newObject = $recurrenceIterator->getEventObject();
+				$recurrenceIterator->next();
+
+				// Compare the Unix timestamp returned by getTimestamp with the previously calculated timestamp.
+				// If they are the same, then this is a matching recurrence, even though its date-time may have
+				// been expressed in a different format/timezone.
+				if (isset($newObject->{'RECURRENCE-ID'}) && $newObject->{'RECURRENCE-ID'}->getDateTime()->getTimestamp() === $recurId) {
+					$found = true;
+				}
+				--$iterations;
+			} while ($recurrenceIterator->valid() && !$found && $iterations);
+
+			// Invalid recurrence id. Skipping this object.
+			if (!$found) {
+				continue;
+			}
+
+			$newObject->remove('RRULE');
+			$newObject->remove('EXDATE');
+			$newObject->remove('RDATE');
+
+			$attendeeFound = false;
+			if (isset($newObject->ATTENDEE)) {
+				foreach ($newObject->ATTENDEE as $attendee) {
+					if ($attendee->getValue() === $itipMessage->sender) {
+						$attendeeFound = true;
+						$attendee['PARTSTAT'] = $partstat;
+						$attendee['SCHEDULE-STATUS'] = $requestStatus;
+						unset($attendee['RSVP']);
+						break;
+					}
+				}
+			}
+			if (!$attendeeFound && !$masterAllowInvitationForwarding) {
+				continue;
+			}
+			if (!$attendeeFound) {
+				// Adding a new attendee
+				$parameters = [
+					'PARTSTAT' => $partstat,
+				];
+				if ($itipMessage->senderName) {
+					$parameters['CN'] = $itipMessage->senderName;
+				}
+				$newObject->add('ATTENDEE', $itipMessage->sender, $parameters);
+			}
+			$existingObject->add($newObject);
+		}
+
+		return $existingObject;
+	}
+
+	protected function getMasterEvent(VCalendar $calendar): ?VEvent {
+		/** @var list<VEvent> $vevents */
+		$vevents = $calendar->select('VEVENT');
+		foreach ($vevents as $vevent) {
+			if (!isset($vevent->{'RECURRENCE-ID'})) {
+				return $vevent;
+			}
+		}
+		return null;
+	}
+
+	/**
+	 * @return int|'master'
+	 */
+	protected function getRecurrenceKey(VEvent $vevent): int|string {
+		/** @var list<Property> $recurrenceIds */
+		$recurrenceIds = $vevent->select('RECURRENCE-ID');
+		foreach ($recurrenceIds as $recurrenceId) {
+			if ($recurrenceId instanceof DateTime) {
+				return $recurrenceId->getDateTime()->getTimestamp();
+			}
+		}
+		return 'master';
+	}
+
+	protected function getFirstAttendee(VEvent $vevent): ?CalAddress {
+		/** @var list<Property> $attendees */
+		$attendees = $vevent->select('ATTENDEE');
+		foreach ($attendees as $attendee) {
+			if ($attendee instanceof CalAddress) {
+				return $attendee;
+			}
+		}
+		return null;
+	}
+
+	protected function allowInvitationForwarding(VEvent $vevent): bool {
+		$properties = $vevent->select(self::INVITATION_FORWARDING_PROPERTY);
+		foreach ($properties as $property) {
+			if ($property instanceof Boolean) {
+				return $property->getValue() === 'TRUE';
+			}
+		}
+		return true;
+	}
+
 	/**
 	 * This method is used in cases where an event got updated, and we
 	 * potentially need to send emails to attendees to let them know of updates
diff --git a/apps/dav/tests/unit/CalDAV/TipBrokerTest.php b/apps/dav/tests/unit/CalDAV/TipBrokerTest.php
index 8843895012598..9d4c27c9bfbc0 100644
--- a/apps/dav/tests/unit/CalDAV/TipBrokerTest.php
+++ b/apps/dav/tests/unit/CalDAV/TipBrokerTest.php
@@ -8,7 +8,9 @@
 namespace OCA\DAV\Tests\unit\CalDAV;
 
 use OCA\DAV\CalDAV\TipBroker;
+use Sabre\VObject;
 use Sabre\VObject\Component\VCalendar;
+use Sabre\VObject\ITip\Message;
 use Test\TestCase;
 
 class TipBrokerTest extends TestCase {
@@ -21,6 +23,8 @@ class TipBrokerTest extends TestCase {
 	protected function setUp(): void {
 		parent::setUp();
 
+		VCalendar::$propertyMap[TipBroker::INVITATION_FORWARDING_PROPERTY] = VObject\Property\Boolean::class;
+
 		$this->broker = new TipBroker();
 
 		$this->templateEventInfo = [
@@ -41,8 +45,8 @@ protected function setUp(): void {
 		$vEvent->add('DTSTART', '20240701T080000', ['TZID' => 'America/Toronto']);
 		$vEvent->add('DTEND', '20240701T090000', ['TZID' => 'America/Toronto']);
 		$vEvent->add('SUMMARY', 'Test Event');
-		$vEvent->add('ORGANIZER', 'mailto:organizer@testing.com', ['CN' => 'Organizer']);
-		$vEvent->add('ATTENDEE', 'mailto:attendee1@testing.com', [
+		$vEvent->add('ORGANIZER', 'mailto:organizer@example.org', ['CN' => 'Organizer']);
+		$vEvent->add('ATTENDEE', 'mailto:attendee1@example.org', [
 			'CN' => 'Attendee One',
 			'CUTYPE' => 'INDIVIDUAL',
 			'PARTSTAT' => 'NEEDS-ACTION',
@@ -65,8 +69,8 @@ protected function setUp(): void {
 		$vEvent->add('DTEND', '20240701T090000', ['TZID' => 'America/Toronto']);
 		$vEvent->add('RRULE', 'FREQ=WEEKLY;COUNT=12;BYDAY=MO');
 		$vEvent->add('SUMMARY', 'Test Event');
-		$vEvent->add('ORGANIZER', 'mailto:organizer@testing.com', ['CN' => 'Organizer']);
-		$vEvent->add('ATTENDEE', 'mailto:attendee1@testing.com', [
+		$vEvent->add('ORGANIZER', 'mailto:organizer@example.org', ['CN' => 'Organizer']);
+		$vEvent->add('ATTENDEE', 'mailto:attendee1@example.org', [
 			'CN' => 'Attendee One',
 			'CUTYPE' => 'INDIVIDUAL',
 			'PARTSTAT' => 'NEEDS-ACTION',
@@ -75,6 +79,12 @@ protected function setUp(): void {
 		]);
 	}
 
+	protected function tearDown(): void {
+		unset(VCalendar::$propertyMap[TipBroker::INVITATION_FORWARDING_PROPERTY]);
+
+		parent::tearDown();
+	}
+
 	/**
 	 * Tests user creating a new singleton or recurring event
 	 */
@@ -159,7 +169,7 @@ public function testParseEventForOrganizerAddAttendee(): void {
 		$mutatedCalendar = clone $this->vCalendar1a;
 		$mutatedCalendar->VEVENT->{'LAST-MODIFIED'}->setValue('20240701T020000Z');
 		$mutatedCalendar->VEVENT->SEQUENCE->setValue(2);
-		$mutatedCalendar->VEVENT->add('ATTENDEE', 'mailto:attendee2@testing.com', [
+		$mutatedCalendar->VEVENT->add('ATTENDEE', 'mailto:attendee2@example.org', [
 			'CN' => 'Attendee Two',
 			'CUTYPE' => 'INDIVIDUAL',
 			'PARTSTAT' => 'NEEDS-ACTION',
@@ -185,7 +195,7 @@ public function testParseEventForOrganizerAddAttendee(): void {
 	public function testParseEventForOrganizerRemoveAttendee(): void {
 		// construct calendar and generate event info for modified event with two attendees
 		$originalCalendar = clone $this->vCalendar1a;
-		$originalCalendar->VEVENT->add('ATTENDEE', 'mailto:attendee2@testing.com', [
+		$originalCalendar->VEVENT->add('ATTENDEE', 'mailto:attendee2@example.org', [
 			'CN' => 'Attendee Two',
 			'CUTYPE' => 'INDIVIDUAL',
 			'PARTSTAT' => 'NEEDS-ACTION',
@@ -197,7 +207,7 @@ public function testParseEventForOrganizerRemoveAttendee(): void {
 		$mutatedCalendar->VEVENT->{'LAST-MODIFIED'}->setValue('20240701T020000Z');
 		$mutatedCalendar->VEVENT->SEQUENCE->setValue(2);
 		$mutatedCalendar->VEVENT->remove('ATTENDEE');
-		$mutatedCalendar->VEVENT->add('ATTENDEE', 'mailto:attendee1@testing.com', [
+		$mutatedCalendar->VEVENT->add('ATTENDEE', 'mailto:attendee1@example.org', [
 			'CN' => 'Attendee One',
 			'CUTYPE' => 'INDIVIDUAL',
 			'PARTSTAT' => 'NEEDS-ACTION',
@@ -214,7 +224,7 @@ public function testParseEventForOrganizerRemoveAttendee(): void {
 		$this->assertEquals($mutatedCalendar->VEVENT->ATTENDEE[0]->getValue(), $messages[0]->recipient);
 		$this->assertEquals('CANCEL', $messages[1]->method);
 		$this->assertEquals($mutatedCalendar->VEVENT->ORGANIZER->getValue(), $messages[1]->sender);
-		$this->assertEquals('mailto:attendee2@testing.com', $messages[1]->recipient);
+		$this->assertEquals('mailto:attendee2@example.org', $messages[1]->recipient);
 	}
 
 
@@ -377,7 +387,7 @@ public function testParseEventForOrganizerModifyInstanceAddAttendee(): void {
 		$originalEventInfo = $this->invokePrivate($this->broker, 'parseEventInfo', [$originalCalendar]);
 		$mutatedInstance = clone $originalInstance;
 		$mutatedInstance->SEQUENCE->setValue(2);
-		$mutatedInstance->add('ATTENDEE', 'mailto:attendee2@testing.com', [
+		$mutatedInstance->add('ATTENDEE', 'mailto:attendee2@example.org', [
 			'CN' => 'Attendee Two',
 			'CUTYPE' => 'INDIVIDUAL',
 			'PARTSTAT' => 'NEEDS-ACTION',
@@ -417,7 +427,7 @@ public function testParseEventForOrganizerModifyInstanceRemoveAttendee(): void {
 		$originalInstance->SEQUENCE->setValue(1);
 		$originalInstance->DTSTART->setValue('20240717T080000');
 		$originalInstance->DTEND->setValue('20240717T090000');
-		$originalInstance->add('ATTENDEE', 'mailto:attendee2@testing.com', [
+		$originalInstance->add('ATTENDEE', 'mailto:attendee2@example.org', [
 			'CN' => 'Attendee Two',
 			'CUTYPE' => 'INDIVIDUAL',
 			'PARTSTAT' => 'NEEDS-ACTION',
@@ -429,7 +439,7 @@ public function testParseEventForOrganizerModifyInstanceRemoveAttendee(): void {
 		$mutatedInstance = clone $originalInstance;
 		$mutatedInstance->SEQUENCE->setValue(2);
 		$mutatedInstance->remove('ATTENDEE');
-		$mutatedInstance->add('ATTENDEE', 'mailto:attendee1@testing.com', [
+		$mutatedInstance->add('ATTENDEE', 'mailto:attendee1@example.org', [
 			'CN' => 'Attendee One',
 			'CUTYPE' => 'INDIVIDUAL',
 			'PARTSTAT' => 'NEEDS-ACTION',
@@ -579,4 +589,364 @@ public function testParseEventForOrganizerScheduleForceSend(): void {
 		$this->assertFalse(isset($messages[0]->message->VEVENT->ATTENDEE['SCHEDULE-FORCE-SEND']));
 	}
 
+	public function testProcessMessageReplyDisallowsInvitationForwarding(): void {
+		$existingCalendar = clone $this->vCalendar1a;
+		$existingCalendar->VEVENT->add(TipBroker::INVITATION_FORWARDING_PROPERTY, 'FALSE');
+		$existingCalendar->VEVENT->ATTENDEE[0]->setValue('mailto:attendee1@example.org');
+		$reply = new Message();
+		$reply->uid = $existingCalendar->VEVENT->UID->getValue();
+		$reply->component = 'VEVENT';
+		$reply->sender = 'mailto:attendee2@example.org';
+		$reply->senderName = 'Attendee Two';
+		$reply->sequence = 1;
+		$reply->message = new VCalendar();
+		/** @var \Sabre\VObject\Component\VEvent $replyEvent */
+		$replyEvent = $reply->message->add('VEVENT', []);
+		$replyEvent->add('UID', $reply->uid);
+		$replyEvent->add('ATTENDEE', $reply->sender, [
+			'PARTSTAT' => 'ACCEPTED',
+		]);
+
+		$result = $this->invokePrivate($this->broker, 'processMessageReply', [$reply, $existingCalendar]);
+
+		$this->assertSame($existingCalendar, $result);
+		$this->assertCount(1, $result->VEVENT->ATTENDEE);
+		$this->assertEquals('mailto:attendee1@example.org', $result->VEVENT->ATTENDEE[0]->getValue());
+	}
+
+	public function testProcessMessageReplyUpdatesExistingAttendeeWhenInvitationForwardingDisabled(): void {
+		$existingCalendar = clone $this->vCalendar1a;
+		$existingCalendar->VEVENT->add(TipBroker::INVITATION_FORWARDING_PROPERTY, 'FALSE');
+		$existingCalendar->VEVENT->ATTENDEE[0]->setValue('mailto:attendee1@example.org');
+		$reply = new Message();
+		$reply->uid = $existingCalendar->VEVENT->UID->getValue();
+		$reply->component = 'VEVENT';
+		$reply->sender = 'mailto:attendee1@example.org';
+		$reply->senderName = 'Attendee One';
+		$reply->sequence = 1;
+		$reply->message = new VCalendar();
+		/** @var \Sabre\VObject\Component\VEvent $replyEvent */
+		$replyEvent = $reply->message->add('VEVENT', []);
+		$replyEvent->add('UID', $reply->uid);
+		$replyEvent->add('ATTENDEE', $reply->sender, [
+			'PARTSTAT' => 'ACCEPTED',
+		]);
+		$replyEvent->add('REQUEST-STATUS', '2.0;Success');
+
+		$result = $this->invokePrivate($this->broker, 'processMessageReply', [$reply, $existingCalendar]);
+
+		$this->assertSame($existingCalendar, $result);
+		$this->assertCount(1, $result->VEVENT->ATTENDEE);
+		$this->assertEquals('mailto:attendee1@example.org', $result->VEVENT->ATTENDEE[0]->getValue());
+		$this->assertEquals('ACCEPTED', $result->VEVENT->ATTENDEE[0]['PARTSTAT']->getValue());
+		$this->assertEquals('2.0', $result->VEVENT->ATTENDEE[0]['SCHEDULE-STATUS']->getValue());
+		$this->assertFalse(isset($result->VEVENT->ATTENDEE[0]['RSVP']));
+	}
+
+	public function testProcessMessageReplyAllowsInvitationForwarding(): void {
+		$existingCalendar = clone $this->vCalendar1a;
+		$existingCalendar->VEVENT->add(TipBroker::INVITATION_FORWARDING_PROPERTY, 'TRUE');
+		$existingCalendar->VEVENT->ATTENDEE[0]->setValue('mailto:attendee1@example.org');
+		$reply = new Message();
+		$reply->uid = $existingCalendar->VEVENT->UID->getValue();
+		$reply->component = 'VEVENT';
+		$reply->sender = 'mailto:attendee2@example.org';
+		$reply->senderName = 'Attendee Two';
+		$reply->sequence = 1;
+		$reply->message = new VCalendar();
+		/** @var \Sabre\VObject\Component\VEvent $replyEvent */
+		$replyEvent = $reply->message->add('VEVENT', []);
+		$replyEvent->add('UID', $reply->uid);
+		$replyEvent->add('ATTENDEE', $reply->sender, [
+			'PARTSTAT' => 'ACCEPTED',
+		]);
+
+		$result = $this->invokePrivate($this->broker, 'processMessageReply', [$reply, $existingCalendar]);
+
+		$this->assertSame($existingCalendar, $result);
+		$this->assertCount(2, $result->VEVENT->ATTENDEE);
+		$this->assertEquals('mailto:attendee2@example.org', $result->VEVENT->ATTENDEE[1]->getValue());
+		$this->assertEquals('ACCEPTED', $result->VEVENT->ATTENDEE[1]['PARTSTAT']->getValue());
+		$this->assertEquals('Attendee Two', $result->VEVENT->ATTENDEE[1]['CN']->getValue());
+	}
+
+	public function testProcessMessageReplyAllowsInvitationForwardingByDefault(): void {
+		$existingCalendar = clone $this->vCalendar1a;
+		$existingCalendar->VEVENT->ATTENDEE[0]->setValue('mailto:attendee1@example.org');
+		$reply = new Message();
+		$reply->uid = $existingCalendar->VEVENT->UID->getValue();
+		$reply->component = 'VEVENT';
+		$reply->sender = 'mailto:attendee2@example.org';
+		$reply->senderName = 'Attendee Two';
+		$reply->sequence = 1;
+		$reply->message = new VCalendar();
+		/** @var \Sabre\VObject\Component\VEvent $replyEvent */
+		$replyEvent = $reply->message->add('VEVENT', []);
+		$replyEvent->add('UID', $reply->uid);
+		$replyEvent->add('ATTENDEE', $reply->sender, [
+			'PARTSTAT' => 'ACCEPTED',
+		]);
+
+		$result = $this->invokePrivate($this->broker, 'processMessageReply', [$reply, $existingCalendar]);
+
+		$this->assertSame($existingCalendar, $result);
+		$this->assertCount(2, $result->VEVENT->ATTENDEE);
+		$this->assertEquals('mailto:attendee2@example.org', $result->VEVENT->ATTENDEE[1]->getValue());
+		$this->assertEquals('ACCEPTED', $result->VEVENT->ATTENDEE[1]['PARTSTAT']->getValue());
+		$this->assertEquals('Attendee Two', $result->VEVENT->ATTENDEE[1]['CN']->getValue());
+	}
+
+	public function testProcessMessageReplyIgnoresReplyWithoutAttendee(): void {
+		$existingCalendar = clone $this->vCalendar1a;
+		$reply = new Message();
+		$reply->uid = $existingCalendar->VEVENT->UID->getValue();
+		$reply->component = 'VEVENT';
+		$reply->sender = 'mailto:attendee1@example.org';
+		$reply->senderName = 'Attendee One';
+		$reply->sequence = 1;
+		$reply->message = new VCalendar();
+		/** @var \Sabre\VObject\Component\VEvent $replyEvent */
+		$replyEvent = $reply->message->add('VEVENT', []);
+		$replyEvent->add('UID', $reply->uid);
+
+		$result = $this->invokePrivate($this->broker, 'processMessageReply', [$reply, $existingCalendar]);
+
+		$this->assertSame($existingCalendar, $result);
+		$this->assertCount(1, $result->VEVENT->ATTENDEE);
+		$this->assertEquals('NEEDS-ACTION', $result->VEVENT->ATTENDEE[0]['PARTSTAT']->getValue());
+		$this->assertTrue(isset($result->VEVENT->ATTENDEE[0]['RSVP']));
+	}
+
+	public function testProcessMessageReplyIgnoresReplyWithoutPartstat(): void {
+		$existingCalendar = clone $this->vCalendar1a;
+		$reply = new Message();
+		$reply->uid = $existingCalendar->VEVENT->UID->getValue();
+		$reply->component = 'VEVENT';
+		$reply->sender = 'mailto:attendee1@example.org';
+		$reply->senderName = 'Attendee One';
+		$reply->sequence = 1;
+		$reply->message = new VCalendar();
+		/** @var \Sabre\VObject\Component\VEvent $replyEvent */
+		$replyEvent = $reply->message->add('VEVENT', []);
+		$replyEvent->add('UID', $reply->uid);
+		$replyEvent->add('ATTENDEE', $reply->sender, []);
+
+		$result = $this->invokePrivate($this->broker, 'processMessageReply', [$reply, $existingCalendar]);
+
+		$this->assertSame($existingCalendar, $result);
+		$this->assertCount(1, $result->VEVENT->ATTENDEE);
+		$this->assertEquals('NEEDS-ACTION', $result->VEVENT->ATTENDEE[0]['PARTSTAT']->getValue());
+		$this->assertTrue(isset($result->VEVENT->ATTENDEE[0]['RSVP']));
+	}
+
+	public function testProcessMessageReplyDisallowsInvitationForwardingForGeneratedRecurringInstance(): void {
+		$existingCalendar = clone $this->vCalendar2a;
+		$existingCalendar->VEVENT->add(TipBroker::INVITATION_FORWARDING_PROPERTY, 'FALSE');
+		$existingCalendar->VEVENT->ATTENDEE[0]->setValue('mailto:attendee1@example.org');
+		$reply = new Message();
+		$reply->uid = $existingCalendar->VEVENT->UID->getValue();
+		$reply->component = 'VEVENT';
+		$reply->sender = 'mailto:attendee2@example.org';
+		$reply->senderName = 'Attendee Two';
+		$reply->sequence = 1;
+		$reply->message = new VCalendar();
+		/** @var \Sabre\VObject\Component\VEvent $replyEvent */
+		$replyEvent = $reply->message->add('VEVENT', []);
+		$replyEvent->add('UID', $reply->uid);
+		$replyEvent->add('RECURRENCE-ID', '20240715T080000', ['TZID' => 'America/Toronto']);
+		$replyEvent->add('ATTENDEE', $reply->sender, [
+			'PARTSTAT' => 'ACCEPTED',
+		]);
+
+		$result = $this->invokePrivate($this->broker, 'processMessageReply', [$reply, $existingCalendar]);
+
+		$this->assertSame($existingCalendar, $result);
+		$this->assertCount(1, $result->VEVENT);
+		$this->assertCount(1, $result->VEVENT->ATTENDEE);
+		$this->assertEquals('mailto:attendee1@example.org', $result->VEVENT->ATTENDEE[0]->getValue());
+	}
+
+	public function testProcessMessageReplyUpdatesExistingAttendeeForGeneratedRecurringInstanceWhenInvitationForwardingDisabled(): void {
+		$existingCalendar = clone $this->vCalendar2a;
+		$existingCalendar->VEVENT->add(TipBroker::INVITATION_FORWARDING_PROPERTY, 'FALSE');
+		$existingCalendar->VEVENT->ATTENDEE[0]->setValue('mailto:attendee1@example.org');
+		$reply = new Message();
+		$reply->uid = $existingCalendar->VEVENT->UID->getValue();
+		$reply->component = 'VEVENT';
+		$reply->sender = 'mailto:attendee1@example.org';
+		$reply->senderName = 'Attendee One';
+		$reply->sequence = 1;
+		$reply->message = new VCalendar();
+		/** @var \Sabre\VObject\Component\VEvent $replyEvent */
+		$replyEvent = $reply->message->add('VEVENT', []);
+		$replyEvent->add('UID', $reply->uid);
+		$replyEvent->add('RECURRENCE-ID', '20240715T080000', ['TZID' => 'America/Toronto']);
+		$replyEvent->add('ATTENDEE', $reply->sender, [
+			'PARTSTAT' => 'ACCEPTED',
+		]);
+		$replyEvent->add('REQUEST-STATUS', '2.0;Success');
+
+		$result = $this->invokePrivate($this->broker, 'processMessageReply', [$reply, $existingCalendar]);
+
+		$this->assertSame($existingCalendar, $result);
+		$this->assertCount(2, $result->VEVENT);
+		$this->assertEquals('20240715T080000', $result->VEVENT[1]->{'RECURRENCE-ID'}->getValue());
+		$this->assertCount(1, $result->VEVENT[1]->ATTENDEE);
+		$this->assertEquals('mailto:attendee1@example.org', $result->VEVENT[1]->ATTENDEE[0]->getValue());
+		$this->assertEquals('ACCEPTED', $result->VEVENT[1]->ATTENDEE[0]['PARTSTAT']->getValue());
+		$this->assertEquals('2.0', $result->VEVENT[1]->ATTENDEE[0]['SCHEDULE-STATUS']->getValue());
+		$this->assertFalse(isset($result->VEVENT[1]->ATTENDEE[0]['RSVP']));
+	}
+
+	public function testProcessMessageReplyAllowsInvitationForwardingForDetachedRecurringExceptionWhenMasterDisallows(): void {
+		$existingCalendar = clone $this->vCalendar2a;
+		$existingCalendar->VEVENT->add(TipBroker::INVITATION_FORWARDING_PROPERTY, 'FALSE');
+		$existingCalendar->VEVENT->ATTENDEE[0]->setValue('mailto:attendee1@example.org');
+		/** @var \Sabre\VObject\Component\VEvent $detachedInstance */
+		$detachedInstance = $existingCalendar->add('VEVENT', []);
+		$detachedInstance->add('UID', $existingCalendar->VEVENT->UID->getValue());
+		$detachedInstance->add('DTSTAMP', '20240701T000000Z');
+		$detachedInstance->add('RECURRENCE-ID', '20240715T080000', ['TZID' => 'America/Toronto']);
+		$detachedInstance->add('DTSTART', '20240715T080000', ['TZID' => 'America/Toronto']);
+		$detachedInstance->add('DTEND', '20240715T090000', ['TZID' => 'America/Toronto']);
+		$detachedInstance->add('SUMMARY', 'Detached Test Event');
+		$detachedInstance->add('ORGANIZER', 'mailto:organizer@example.org', ['CN' => 'Organizer']);
+		$detachedInstance->add(TipBroker::INVITATION_FORWARDING_PROPERTY, 'TRUE');
+		$detachedInstance->add('ATTENDEE', 'mailto:attendee1@example.org', [
+			'CN' => 'Attendee One',
+			'CUTYPE' => 'INDIVIDUAL',
+			'PARTSTAT' => 'NEEDS-ACTION',
+			'ROLE' => 'REQ-PARTICIPANT',
+			'RSVP' => 'TRUE',
+		]);
+
+		$reply = new Message();
+		$reply->uid = $existingCalendar->VEVENT->UID->getValue();
+		$reply->component = 'VEVENT';
+		$reply->sender = 'mailto:attendee2@example.org';
+		$reply->senderName = 'Attendee Two';
+		$reply->sequence = 1;
+		$reply->message = new VCalendar();
+		/** @var \Sabre\VObject\Component\VEvent $replyEvent */
+		$replyEvent = $reply->message->add('VEVENT', []);
+		$replyEvent->add('UID', $reply->uid);
+		$replyEvent->add('RECURRENCE-ID', '20240715T080000', ['TZID' => 'America/Toronto']);
+		$replyEvent->add('ATTENDEE', $reply->sender, [
+			'PARTSTAT' => 'ACCEPTED',
+		]);
+
+		$result = $this->invokePrivate($this->broker, 'processMessageReply', [$reply, $existingCalendar]);
+
+		$this->assertSame($existingCalendar, $result);
+		$this->assertCount(2, $result->VEVENT);
+		$this->assertCount(2, $result->VEVENT[1]->ATTENDEE);
+		$this->assertEquals('mailto:attendee2@example.org', $result->VEVENT[1]->ATTENDEE[1]->getValue());
+	}
+
+	public function testProcessMessageReplyDisallowsInvitationForwardingForDetachedRecurringExceptionWhenMasterAllows(): void {
+		$existingCalendar = clone $this->vCalendar2a;
+		$existingCalendar->VEVENT->add(TipBroker::INVITATION_FORWARDING_PROPERTY, 'TRUE');
+		$existingCalendar->VEVENT->ATTENDEE[0]->setValue('mailto:attendee1@example.org');
+		/** @var \Sabre\VObject\Component\VEvent $detachedInstance */
+		$detachedInstance = $existingCalendar->add('VEVENT', []);
+		$detachedInstance->add('UID', $existingCalendar->VEVENT->UID->getValue());
+		$detachedInstance->add('DTSTAMP', '20240701T000000Z');
+		$detachedInstance->add('RECURRENCE-ID', '20240715T080000', ['TZID' => 'America/Toronto']);
+		$detachedInstance->add('DTSTART', '20240715T080000', ['TZID' => 'America/Toronto']);
+		$detachedInstance->add('DTEND', '20240715T090000', ['TZID' => 'America/Toronto']);
+		$detachedInstance->add('SUMMARY', 'Detached Test Event');
+		$detachedInstance->add('ORGANIZER', 'mailto:organizer@example.org', ['CN' => 'Organizer']);
+		$detachedInstance->add(TipBroker::INVITATION_FORWARDING_PROPERTY, 'FALSE');
+		$detachedInstance->add('ATTENDEE', 'mailto:attendee1@example.org', [
+			'CN' => 'Attendee One',
+			'CUTYPE' => 'INDIVIDUAL',
+			'PARTSTAT' => 'NEEDS-ACTION',
+			'ROLE' => 'REQ-PARTICIPANT',
+			'RSVP' => 'TRUE',
+		]);
+
+		$reply = new Message();
+		$reply->uid = $existingCalendar->VEVENT->UID->getValue();
+		$reply->component = 'VEVENT';
+		$reply->sender = 'mailto:attendee2@example.org';
+		$reply->senderName = 'Attendee Two';
+		$reply->sequence = 1;
+		$reply->message = new VCalendar();
+		/** @var \Sabre\VObject\Component\VEvent $replyEvent */
+		$replyEvent = $reply->message->add('VEVENT', []);
+		$replyEvent->add('UID', $reply->uid);
+		$replyEvent->add('RECURRENCE-ID', '20240715T080000', ['TZID' => 'America/Toronto']);
+		$replyEvent->add('ATTENDEE', $reply->sender, [
+			'PARTSTAT' => 'ACCEPTED',
+		]);
+
+		$result = $this->invokePrivate($this->broker, 'processMessageReply', [$reply, $existingCalendar]);
+
+		$this->assertSame($existingCalendar, $result);
+		$this->assertCount(2, $result->VEVENT);
+		$this->assertCount(1, $result->VEVENT[1]->ATTENDEE);
+		$this->assertEquals('mailto:attendee1@example.org', $result->VEVENT[1]->ATTENDEE[0]->getValue());
+	}
+
+	public function testProcessMessageReplyAllowsInvitationForwardingForGeneratedRecurringInstance(): void {
+		$existingCalendar = clone $this->vCalendar2a;
+		$existingCalendar->VEVENT->add(TipBroker::INVITATION_FORWARDING_PROPERTY, 'TRUE');
+		$existingCalendar->VEVENT->ATTENDEE[0]->setValue('mailto:attendee1@example.org');
+		$reply = new Message();
+		$reply->uid = $existingCalendar->VEVENT->UID->getValue();
+		$reply->component = 'VEVENT';
+		$reply->sender = 'mailto:attendee2@example.org';
+		$reply->senderName = 'Attendee Two';
+		$reply->sequence = 1;
+		$reply->message = new VCalendar();
+		/** @var \Sabre\VObject\Component\VEvent $replyEvent */
+		$replyEvent = $reply->message->add('VEVENT', []);
+		$replyEvent->add('UID', $reply->uid);
+		$replyEvent->add('RECURRENCE-ID', '20240715T080000', ['TZID' => 'America/Toronto']);
+		$replyEvent->add('ATTENDEE', $reply->sender, [
+			'PARTSTAT' => 'ACCEPTED',
+		]);
+
+		$result = $this->invokePrivate($this->broker, 'processMessageReply', [$reply, $existingCalendar]);
+
+		$this->assertSame($existingCalendar, $result);
+		$this->assertCount(2, $result->VEVENT);
+		$this->assertEquals('20240715T080000', $result->VEVENT[1]->{'RECURRENCE-ID'}->getValue());
+		$this->assertFalse(isset($result->VEVENT[1]->RRULE));
+		$this->assertCount(2, $result->VEVENT[1]->ATTENDEE);
+		$this->assertEquals('mailto:attendee2@example.org', $result->VEVENT[1]->ATTENDEE[1]->getValue());
+		$this->assertEquals('ACCEPTED', $result->VEVENT[1]->ATTENDEE[1]['PARTSTAT']->getValue());
+		$this->assertEquals('Attendee Two', $result->VEVENT[1]->ATTENDEE[1]['CN']->getValue());
+	}
+
+	public function testProcessMessageReplyAllowsInvitationForwardingByDefaultForGeneratedRecurringInstance(): void {
+		$existingCalendar = clone $this->vCalendar2a;
+		$existingCalendar->VEVENT->ATTENDEE[0]->setValue('mailto:attendee1@example.org');
+		$reply = new Message();
+		$reply->uid = $existingCalendar->VEVENT->UID->getValue();
+		$reply->component = 'VEVENT';
+		$reply->sender = 'mailto:attendee2@example.org';
+		$reply->senderName = 'Attendee Two';
+		$reply->sequence = 1;
+		$reply->message = new VCalendar();
+		/** @var \Sabre\VObject\Component\VEvent $replyEvent */
+		$replyEvent = $reply->message->add('VEVENT', []);
+		$replyEvent->add('UID', $reply->uid);
+		$replyEvent->add('RECURRENCE-ID', '20240715T080000', ['TZID' => 'America/Toronto']);
+		$replyEvent->add('ATTENDEE', $reply->sender, [
+			'PARTSTAT' => 'ACCEPTED',
+		]);
+
+		$result = $this->invokePrivate($this->broker, 'processMessageReply', [$reply, $existingCalendar]);
+
+		$this->assertSame($existingCalendar, $result);
+		$this->assertCount(2, $result->VEVENT);
+		$this->assertEquals('20240715T080000', $result->VEVENT[1]->{'RECURRENCE-ID'}->getValue());
+		$this->assertFalse(isset($result->VEVENT[1]->RRULE));
+		$this->assertCount(2, $result->VEVENT[1]->ATTENDEE);
+		$this->assertEquals('mailto:attendee2@example.org', $result->VEVENT[1]->ATTENDEE[1]->getValue());
+		$this->assertEquals('ACCEPTED', $result->VEVENT[1]->ATTENDEE[1]['PARTSTAT']->getValue());
+		$this->assertEquals('Attendee Two', $result->VEVENT[1]->ATTENDEE[1]['CN']->getValue());
+	}
+
 }