diff --git a/apps/dav/lib/CalDAV/InvitationResponse/InvitationResponseServer.php b/apps/dav/lib/CalDAV/InvitationResponse/InvitationResponseServer.php
index d1ea6902af39a..7910eb614b71f 100644
--- a/apps/dav/lib/CalDAV/InvitationResponse/InvitationResponseServer.php
+++ b/apps/dav/lib/CalDAV/InvitationResponse/InvitationResponseServer.php
@@ -121,4 +121,10 @@ public function handleITipMessage(Message $iTipMessage) {
 		$schedulingPlugin = $this->server->getPlugin('caldav-schedule');
 		$schedulingPlugin->scheduleLocalDelivery($iTipMessage);
 	}
+
+	public function isExternalAttendee(string $principalUri): bool {
+		/** @var \Sabre\DAVACL\Plugin $aclPlugin */
+		$aclPlugin = $this->server->getPlugin('acl');
+		return $aclPlugin->getPrincipalByUri($principalUri) === null;
+	}
 }
diff --git a/apps/dav/lib/Controller/InvitationResponseController.php b/apps/dav/lib/Controller/InvitationResponseController.php
index 9dbe43199d896..de22e3ba6a9e6 100644
--- a/apps/dav/lib/Controller/InvitationResponseController.php
+++ b/apps/dav/lib/Controller/InvitationResponseController.php
@@ -198,7 +198,12 @@ private function buildITipResponse(array $row, string $partStat, int $guests = n
 		$iTipMessage->method = 'REPLY';
 		$iTipMessage->sequence = $row['sequence'];
 		$iTipMessage->sender = $row['attendee'];
-		$iTipMessage->recipient = $row['attendee'];
+
+		if ($this->responseServer->isExternalAttendee($row['attendee'])) {
+			$iTipMessage->recipient = $row['organizer'];
+		} else {
+			$iTipMessage->recipient = $row['attendee'];
+		}
 
 		$message = <<<EOF
 BEGIN:VCALENDAR