diff --git a/apps/dav/lib/CalDAV/InvitationResponse/InvitationResponseServer.php b/apps/dav/lib/CalDAV/InvitationResponse/InvitationResponseServer.php
index 7b0ee1bbef243..a24fc6856b660 100644
--- a/apps/dav/lib/CalDAV/InvitationResponse/InvitationResponseServer.php
+++ b/apps/dav/lib/CalDAV/InvitationResponse/InvitationResponseServer.php
@@ -122,4 +122,10 @@ 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 726ee1c498a3d..b021878655e88 100644
--- a/apps/dav/lib/Controller/InvitationResponseController.php
+++ b/apps/dav/lib/Controller/InvitationResponseController.php
@@ -200,7 +200,12 @@ private function buildITipResponse(array $row, string $partStat, int $guests = n
 		$iTipMessage->method = 'REPLY';
 		$iTipMessage->sequence = $row['sequence'];
 		$iTipMessage->sender = $row['attendee'];
-		$iTipMessage->recipient = $row['organizer'];
+
+		if ($this->responseServer->isExternalAttendee($row['attendee'])) {
+			$iTipMessage->recipient = $row['organizer'];
+		} else {
+			$iTipMessage->recipient = $row['attendee'];
+		}
 
 		$message = <<<EOF
 BEGIN:VCALENDAR
diff --git a/apps/dav/tests/unit/Controller/InvitationResponseControllerTest.php b/apps/dav/tests/unit/Controller/InvitationResponseControllerTest.php
index 2fb97969655d4..f959f45a8a578 100644
--- a/apps/dav/tests/unit/Controller/InvitationResponseControllerTest.php
+++ b/apps/dav/tests/unit/Controller/InvitationResponseControllerTest.php
@@ -76,7 +76,17 @@ protected function setUp(): void {
 		);
 	}
 
-	public function testAccept() {
+	public function attendeeProvider(): array {
+		return [
+			'local attendee' => [false],
+			'external attendee' => [true]
+		];
+	}
+
+	/**
+	 * @dataProvider attendeeProvider
+	 */
+	public function testAccept(bool $isExternalAttendee): void {
 		$this->buildQueryExpects('TOKEN123', [
 			'id' => 0,
 			'uid' => 'this-is-the-events-uid',
@@ -109,21 +119,26 @@ public function testAccept() {
 		$called = false;
 		$this->responseServer->expects($this->once())
 			->method('handleITipMessage')
-			->willReturnCallback(function (Message $iTipMessage) use (&$called, $expected) {
+			->willReturnCallback(function (Message $iTipMessage) use (&$called, $isExternalAttendee, $expected) {
 				$called = true;
 				$this->assertEquals('this-is-the-events-uid', $iTipMessage->uid);
 				$this->assertEquals('VEVENT', $iTipMessage->component);
 				$this->assertEquals('REPLY', $iTipMessage->method);
 				$this->assertEquals(null, $iTipMessage->sequence);
 				$this->assertEquals('mailto:attendee@foo.bar', $iTipMessage->sender);
-				$this->assertEquals('mailto:organizer@foo.bar', $iTipMessage->recipient);
+				if ($isExternalAttendee) {
+					$this->assertEquals('mailto:organizer@foo.bar', $iTipMessage->recipient);
+				} else {
+					$this->assertEquals('mailto:attendee@foo.bar', $iTipMessage->recipient);
+				}
 
 				$iTipMessage->scheduleStatus = '1.2;Message delivered locally';
 
 				$this->assertEquals($expected, $iTipMessage->message->serialize());
 			});
-
-
+		$this->responseServer->expects($this->once())
+			->method('isExternalAttendee')
+			->willReturn($isExternalAttendee);
 
 		$response = $this->controller->accept('TOKEN123');
 		$this->assertInstanceOf(TemplateResponse::class, $response);
@@ -132,7 +147,10 @@ public function testAccept() {
 		$this->assertTrue($called);
 	}
 
-	public function testAcceptSequence() {
+	/**
+	 * @dataProvider attendeeProvider
+	 */
+	public function testAcceptSequence(bool $isExternalAttendee): void {
 		$this->buildQueryExpects('TOKEN123', [
 			'id' => 0,
 			'uid' => 'this-is-the-events-uid',
@@ -165,21 +183,26 @@ public function testAcceptSequence() {
 		$called = false;
 		$this->responseServer->expects($this->once())
 			->method('handleITipMessage')
-			->willReturnCallback(function (Message $iTipMessage) use (&$called, $expected) {
+			->willReturnCallback(function (Message $iTipMessage) use (&$called, $isExternalAttendee, $expected) {
 				$called = true;
 				$this->assertEquals('this-is-the-events-uid', $iTipMessage->uid);
 				$this->assertEquals('VEVENT', $iTipMessage->component);
 				$this->assertEquals('REPLY', $iTipMessage->method);
 				$this->assertEquals(1337, $iTipMessage->sequence);
 				$this->assertEquals('mailto:attendee@foo.bar', $iTipMessage->sender);
-				$this->assertEquals('mailto:organizer@foo.bar', $iTipMessage->recipient);
+				if ($isExternalAttendee) {
+					$this->assertEquals('mailto:organizer@foo.bar', $iTipMessage->recipient);
+				} else {
+					$this->assertEquals('mailto:attendee@foo.bar', $iTipMessage->recipient);
+				}
 
 				$iTipMessage->scheduleStatus = '1.2;Message delivered locally';
 
 				$this->assertEquals($expected, $iTipMessage->message->serialize());
 			});
-
-
+		$this->responseServer->expects($this->once())
+			->method('isExternalAttendee')
+			->willReturn($isExternalAttendee);
 
 		$response = $this->controller->accept('TOKEN123');
 		$this->assertInstanceOf(TemplateResponse::class, $response);
@@ -188,7 +211,10 @@ public function testAcceptSequence() {
 		$this->assertTrue($called);
 	}
 
-	public function testAcceptRecurrenceId() {
+	/**
+	 * @dataProvider attendeeProvider
+	 */
+	public function testAcceptRecurrenceId(bool $isExternalAttendee): void {
 		$this->buildQueryExpects('TOKEN123', [
 			'id' => 0,
 			'uid' => 'this-is-the-events-uid',
@@ -222,21 +248,26 @@ public function testAcceptRecurrenceId() {
 		$called = false;
 		$this->responseServer->expects($this->once())
 			->method('handleITipMessage')
-			->willReturnCallback(function (Message $iTipMessage) use (&$called, $expected) {
+			->willReturnCallback(function (Message $iTipMessage) use (&$called, $isExternalAttendee, $expected) {
 				$called = true;
 				$this->assertEquals('this-is-the-events-uid', $iTipMessage->uid);
 				$this->assertEquals('VEVENT', $iTipMessage->component);
 				$this->assertEquals('REPLY', $iTipMessage->method);
 				$this->assertEquals(0, $iTipMessage->sequence);
 				$this->assertEquals('mailto:attendee@foo.bar', $iTipMessage->sender);
-				$this->assertEquals('mailto:organizer@foo.bar', $iTipMessage->recipient);
+				if ($isExternalAttendee) {
+					$this->assertEquals('mailto:organizer@foo.bar', $iTipMessage->recipient);
+				} else {
+					$this->assertEquals('mailto:attendee@foo.bar', $iTipMessage->recipient);
+				}
 
 				$iTipMessage->scheduleStatus = '1.2;Message delivered locally';
 
 				$this->assertEquals($expected, $iTipMessage->message->serialize());
 			});
-
-
+		$this->responseServer->expects($this->once())
+			->method('isExternalAttendee')
+			->willReturn($isExternalAttendee);
 
 		$response = $this->controller->accept('TOKEN123');
 		$this->assertInstanceOf(TemplateResponse::class, $response);
@@ -272,7 +303,10 @@ public function testAcceptExpiredToken() {
 		$this->assertEquals([], $response->getParams());
 	}
 
-	public function testDecline() {
+	/**
+	 * @dataProvider attendeeProvider
+	 */
+	public function testDecline(bool $isExternalAttendee): void {
 		$this->buildQueryExpects('TOKEN123', [
 			'id' => 0,
 			'uid' => 'this-is-the-events-uid',
@@ -305,21 +339,26 @@ public function testDecline() {
 		$called = false;
 		$this->responseServer->expects($this->once())
 			->method('handleITipMessage')
-			->willReturnCallback(function (Message $iTipMessage) use (&$called, $expected) {
+			->willReturnCallback(function (Message $iTipMessage) use (&$called, $isExternalAttendee, $expected) {
 				$called = true;
 				$this->assertEquals('this-is-the-events-uid', $iTipMessage->uid);
 				$this->assertEquals('VEVENT', $iTipMessage->component);
 				$this->assertEquals('REPLY', $iTipMessage->method);
 				$this->assertEquals(null, $iTipMessage->sequence);
 				$this->assertEquals('mailto:attendee@foo.bar', $iTipMessage->sender);
-				$this->assertEquals('mailto:organizer@foo.bar', $iTipMessage->recipient);
+				if ($isExternalAttendee) {
+					$this->assertEquals('mailto:organizer@foo.bar', $iTipMessage->recipient);
+				} else {
+					$this->assertEquals('mailto:attendee@foo.bar', $iTipMessage->recipient);
+				}
 
 				$iTipMessage->scheduleStatus = '1.2;Message delivered locally';
 
 				$this->assertEquals($expected, $iTipMessage->message->serialize());
 			});
-
-
+		$this->responseServer->expects($this->once())
+			->method('isExternalAttendee')
+			->willReturn($isExternalAttendee);
 
 		$response = $this->controller->decline('TOKEN123');
 		$this->assertInstanceOf(TemplateResponse::class, $response);
@@ -335,7 +374,10 @@ public function testOptions() {
 		$this->assertEquals(['token' => 'TOKEN123'], $response->getParams());
 	}
 
-	public function testProcessMoreOptionsResult() {
+	/**
+	 * @dataProvider attendeeProvider
+	 */
+	public function testProcessMoreOptionsResult(bool $isExternalAttendee): void {
 		$this->request->expects($this->at(0))
 			->method('getParam')
 			->with('partStat')
@@ -383,20 +425,26 @@ public function testProcessMoreOptionsResult() {
 		$called = false;
 		$this->responseServer->expects($this->once())
 			->method('handleITipMessage')
-			->willReturnCallback(function (Message $iTipMessage) use (&$called, $expected) {
+			->willReturnCallback(function (Message $iTipMessage) use (&$called, $isExternalAttendee, $expected) {
 				$called = true;
 				$this->assertEquals('this-is-the-events-uid', $iTipMessage->uid);
 				$this->assertEquals('VEVENT', $iTipMessage->component);
 				$this->assertEquals('REPLY', $iTipMessage->method);
 				$this->assertEquals(null, $iTipMessage->sequence);
 				$this->assertEquals('mailto:attendee@foo.bar', $iTipMessage->sender);
-				$this->assertEquals('mailto:organizer@foo.bar', $iTipMessage->recipient);
+				if ($isExternalAttendee) {
+					$this->assertEquals('mailto:organizer@foo.bar', $iTipMessage->recipient);
+				} else {
+					$this->assertEquals('mailto:attendee@foo.bar', $iTipMessage->recipient);
+				}
 
 				$iTipMessage->scheduleStatus = '1.2;Message delivered locally';
 
 				$this->assertEquals($expected, $iTipMessage->message->serialize());
 			});
-
+		$this->responseServer->expects($this->once())
+			->method('isExternalAttendee')
+			->willReturn($isExternalAttendee);
 
 
 		$response = $this->controller->processMoreOptionsResult('TOKEN123');