diff --git a/apps/dav/lib/CalDAV/Reminder/NotificationProvider/EmailProvider.php b/apps/dav/lib/CalDAV/Reminder/NotificationProvider/EmailProvider.php
index e3c471521f469..7a62ba19d4fb3 100644
--- a/apps/dav/lib/CalDAV/Reminder/NotificationProvider/EmailProvider.php
+++ b/apps/dav/lib/CalDAV/Reminder/NotificationProvider/EmailProvider.php
@@ -262,7 +262,10 @@ private function getAllEMailAddressesFromEvent(VEvent $vevent):array {
 					$emailAddressesOfDelegates = $delegates->getParts();
 					foreach($emailAddressesOfDelegates as $addressesOfDelegate) {
 						if (strcasecmp($addressesOfDelegate, 'mailto:') === 0) {
-							$emailAddresses[substr($addressesOfDelegate, 7)] = [];
+							$delegateEmail = substr($addressesOfDelegate, 7);
+							if ($delegateEmail !== false && $this->mailer->validateMailAddress($delegateEmail)) {
+								$emailAddresses[$delegateEmail] = [];
+							}
 						}
 					}
 
@@ -334,8 +337,12 @@ private function getEMailAddressOfAttendee(VObject\Property $attendee):?string {
 		if (!$this->hasAttendeeMailURI($attendee)) {
 			return null;
 		}
+		$attendeeEMail = substr($attendee->getValue(), 7);
+		if ($attendeeEMail === false || !$this->mailer->validateMailAddress($attendeeEMail)) {
+			return null;
+		}
 
-		return substr($attendee->getValue(), 7);
+		return $attendeeEMail;
 	}
 
 	/**
diff --git a/apps/dav/tests/unit/CalDAV/Reminder/NotificationProvider/EmailProviderTest.php b/apps/dav/tests/unit/CalDAV/Reminder/NotificationProvider/EmailProviderTest.php
index c07259f9cd71d..4b2aa463874c4 100644
--- a/apps/dav/tests/unit/CalDAV/Reminder/NotificationProvider/EmailProviderTest.php
+++ b/apps/dav/tests/unit/CalDAV/Reminder/NotificationProvider/EmailProviderTest.php
@@ -250,30 +250,20 @@ public function testSendWithAttendees(): void {
 		$deL10N->method('l')
 			->will($this->returnArgument(0));
 
-		$this->l10nFactory->expects($this->at(0))
+		$this->l10nFactory->expects(self::exactly(3))
 			->method('findLanguage')
 			->with()
 			->willReturn('en');
 
-		$this->l10nFactory->expects($this->at(1))
+		$this->l10nFactory->expects(self::exactly(3))
 			->method('languageExists')
-			->with('dav', 'de')
+			->withConsecutive(['dav', 'en'], ['dav', 'de'], ['dav', 'de'])
 			->willReturn(true);
 
-		$this->l10nFactory->expects($this->at(2))
+		$this->l10nFactory->expects(self::exactly(2))
 			->method('get')
-			->with('dav', 'de')
-			->willReturn($enL10N);
-
-		$this->l10nFactory->expects($this->at(3))
-			->method('languageExists')
-			->with('dav', 'en')
-			->willReturn(true);
-
-		$this->l10nFactory->expects($this->at(4))
-			->method('get')
-			->with('dav', 'en')
-			->willReturn($deL10N);
+			->withConsecutive(['dav', 'de'], ['dav', 'de'])
+			->willReturnOnConsecutiveCalls([$deL10N],[$deL10N]);
 
 		$template1 = $this->getTemplateMock();
 		$message11 = $this->getMessageMock('foo1@example.org', $template1);
@@ -284,7 +274,7 @@ public function testSendWithAttendees(): void {
 		$message22 = $this->getMessageMock('foo4@example.org', $template2);
 		$message23 = $this->getMessageMock('uid1@example.com', $template2);
 
-		$this->mailer->expects($this->at(0))
+		$this->mailer->expects(self::exactly(2))
 			->method('createEMailTemplate')
 			->with('dav.calendarReminder')
 			->willReturn($template1);
@@ -341,9 +331,14 @@ public function testSendWithAttendees(): void {
 			->willReturn($message23);
 		$this->mailer->expects($this->at(13))
 			->method('send')
-			->with($message23)
-			->willReturn([]);
-
+			->withConsecutive(
+				[$message11],
+				[$message12],
+				[$message13],
+				[$message21],
+				[$message22],
+				[$message23]
+			)->willReturn([]);
 		$this->setupURLGeneratorMock(2);
 
 		$vcalendar = $this->getAttendeeVCalendar();