diff --git a/lib/private/Mail/Mailer.php b/lib/private/Mail/Mailer.php
index 5a054d2efd..d55546769e 100644
--- a/lib/private/Mail/Mailer.php
+++ b/lib/private/Mail/Mailer.php
@@ -205,7 +205,7 @@ class Mailer implements IMailer {
 	 * @return string Converted mail address if `idn_to_ascii` exists
 	 */
 	protected function convertEmail(string $email): string {
-		if (!function_exists('idn_to_ascii') || strpos($email, '@') === false) {
+		if (!function_exists('idn_to_ascii') || !defined('INTL_IDNA_VARIANT_UTS46') || strpos($email, '@') === false) {
 			return $email;
 		}
 
diff --git a/lib/private/Mail/Message.php b/lib/private/Mail/Message.php
index 622c18a911..7c7ecae857 100644
--- a/lib/private/Mail/Message.php
+++ b/lib/private/Mail/Message.php
@@ -66,7 +66,7 @@ class Message implements IMessage {
 	 * @return array Converted addresses if `idn_to_ascii` exists
 	 */
 	protected function convertAddresses(array $addresses): array {
-		if (!function_exists('idn_to_ascii')) {
+		if (!function_exists('idn_to_ascii') || !defined('INTL_IDNA_VARIANT_UTS46')) {
 			return $addresses;
 		}