diff --git a/src/Symfony/Component/Mailer/Bridge/Mailjet/Transport/MailjetApiTransport.php b/src/Symfony/Component/Mailer/Bridge/Mailjet/Transport/MailjetApiTransport.php
index edb7c8e8c589c..49119caedc7cb 100644
--- a/src/Symfony/Component/Mailer/Bridge/Mailjet/Transport/MailjetApiTransport.php
+++ b/src/Symfony/Component/Mailer/Bridge/Mailjet/Transport/MailjetApiTransport.php
@@ -201,7 +201,7 @@ private function castCustomHeader(string $value, string $type)
         return match ($type) {
             'bool' => filter_var($value, \FILTER_VALIDATE_BOOLEAN),
             'int' => (int) $value,
-            'json' => json_decode($value, true, 2, \JSON_THROW_ON_ERROR),
+            'json' => json_decode($value, true, 512, \JSON_THROW_ON_ERROR),
             'string' => $value,
         };
     }