diff --git a/src/Symfony/Component/Mailer/Transport/Smtp/Stream/ProcessStream.php b/src/Symfony/Component/Mailer/Transport/Smtp/Stream/ProcessStream.php
index a8a8603807d27..bc721ad0cd85f 100644
--- a/src/Symfony/Component/Mailer/Transport/Smtp/Stream/ProcessStream.php
+++ b/src/Symfony/Component/Mailer/Transport/Smtp/Stream/ProcessStream.php
@@ -35,7 +35,7 @@ public function initialize(): void
         $descriptorSpec = [
             0 => ['pipe', 'r'],
             1 => ['pipe', 'w'],
-            2 => ['pipe', 'w'],
+            2 => ['pipe', '\\' === \DIRECTORY_SEPARATOR ? 'a' : 'w'],
         ];
         $pipes = [];
         $this->stream = proc_open($this->command, $descriptorSpec, $pipes);