[Mailer] STDOUT blocks infinitely under Windows when STDERR is filled

Author: TemaYud

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);