Update \Magento\Email\Model\Transport::setReturnPath to use the existing logic defined in [Headers.php v6.4.19](https://github.com/symfony/mime/blob/v6.4.19/Header/Headers.php)

Author: TomashKhamlai

app/code/Magento/Email/Model/Transport.php

@@ -242,11 +242,15 @@ public function sendMessage(): void
     private function setReturnPath(SymfonyMessage $email): void
     {
         if ($this->isSetReturnPath === 2 && $this->returnPathValue) {
-            $email->getHeaders()->addMailboxListHeader('Sender', [$this->returnPathValue]);
+            $email->getHeaders()->addMailboxHeader('Sender', $this->returnPathValue);
         } elseif ($this->isSetReturnPath === 1 &&
-            !empty($fromAddresses = $email->getHeaders()->get('From')?->getAddresses())) {
+            !empty(
+                /** @var \Symfony\Component\Mime\Address[] $fromAddresses */
+                $fromAddresses = $email->getHeaders()->get('From')?->getAddresses()
+            )
+        ) {
             reset($fromAddresses);
-            $email->getHeaders()->addMailboxListHeader('Sender', [current($fromAddresses)->getAddress()]);
+            $email->getHeaders()->addMailboxHeader('Sender', current($fromAddresses));
         }
     }