Issue #3370855: Sending mails to multiple email addresses does not work

src/Plugin/Mail/SymfonyMailer.php

@@ -261,10 +261,7 @@ class SymfonyMailer implements MailInterface, ContainerFactoryPluginInterface {
         }
       }
 
-      $to = $message['to'];
-      if (!is_array($message['to'])) {
-        $to = [$message['to']];
-      }
+      $to = $this->parseMailboxes($message['to']);
       $email->to(...$to);
       if (!empty($message['headers']['From'])) {
         $email->from($message['headers']['From']);