Unfortunately, having the list software create a new message breaks threading. It is common practice to Cc people (subscribed to the list or not) that may need

Unfortunately, having the list software create a new message breaks threading. It is common practice to Cc people (subscribed to the list or not) that may need to interact with the thread, but they get the message directly from the sender while list recipients get a different message (different From, different Message-ID). Similarly, if the original sender replies to their own outgoing message, it will not thread for list recipients.