I apologize if I am missing a setting somewhere, and I couldn't find a post covering this specific topic, but if I have threads enabled and collapsed, it does not indicate when a new message is received to that thread. Currently, with threads disabled, new messages (when sorted by date) are at the top and in bold. If the thread is collapsed, and a new message is received, the only way to know is to expand that thread and go to the end of the thread. I would think that even with threads collapsed, if a new message for that thread is received, it should move the entire conversation to the top of the list and mark the conversation in bold indicating that there is a new/unread message.
