I replied to a message via email, but it didnt appear in the correct topic. Why?
Messages are grouped in topics primarily based on the subject of the message. So if you change the subject of the message you are replying to, it likely will not appear in the correct topic.
Note: The Groups system will still attempt to place it in the right topic based on the message threading information in the email header, but since only some email clients provide this data, this matching process will only work some of the time.