Look at the bottom of your messages.

cougar will be notified if you reply to this message!
cougar will be notified if you reply to any message in this thread!

When you compose a message- you are clicking on the boxes at the bottom to be notified.

Allow Emails : allow visitors to email you (your email will stay hidden)
Reply Notification : receive an email notification every time someone replies to your message
Thread Notification : receive an email notification every time a new message is posted inside this thread.

If you notice Newport's messages, at the bottom of his posts, it says he will be notified if anyone responds to his posts.

If you check those boxes, it can be a year later and someone can respond to a messages, or with the box you also checked, to any replies in that thread.