Today when I opened windows live mail all the messages in my inbox that had already been read showed up as unread. I was storing over 450 messages in there and they all show up as new now.
So I'm going through and deleting ones I don't need, then I'll go quickly through the list so they show up as read again, but why did this happen and I hope it won't happen again?

Oh wait.. I just found an icon for marking all the messages as read, that at least saved me some time, but still not sure why it happened.

I don't know what version of live mail I have, it is with windows 7.