A stray email

My email programme was stating I had one email unread; yet no recent ones remained as such. Switching views I discovered an unread email from my provider stating that my mailbox was getting full. That's odd for several reasons.

1. I'm using around 11% of my limit
2. The email is dated 2/12/2011
3. I've archived off all the 2011 emails

So somehow an email has moved from my 2011 folder into my main folder, and marked itself as unread. How bizarre.