BB Bold :: Losing Emails Off PC?

For the last couple of weeks, every time I've opened an email on my Blackberry it has been automatically deleted from my PC. I've had the phone a year and this has only recently started happening. I have been all over the Blackberry, checking settings, trying to find a solution and just cannot work out what the problem is.

I've seen from various internet forums that other people have this problem too, but no one seems to be able to offer a solution. It's maddening because I need some of these emails to be on my PC! Does anyone know how I can resolve this issue?