Hi Stephen,
It may be that your system has some sort of Rule set up to BCC all messages.
If you look in Tools > Rules & alerts, you may find the rule that is being causing all your mail to be sent BCC.
I think there is also a small application that can be downloaded and installed to do this so it may be worth investigating to see if something similar is being used.
I found this link which may provide you with some more information.
http://office.microsoft.com/en-gb/ma...587501033.aspx
Kind regards,
Zade