Email was cc to unspecified recipient in Outlook 2003

Hi all,

Can anyone help? We are using outlook 2003 with Exchange 2010.  I have a user who would sent to a fixed group of external contacts.  However, on several occasions this user happen to to have his mail cc to unspecified recipient which is within the company.  How can I prevent this from happening?  Is there anyway to troubleshoot or monitor and keep track the message that was sent to the unspecified recipients?

Thanks