I am sure this has happened to you before. You were on BCC, you did not realize it and did a REPLY ALL. That exposes you and the sender .
Well, if you use Outlook 2010 and Exchange server 2010 (or above), it is a simple matter of LOOKING at what Outlook is telling you. Outlook detects this problem AUTOMATICALLY and shows you a warning in RED color – just above the TO box. Notice it and act on it. Cancel the mail and just use REPLY – so that only the sender gets the mail.
If you don’t know which version of Exchange Server you are using, talk to your IT team.