Firstly HR should have a non-disclosure clause in the appointment letters or a policy framed on confidentiality which should be explained while a new hire is done.
Then the process would be clear.
Contact your company IT department or else Administrator.
Is your company having a mail server, if yes you can restrict the users through the IT wing.
11th April 2018 From India, Hyderabad
involved in information/data leakage offence. Yes you can go legal only when you are in a position to hold one for doing illegal. Use of Email or saring of information is not illegal unless untill you make policy. To go for a legal action you must have proof of 'illegality' against the concern. Your establishment should have the written policies and action for violation of principles. If it is not in place, ask your management to have one and get it drafted by a lawer. Does your establishment have it's own EMail then block other popups and put filter and check all that mails are going out. At an immidiate step your take the fidelity guarantee from the employee whom you think prime suspect.
12th April 2018 From India, Mumbai