What is the harm in telling a friend about a new project at work that your supervisor has assigned to you? Sending a personal e-mail that reveals proprietary information (ideas the company owns) might cost you your good standing with your colleagues or even your job. It might subject you to legal action as well. Do not say anything in an e-mail that you would not say directly to a person, and do not write anything that might embarrass you or your organization, regardless of who reads it. E-mail can be deleted, but it never really disappears and may be forwarded anywhere to anyone.

Most networks (the sender’s and the receiver’s) archive and back up all e-mail.