From time to time we all fall into thinking that our emails are private communications between ourselves and whoever we are emailing. That is dangerous thinking, as Hillary Clinton has learned. And although national security interests may not be at stake, the content of your business and personal emails can cause you great heartache and money.
One of the first things I do in any case is request business and personal emails, usually in native format so that I can not only read them, but I can also mine them for metadata. You would be surprised how much more honest people are in their emails than they are giving testimony under oath. In other words, there is often information in those emails that a witness would never disclose otherwise. Also, when people write emails they often do not write them with the same careful thought process that they would use in testimony. So what is the key? DO NOT put anything into an email that you would not want to testify to in Court! That rule goes for business litigation, personal injury, divorce -- any and every kind of litigation or trial. And don't think that you can just delete your harmful emails when you get involved in a legal action. There is a legal doctrine known as "spoliation of evidence" whereby under certain circumstances the court can dismiss your case if you are the plaintiff, or tell the jurors they can make adverse inferences as a result of deleting the emails if you are a defendant.
Unfortunately, almost nothing is private with electronic communications and media, so beware how you use them.