ASK THE EXPERT: Can I get sued for invasion of privacy for reading my friend’s email?

Candace Payne from the United States (Brooklyn) asks:

One of my friends left his email inbox open on my computer. I read an email that he did not want me to read. Now he is threatening to sue me with invasion of privacy. I am thinking that legally it is not wrong because he left his email inbox open on my computer. I did not hack into his email. He left it open. Please inform me.