Not sure how much compliant this is with the law, but in this case the law should be more protective towards employees.
I imagine the following situation
I write on a company-owned piece of paper - "My boss is an idiot". Then take this piece of paper put it in an envelope ( owned by the company as well ), write the name of my colleague and seal the envelope. Then put the envelope on the recipient's desk.
I bet it would be illegal for my boss to take that letter, open it and read it.
P.S.
Looks like with e-mails the law is more protective towards employees :
[1] : https://www.reuters.com/article/us-privacy-emails-echr/europ...
[2] : http://www.internationallawoffice.com/Newsletters/Employment...
[3] : https://www.womblebonddickinson.com/uk/insights/articles-and...