Prying into workers' e-mails has become harder since a European Court of Human Rights ruling that a company violated a Romanian man's privacy by firing him after spying on his personal chats.
Romanian authorities had not properly protected the employee's "right to respect for his private life and correspondence", the court's Grand Chamber ruled on Tuesday.
The ruling focused on the fact the Romanian courts had failed to look at whether the employee had been warned that his communications might be monitored.
A lower court ruled in January 2016 that it was not unreasonable for companies to monitor private communications made during working hours.
The ruling highlights the dilemma that countries around the world have in balancing citizens' rights and the need to protect national security. Privacy campaigners welcomed the decision, saying it offered important protections to workers' rights. However, Privacy International sounded a note of caution.
The ruling "doesn't give an absolute right to privacy to employees", the advocacy group said.
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