The Strasbourg court is the global court established by the ECHR, which is a separate institution from the European Union, and was founded in 1959.
"The right to respect for private life and for the privacy of correspondence continued to exist, even if these might be restricted in so far as necessary", it writes.
The company had presented him with printouts of his private messages to his brother and fiancée on Yahoo Messenger as evidence of his breach of a company ban on such personal use.
"The usual reasons are to ensure that they need to monitor an employee's work emails to ensure they are complying with the law and their obligations and this is likely to be a legitimate reason", she explained.
The decision marks the end of a lengthy appeal brought by a Romanian national centered on private correspondence that his former employer had intercepted and subsequently used in disciplinary proceedings that resulted in his termination in 2007.
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The ECHR judges agreed that the Romanian courts had not struck a "fair balance" between Bărbulescu's right to a private life and his employer's right to ensure he was following work rules. As part of his role, he was asked to set up a Yahoo Messenger account to handle customers' queries.
Bărbulescu was sacked on August 1, 2007 for breach of the company's internal regulations that prohibited the use of company resources for personal purposes.
In 2016, the European Court of Human Rights also ruled against him.
By an 11 to six majority, judges in the grand chamber sided with Bogdan Bărbulescu, who claimed his right to a private life was not properly upheld by Romania's courts. The judges found that there had been no violation of his rights and his employer had acted reasonably. The court concluded that the employer had not been informed ahead of time of the exact extent or nature of his employer's email monitoring, or that the employer could access the content of his messages.
Mr Barbulescu argued that his employer invaded his right to privacy by spying on messages which included details about his health and sex life, but both a Romanian court and the ECHR had previously ruled in support of the employer.
However, the ECHR did say that it was acceptable for employers to monitor their staff members' online activity, and even in some cases fire them for using private messages. Britain enacted regulations in 2000, giving employers broad power to record or monitor employees' communications without consent, as long as they took reasonable steps to inform employees that their communications might be intercepted.