Bogdan Mihai Barbulescu was sacked for sending the messages via the Yahoo messaging system in 2007.
His employer had used surveillance software to monitor his computer activity.
A Romanian court ruled in 2016 that the firm was within its rights but this has now been overturned.
Mr Barbulescu successfully challenged the original decision.
Some of the communications he had sent were "intimate in nature" and were sent to his brother and his fiancee, the court heard.