Microsoft is to begin warning users of the Outlook email service when it suspects a government has tried to hack their accounts, the company said on Wednesday.
The policy change at the world's largest software company follows similar moves since October by Facebook, Twitter and Yahoo. Google pioneered the practice in 2012 and said it now alerts tens of thousands of users every few months.
Microsoft has offered alerts about potential security breaches for two years without specifying the likely suspect.
In the statement, the company said: "As the threat landscape has evolved our approach has too, and we'll now go beyond notification and guidance to specify if we reasonably believe the attacker is 'state-sponsored'."
The announcement came after it emerged Microsoft did not tell Hotmail users it suspected Chinese authorities of hacking into more than 1,000 accounts several years ago.