"We?re taking this additional step of specifically letting you know if we have evidence that the attacker may be "state-sponsored" because it is likely that the attack could be more sophisticated or more sustained than attacks from cybercriminals and others," Charney said.
Microsoft's statement follows the disclosure made by two former employees of Microsoft about a hacking campaign that the company failed to bring to the surface. The employees said that the company's own experts had concluded several years ago that Chinese authorities had been behind a hacking campaign. However, Microsoft had failed to notify users.
Those Hotmail attacks targeted diplomats, media workers, human rights lawyers, and others in sensitive positions inside China.
The policy shift at the world's largest software company follows similar moves since October by Internet giants Facebook Inc, Twitter Inc and most recently Yahoo Inc.
Google pioneered the practice in 2012 and said it now alerts tens of thousands of users every few months.
For two years, Microsoft has offered alerts about potential security breaches without specifying the likely suspect.