Microsoft has joined other online companies with a new policy alerting users if their accounts are being targeted by governments.
"A key part of our work is identifying and preventing unauthorized access to your Microsoft account (including Outlook.com email and OneDrive) by anyone other than you," Microsoft vice president Scott Charney said in a blog on Wednesday.
"We're taking an additional step today. We will now notify you if we believe your account has been targeted or compromised by an individual or group working on behalf of a nation state."
Microsoft said it already notifies users if accounts have been targeted or compromised by a third party but is taking the additional step of letting people know if it is likely 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.
"If you receive one of these notifications it doesn't necessarily mean that your account has been compromised, but it does mean we have evidence your account has been targeted, and it's very important you take additional measures to keep your account secure," he added.
Twitter and Facebook announced similar measures earlier this year and Google implemented such warnings in 2012.