From Microsoft on the Issues, Additional steps to help keep your personal information secure (http://blogs.microsoft.com/on-the-issues/2015/12/30/additional-steps-to-help-keep-your-personal-information-secure/)
QuoteWe’re committed to helping our users keep their personal information secure and private. 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.
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.