Some 17,355 people lost their e-mails starting on Thursday, but by Sunday evening, the messages had been restored, according to the Washington Post.
The paper reported Microsoft said the problem and steps would be taken to prevent it from happening again, but did not disclose any details.
Though his team has identified what went wrong, Chris Jones, senior vice president of Microsoft's Windows Live busines, did not disclose any details.
Microsoft's free Hotmail web-based e-mail service has about 360 million users, according to web traffic measurement group comScore.
This story was first published by Computer Weekly