Thought to be result of a phishing attack the list of around 10,000 Microsoft Hotmail, Windows Live and MSN accounts were posted on Monday on technology website Neowin.net by an anonymous user, Daily Telegraph reported.
Neowin said the details appeared legitimate and most of the accounts exposed by the leak belonged to European web users.
Microsoft on Tuesday confirmed the phishing attack against users of the popular email service. But said in a statement that the culprit had not breached its security but had fooled e-mail users into handing over their details by creating an identical Hotmail website.
"Over the weekend, Microsoft learned that several thousand Windows Live Hotmail customers' credentials were exposed on a third-party site due to a likely phishing scheme," it said, according media reports.
Users of Hotmail, Windows Live and MSN email accounts are advised to change their passwords and security questions immediately to block unauthorised access to accounts.
Hotmail is the largest web-based email service in the world, boasting an estimated 500 million users, with 14 million in the UK alone.