Thousands of Hotmail passwords have been hacked and posted online, BBC News has learnt.
Microsoft, which owns the popular web-based e-mail system, said that it was aware of the claims and that it was "investigating the situation".
BBC News has seen a list of more than 10,000 accounts, which technology blog Neowin.net said had been posted online.
The blog suggested the accounts had been hacked or had been collected as part of a phishing scheme.
Phishing involves using fake websites to lure people into revealing personal details such as bank accounts or login names and passwords.
'Rapid response'
Neowin claims the details were posted on 1 October to pastebin.com, a website commonly used by developers to share code.
Although the details have since been removed, BBC News and Neowin has seen a list of 10,027 names beginning with the letters A and B.
"[We] can confirm the accounts are genuine and most appear to be based in Europe," Tom Warren, a neowin blogger, wrote on the site.
The list included details of Microsoft's Windows Live Hotmail accounts with email addresses ending hotmail.com, msn.com and live.com.
Microsoft said it had "been made aware of the claims that Windows Live IDs and passwords have been made available on the web".
"We're actively investigating the situation and will take appropriate steps as rapidly as possible," a spokesperson said.
Neowin said that it recommended Windows Live Hotmail users to change their "password and security question immediately".
Hotmail is currently the largest web-based email service.
Do you have a Hotmail email account? Have you been affected by the issues in this story? Send us your experiences using the form below.
__________________ "I won't come on tv and speak or write about anything and say words only half the people listening understand. In our language there's a similarity, words that are spelt differently but mean the same thing. Words that mean the same thing and the big men use these words, knowing full well that maybe only 10% of the people listening will understand. Well we don't. We speak the language that everybody understands. Instead of me saying somebody was avaricious, I'd say he was bloody greedy."
__________________ "I won't come on tv and speak or write about anything and say words only half the people listening understand. In our language there's a similarity, words that are spelt differently but mean the same thing. Words that mean the same thing and the big men use these words, knowing full well that maybe only 10% of the people listening will understand. Well we don't. We speak the language that everybody understands. Instead of me saying somebody was avaricious, I'd say he was bloody greedy."
Set up a gmail account, forward all your hotmail mail to the gmail account. Job done.
I don't think this works anylonger. I have tried to forward my emails to gmail, but it stops me from doing this! It says that I can only forward on emails to either *@hotmail.com, or *@live.com or *@msn.com
__________________ "I won't come on tv and speak or write about anything and say words only half the people listening understand. In our language there's a similarity, words that are spelt differently but mean the same thing. Words that mean the same thing and the big men use these words, knowing full well that maybe only 10% of the people listening will understand. Well we don't. We speak the language that everybody understands. Instead of me saying somebody was avaricious, I'd say he was bloody greedy."
