Users of Microsoft's Windows Live email service (also known as Hotmail) have been reporting problems accessing their accounts since yesterday.
Microsoft has subsequently confirmed that there is a problem for some users and it is investigating, but in the meantime some, but not all users will continue to suffer access issues.
"We're actively investigating the cause and are working to take the appropriate steps to remedy the situation as rapidly as possible," said a Microsoft spokesman.
UK, US and Canadian users are reporting access failures of the free web-based email service, which has more than 300 million active users worldwide.
Microsoft bought Hotmail in 1998 when it had only nine million users. It is now the most popular free webmail service, surpassing Yahoo Mail and Google's Gmail.
