Microsoft has said the users of Outlook and Hotmail webmail services have been hit by a lengthy fault.
A status message informed users about the issues, with the company adding it was 'working to restore service right now'.
According to the BBC, users reported not seeing emails that should have reached their accounts.
Microsoft is in the process of migrating millions of users from Hotmail, which launched in 1998, to new Outlook.com accounts.
The move has been seen as an attempt by the Windows-maker to claw back ground lost to competitors such as Google, whose Gmail service has proved immensely popular since its launch in 2004.
Problems began to hit the services on Tuesday evening, with Microsoft acknowledging it had a problem at 21:35 GMT.
According to reports, some users noted being unable to see certain emails in the accounts, while others reported email was simply inaccessible altogether.