Microsoft is struggling to fix an intermittent Hotmail network outage that is stopping users accessing their e-mail accounts.
The stoppage is affecting both users of free standard Hotmail accounts and those who have paid for extra e-mail storage.
Microsoft has not said how many of its 185 million Hotmail users are affected by the problem.
The outage is a blow to Microsoft as it comes in the middle of a major advertising campaign to promote the company's new MSN Search engine.
Hotmail users, who access their messages through the MSN service, are a key potential traffic and cash generator for MSN Search, as well as for standard web advertising.