The Hotmail online e-mail service, operated by Microsoft Corp, was down for most of the working day on Friday, affecting "a significant portion of MS customers."
The world's largest software maker said that Hotmail, as well as MSN Messenger and other related services operated by its MSN Internet business, were down from around 8:30 am Pacific time (1630 GMT).
"We sincerely apologise for any inconvenience and disruption this may be causing our customers," a Microsoft spokesman said.
Redmond, Washington-based Microsoft said it had identified the cause of the problem, adding that service would be fully restored within a few hours before the end of Friday. No customers are at risk of losing data, Microsoft said.