Outlook.com and Hotmail email services were cut off in Europe on Monday, preventing users from sending and receiving emails for 12 hours.
The outage began at 4:20 am (GMT) and continued to affect users in the region almost 12 hours later, Microsoft said in a message on its Office 365 security site.
Microsoft said it was working to resolve the outage and would issue an update on Monday about the status of its services.
"We continue to investigate to determine the source of the problem and identify service recovery steps," the company said.
Microsoft said the problem involved part of the internet traffic load balancing system, which was swallowing server capacity, though there was no apparent increase in user traffic.
Reports of outages were concentrated in Western Europe and the United Kingdom, according to DownDetector.co.uk. No other Microsoft online service appears to have been affected.
