Microsoft Corp's Hotmail service, the world's most-used online email system, is back to normal operations on Monday after some users over the weekend lost access to emails or found them transferred to a deleted mail folder.

The world's largest software company, which has more than 360 million Hotmail users, said it has restored full email access and recovered content to those who were affected.

Microsoft's online message boards and Twitter were abuzz with complaints Sunday about the Hotmail glitch. Microsoft said on Monday it was still investigating the root cause of the problem, which started four days ago.

Beginning on Dec 30, we had an issue with Windows Live Hotmail that impacted 17,355 accounts, Chris Jones, a Microsoft executive, said in a company blog on Monday. Customers impacted temporarily lost the contents of their mailbox through the course of mailbox load balancing between servers.

He said Microsoft will take steps to prevent a similar occurrence. We're very sorry for the inconvenience this may have caused to you, our customers and partners, he added.

(Reporting by Bill Rigby, editing by Maureen Bavdek, Gary Hill)

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