Microsoft's Exchange Online service went offline for several hours on Tuesday, preventing users from sending or receiving emails through Microsoft Outlook. The outage did not affect all users, but reactions on Twitter and in Microsoft forums suggest a large number of customers were unable to use Outlook.

Although Google's Gmail has a firm grasp on individual email accounts, Microsoft Outlook is still widely used in the business world. In fact, enterprise customers currently account for more than half of Microsoft's revenue. The Exchange Online service is currently available as a standalone product or as a part of Office 365, a subscription-based productivity suite that provides access to Microsoft Word and related programs for a monthly fee.

Exchange Online is promoted with a number of features, including tools for easier collaboration between business associates as well as reliability. The service states that it guarantees 99.9 percent uptime. While the outage may not disprove that claim, customers are upset that the supposedly reliable service was offline for such a long time. The outage began shortly after 9 a.m. Eastern time, and functionality was not completely restored for all users until 6 p.m.

When complaints started pouring in, Microsoft responded by saying that some Outlook users may be experiencing "delays" and that users should consult the status dashboard in the Office 365 package for more information. However, many users reported that the dashboard did not reflect the outage and claimed that the service was operational.

Details from Microsoft have been scarce so far. It declined to comment on how many users were affected, and the little information available about the cause of the outage comes from a post by support agent David Zhang in the Office 365 forum.

"Investigation determined that a portion of the networking infrastructure entered into a degraded state," Zhang writes. "Engineers made configuration changes on the affected capacity to remediate end-user impact."

Microsoft is currently competing with Google for control of the cloud market. Services for cloud storage, productivity and collaboration are available from both companies. Microsoft's related Lync Online service also suffered an outage earlier in June, and if these suspensions of service continue, the company is sure to lose ground to Google in the battle for the cloud.

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