A software bug caused email queues of some Exchange Online customers in the Americas to clog up on Thursday, leading to delivery delays that lasted in some cases almost seven hours.
The problem, which began at close to noon U.S. Eastern Time and wasn't fully resolved until past 6:30 p.m., initially caused 30 percent of Exchange Online hub servers in the Americas to experience backed-up mail queues.
Most of those affected servers returned to normal queue levels after about an hour, but one server required more extensive repair work, according to a Microsoft explanation posted to a company discussion forum on Friday.
"Microsoft is working to fix and deploy the underlying software problem to prevent the problem from occurring again," the message reads.
Thursday's problem, which Microsoft said affected about 1 percent of Exchange Online users, was the latest in a series of glitches that have affected the cloud email service's uptime and speed in the past week.
Microsoft doesn't disclose the number of Exchange Online end users, but last year a Microsoft official told IDG News Service that there are 40 million paid end-user seats of Microsoft Online Services, of which Exchange Online is a part.
Speed and uptime continue to concern CIOs and IT managers who have signed up for enterprise cloud applications, which are hosted by vendors.
When outages occur, IT staffers field complaints from their end users but have no control over repairs and must wait for the vendor in question to fix the problem, hoping that their job security isn't endangered by the downtime.
Microsoft is far from alone in facing downtime for its enterprise cloud services, as this problem has hit virtually all major cloud software vendors at one point or another.
However, for Microsoft, the timing of Exchange Online's recent problems is particularly bad, because the issues have sandwiched the company's announcement on Wednesday that it won a big contract from the city and county of San Francisco, which chose Exchange Online over competing cloud-hosted email options from Google and IBM.
Moreover, Microsoft plans to release this year a major upgrade to its Business Productivity Online Suite, which includes Exchange Online, called Office 365. With Office 365, Microsoft plans to sharpen the competition against Google's Apps hosted collaboration and communication suite.
A big part of Microsoft's competitive pitch, especially against Google, is that Microsoft has a longer track record in enterprise software provisioning, which the company argues makes it a better choice for CIOs and IT managers looking for cloud-based applications.
Seeing Microsoft struggle with uptime for Exchange Online erodes the confidence it's trying to build in the market, according to Rebecca Wettemann, a Nucleus Research analyst.
"I'd argue that Microsoft should be held to a higher standard than anyone else, and this certainly doesn't bode well for Microsoft's image as a cloud provider, especially as it tries to push customers toward the still-cloudy promise of Azure," she said via email, referring to Microsoft's cloud platform-as-a-service offering for building, hosting and managing applications on Microsoft data centers.
"Email outages for a few hours are frustrating. If my entire productive world is online, downtime has an even more significant productivity and economic impact," Wettemann added.