Thursday's problems apparently began at around noon Eastern Time, affecting users in North America, and at close to 5 p.m. seemed still unresolved, prompting frustrated IT administrators to post complaints on various discussion forums.
A Microsoft official posted the following message to some of the discussion threads: "As of this morning, we are investigating reports of intermittent mail flow issues affecting Exchange Online users served from the Americas data center."
The outages affected 1 percent of all Exchange users, Microsoft said in an e-mail shortly after 5:30 p.m.
"Currently all new inbound and outbound messages are delivering as expected; however there is a small percentage that remain in the queue awaiting delivery. Full resolution of the issue is expected shortly," the company said.
A spokeswoman said Microsoft doesn't disclose how many Exchange Online users there are. In October last year, Betsy Webb, general manager of Microsoft's Online Services division, told IDG News Service there were 40 million paid end-user seats of Microsoft Online Services, of which Exchange Online is a part.
The latest problems come a day after Microsoft trumpeted in a press conference that the city and county of San Francisco chose Exchange Online over competing cloud-hosted options from Google and IBM.
Two days ago, Microsoft was still doing damage control after the series of technical problems that rocked Exchange Online last week.
"I want to again sincerely apologize for the inconvenience last week's issues caused you, our customers and partners," wrote Dave Thompson, corporate vice president of Microsoft Online Services and chief engineer of Microsoft's Business Productivity Online Suite (BPOS), which includes Exchange Online.
Microsoft is getting ready to release a major upgrade to BPOS, called Office 365, before the end of the year. It is designed to mount a stronger front against Google's Apps hosted collaboration and communication suite.
IT professionals get nervous when they hear about performance and availability problems with cloud-hosted services. CIOs, IT managers and network administrators have little or no control over the downtime of hosted applications, but that doesn't stop end users from complaining loudly to them. IT departments sometimes campaign hard for their companies to adopt cloud applications, and their reputations are tied to how those applications perform.
Channel partners that resell and implement these applications also often find themselves in the crosshairs when outages occur.