Microsoft on Saturday apologized for a three-day partial outage of Outlook.com and said the email service was back up and running, only to note hours later that problems still plagued some customers.
Early Saturday, the Redmond, Wash., company said it had finally resolved Outlook.com's issues, which stemmed from a failure in a caching service of Exchange ActiveSync (EAS), the popular synchronization service widely used to sync smartphones and tablets with company email, contacts and calendars stored on Microsoft Exchange Server systems.
"We want to apologize to everyone who was affected by the outage, and we appreciate the patience you have shown us as we worked through the issues," Microsoft said in a note appended to its services status board.
On Wednesday, Outlook.com, the SkyDrive cloud storage service and the Peoples contacts application suffered partial outages that began around 10 a.m. ET. While the Peoples problem was fixed about five hours later, SkyDrive's was not fully resolved until Thursday around 4 p.m. ET.
Some users of Outlook.com, however, were unable to access email on mobile devices that relied on EAS -- a category that includes iPhones, whose iOS uses EAS for synchronization -- until around 4:30 a.m. ET Saturday.
During the outage, Microsoft said, the cache service failure "caused these devices to receive an error and continuously try to connect to our service. This resulted in a flood of traffic that our services did not handle properly."
Microsoft said it had already taken steps to prevent similar problems in the future. "[We] have made two key changes ... one that involved increasing network bandwidth in the affected part of the system, and one that involved changing the way error handling is done for devices using Exchange ActiveSync."
The cache flood problem Microsoft described sounded reminiscent of the trouble earlier this year that the company documented in iOS 6.1-powered iPhones and iPads which affected not only on-premise Exchange servers within enterprises, but also Microsoft's own infrastructure, including its Office 365 subscription service.
But Outlook.com's problems were not completely behind it Saturday morning. Around 2:30 p.m. ET, Microsoft again logged a problem on the status board, saying, "A small percentage of mobile users may experience intermittent issues while syncing email."
Microsoft said the problem was resolved at around 8 a.m. ET Sunday.
The Outlook.com outages were an embarrassment to Microsoft for multiple reasons, including a boast the week before that its Office 365 cloud-based service exceeded 99.9% uptime each of the last four quarters, and new attacks against rival Google Gmail in another run of its "Scroogled" campaign that kicked off Aug. 9.
Microsoft touts its own Outlook.com as an alternative to Gmail.
In fact, the latest Outlook.com outage was the second this year within weeks of a new Scroogled attack. In mid-March, about five weeks after a different Scroogled round, Microsoft's online email service went dark for about 15 hours.
Outlook.com has logged numerous interruptions in the last 60 days, including on June 14 and 27; July 1, 2, 3, 11, 12-14 and 25-26; and the event that began Aug. 14.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed . His email address is firstname.lastname@example.org.