Microsoft on Saturday provided a detailed rundown of an internal error that triggered an outage of its Outlook.com service last week. Although the full impact of the outage is currently unclear, Microsoft said that most users experienced “a few hours” of downtime, while some users on mobile devices might have been unable to access the service for a few days.

“We want to apologize to our customers who were affected by the outage on Outlook.com this week,” Microsoft explained in a detailed problem report on Outlook.com. “We have restored access to all accounts and have made changes so that the service will be more resilient in the future.”

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Outlook.com went dark for the few hours Microsoft noted last Wednesday, but access problems via mobile devices persisted for some users over the next two days as well. Microsoft attributed the issue to a failure in a caching service that interfaces with devices using Exchange ActiveSync (EAS), including most smartphones.

“The failure caused these devices to receive an error and continuously try to connect to our service,” the company explained. “This resulted in a flood of traffic that our services did not handle properly, with the effect that some customers were unable to access their Outlook.com email and unable to share their SkyDrive files via email.” To restore service, Microsoft temporarily blocked EAS access, stabilized the system within “a few hours,” and then began restoring EAS access for mobile devices. This meant that “some customers remained impacted for a longer period of time,” Microsoft explained.

Microsoft further explained that it made changes to “harden” its systems against a similar collapse in the future. It increased network bandwidth for the impacted part of the system and changed how EAS handles errors.

“We realize that we have a responsibility to the customers who use our services to communicate and share with the people they care most about, and we apologize for letting those customers down this week,” the firm added.

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