Microsoft apologized to all those who were affected by the outage on Outlook.com earlier this week, and explained the reasons behind the issue.
Microsoft said the incident was a result of a failure in a caching service that interfaces with devices using Exchange ActiveSync, including most smart phones. The 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, with the effect that some users were unable to access their Outlook.com email and unable to share their SkyDrive files via email.
In order to stabilize the overall email service, Microsoft temporarily blocked access via Exchange ActiveSync and restored access to Outlook.com via the web and restore the sharing features of SkyDrive. These parts of the service were fully stabilized within a few hours of the initial incident. However, a significant backlog of Exchange ActiveSync requests accumulated as Microsoft's engineeers worked to stabilize access. To avoid another flood of traffic, Microsoft had to restore access to Exchange ActiveSync slowly, which meant that some users remained impacted for a longer period of time.
Microsoft said that it had increased network bandwidth in the affected part of the system, and also changed the way error handling was done for devices using Exchange ActiveSync.
"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," the company said.