Microsoft has apologised for an Outlook and SkyDrive disruption which lasted up to three days for some users.
The outage was first reported on 14 August, with a large number of users reporting problems accessing their Outlook email and SkyDrive data for a number of hours. Others were still experiencing issues until early on Saturday.
Microsoft have since apologised for the disruption. The company has said that it was the result of a failure in a caching service applied to devices using Exchange ActiveSync, which includes most smart phones.
The failure caused the mobile devices to receive an error and continuously attempt to connect to the Outlook service. This resulted in a flood of traffic that crashed Microsoft's servers.
To avoid another flood of traffic, engineers then had to slowly restore access to Exchange ActiveSync, leading to the extensive outage.
"We want to apologise to our customers who were affected by the outage on Outlook.com this week. We have restored access to all accounts and have made changes so that the service will be more resilient in the future," Microsoft said, in a service status update.
"We realise 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 apologise for letting those customers down this week.
"We have learned from this incident, and have made two key changes to harden our systems against future failure – 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."