Microsoft has issued a full and frank explanation for the Outlook.com outage which left many users without email access last week.

Following the problems, Outlook.com Group Program Manager Dick Craddock was forced into an emailed apology to users, offering a personal apology for any inconvenience caused.

"We realise how critical your email is for staying in touch with your personal network and for being productive with the things you need to get done," he wrote.

"And we are truly sorry for any issues you had in accessing Outlook.com.

"We want you to know that Outlook.com has earned a leading reputation as a reliable and trustworthy email experience and to that end, we deeply investigate any issue to ensure that we fully understand the root cause and can prevent a similar occurrence in the future.

"And we pledge to you that we are dedicated to providing the reliable and trustworthy mail experience that you expect."

With many users demanding an explanation for the problem, Craddock was quick to provide a more detailed explanation of what happened last week.

The incident was caused by a failure for some of Microsoft's servers in the functionality that syncs information on some mobile devices using Exchange ActiveSync.

As a result this failure caused these devices to receive an error and continuously retry connecting with our service, resulting in a flood of traffic that some servers did not handle properly, with the effect that many customers on those servers were unable to access their Outlook.com email.

In order to stabilise the impacted servers, Craddock said the software giant temporarily blocked access over Exchange ActiveSync for all of those servers and then worked to restore this gradually.

Because of the significant backlog of traffic that had accumulated and in order to avoid another flood of traffic, we restored access slowly, meaning some of our mobile customers remained impacted for a longer period of time.

"We have learned from this incident, and have made two key changes to harden our systems against future failure," Craddock wrote.

"The most important is in updating the way we handle Exchange ActiveSync traffic to avoid a flood of requests hitting our servers and to ensure more reliable connections and the second involves increasing network bandwidth in the affected part of the system to ensure we have a greater capacity for these requests.

"It is our goal to provide exceptional service to every person using Outlook.com, and I hope you will give us an opportunity to restore your confidence in Outlook.com.

"Your support and dedication to our email service is very important to us.

"And so once again, on behalf of the entire Outlook.com team, I want to apologise for any inconvenience you experience and want to thank you for any patience you have shown us as we worked through the issues.

"We look forward to your continued Outlook.com usage and hope to have the privilege of being your primary personal email service in the future."

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