"This spike was significant enough before it was mitigated that it caused our safeguards to come in to place for a large number of servers in this part of the datacenter," he wrote.
As a result, access to mailboxes on these servers was shut down, and the failover process was blocked.
"Based on the failure scenario, there was a mix of infrastructure software and human intervention that was needed to bring the core infrastructure back online. Requiring this kind of human intervention is not the norm for our services and added significant time to the restoration," he wrote.
Microsoft is taking steps to ensure this scenario doesn't materialize again, according to de Haan.
De Haan didn't say how many people were affected by the outage and the company hasn't responded to a request for that information. The site that shows the status of its online services states that it lists problems that affect a "large number of customers."
He also didn't address the three straight days of maintenance work on Hotmail's Calendar component that began Monday and that the company warned could slow down the viewing and editing of calendars and trigger error messages. Users could also have experienced delays when syncing mail, contacts and calendar entries to their mail applications or phones, according to a status update.
Juan Carlos Perez covers enterprise communication/collaboration suites, operating systems, browsers and general technology breaking news for The IDG News Service. Follow Juan on Twitter at @JuanCPerezIDG.