Although the first fault was reported on the evening of Tuesday, March 5, Microsoft is still in the process of repairing full functionality of users’ inboxes.
Despite rumours of similar faults on SkyDrive, Microsoft has declined to comment.
Apologies appeared on the Outlook blog, without offering any indication of when the system will be restored.
While Hotmail users are moved to Outlook for a temporary solution, the world leading software developer is still confident about its services.
David Law, manager at Microsoft, wrote on the company’s blog: “When we say that we're confident Outlook.com is the best email choice for consumers, we mean it.”
He described the forced migration as “seamless and instant”.
Microsoft reported in late February on Outlook’s growth as the global dominator among email options.
After Outlook’s new features were launched last summer, the user count increased to six million new subscribers within six months.