Numerous users of Outlook.com were unable to access emails via the service on Monday after they were locked out of their accounts.
The problem left certain users unable to log onto the online email service via desktop or mobile from at least 10am until at least 5pm GMT. There are also reports of sluggish loading on the Outlook.com site from those able to log into the service.
Microsoft confirmed there is a problem with Outlook.com via its Microsoft Support Twitter account, which at about 2pm GMT posted "Hotmail/Outlook services are down and we are currently looking into the issue". The account posted a message saying Microsoft is "continuing to investigate" the problem at about 4.20pm today.
However Microsoft had not responded to a request for comment about the outage and was yet to acknowledge the problem on its Outlook.com service status page at the time of publication.
There are reports that the problems are only affecting Outlook.com users who upgraded from Hotmail.
While the cause remains unconfirmed Microsoft last week announced it is to closing Hotmail and migrating the hundreds of millions of people still using the service to Outlook.com.