In a notice, Microsoft said users were unable to connect to the Exchange Online service through a number of protocols.
It said traffic was being rerouted to alternate infrastructure to address potential packet loss and to rebalance the load that was not being processed as expected.
4 hours into Office365 outage, nothing on the public O365 portal. https://t.co/EYvqj3f67v— Kevin Beaumont ? (@GossiTheDog) November 26, 2018
The issues with Exchange Online come a week after the company's cloud customers found themselves locked out of their accounts.
With regard to the current outage, Microsoft said it was also continuing to review its network infrastructure to see if other factors were adding to the problem.
The company has now released details about last week's outage. It said there were three root causes, two of which were issues on the multi-factor authentication front-end server, caused by a code update.
Details of the issues are available here.