The exact cause of the outage has not been confirmed, but Microsoft has ruled out a cyber-attack.
The company said it was “investigating reports of users experiencing issues accessing Exchange Online mailboxes through multiple connection methods in the Americas” at 22:00 GMT on Thursday.
It said that the issue was a “portion” of its infrastructure experiencing “elevated load” across multiple regions.
The term refers to when servers are handling a much higher volume of stress than they usually do.
In the case of the Microsoft outage, a large number of servers became “unexpectedly unavailable”, the company said, which sent added traffic to remaining servers, straining these.
Shortly after the initial reports, the company said that its team was “seeing positive results” in its efforts to restart affected systems and was actively monitoring their performance.
But in the early hours of Friday morning, the company said the fixes were “not taking as expected”.