Microsoft 365 is used by companies worldwide and was expected to have a knock-on effect on other services. Because the outage hit integrated organisational and communication platforms, many other workplaces were affected. It remained difficult to know how many people had been hit, and one assessment was that a business email outage had rarely taken this long to return to normal.
Royal Mail said some of its online services were unavailable and that email notifications about deliveries may be affected. It said post was still being collected and delivered as normal.
A major global Microsoft outage has disrupted email, workplace software and some public services, with problems lasting more than 20 hours.
Microsoft 365 services, including Teams, Outlook and the artificial intelligence tool Copilot, were experiencing connectivity problems, the company said. More than 430 million people use Microsoft 365 apps, according to Microsoft's website.
Microsoft said the outage was not caused by a cyber-security incident. The company and cyber-security experts ruled out a cyber-attack and attributed the disruption to elevated load on its infrastructure.
A large number of servers became unexpectedly unavailable, the company said, sending traffic to the servers that remained online and putting them under strain. It linked the failure to higher-than-usual stress on its servers.
According to one account of what Microsoft has said, the part of its servers that sets up secure connections was getting swamped, and once that happens the server stops answering.
Microsoft said services were gradually being restored globally, but warned that some users may still face intermittent issues as recovery continues. It said it was focused on addressing the remaining areas of disruption. Efforts to fix the problems have so far been largely unsuccessful, and the company has said fixes were not going as expected.
Outlook appeared to be resuming for some users, but an outage of more than 20 hours is considered a long one. Some people have been able to use Teams messaging and online meetings without issues. Others have been left struggling without Microsoft tools.
Staff working in 111 and 999 services were unable to refer patient details in the normal way, creating a large backlog with the potential for chaos, though emergency and life-threatening call-outs appeared to be unaffected. Some GP surgeries urged patients not to send messages through their websites while the problem remained. Several large police forces urged people to call if they wanted to report a crime.
Much of the NHS calendar is linked to diaries on the Microsoft system, though workflow practices differ between hospitals and trusts. The overall impact on health services remained unclear. Downing Street said it was looking to get a full picture of the effect on government services.
The outage reached well beyond Britain. In Canada, CBC reported that a healthcare network running several hospitals in Toronto had email problems, but that patient care was not affected. SickKids, Canada's largest children's hospital, reported that staff were having trouble with phone calls and emails. Via Rail said passengers may not receive emails about their trips.
Singapore's courts said they were experiencing problems with incoming and outgoing emails and urged people to telephone about urgent matters. Singapore also said some government systems were affected, while stressing that emergency services remained fully operational.
OpenAI and Anthropic both reported problems linked to the outage. OpenAI, which makes ChatGPT, said it was investigating an issue with its connections to Microsoft's services that caused some tools not to work as expected.
Australian cyber-security expert Troy Hunt told the BBC the outage could have far-reaching consequences."Microsoft is a massive part of the internet backbone and reminds us how critical email still is,"he said. The disruption has also hit the ability to arrange appointments, complete purchases that need verification, and create or access accounts and services.