More than 430 million people use Microsoft 365 apps according to Microsoft’s website.
The full scale of the outage has not been confirmed, but monitoring website Downdetector had 140,000 people from 74 countries report issues with Outlook on Thursday evening.
The real figure affected is likely much higher because Downdetector relies primarily on users self-reporting.
Microsoft 365 is also used by companies worldwide and is expected to have a knock-on effect to other services.
Tech giants OpenAI and Anthropic have both reported problems as a result of the outage.
ChatGPT-maker OpenAI said it was investigating an issue affecting its connections to Microsoft’s services, which has caused some tools to not work as expected.
From city governments in the US to Australian spy agencies, disruption across the globe has been widespread.
Courts from Los Angeles to Texas warned that communications might be delayed.
In Canada, the public broadcaster CBC reported that a healthcare network running several hospitals in Toronto was experiencing email issues, but that patient care was not affected.
The government of Japan also said its shared IT platform had been disrupted.
Across the UK, GP surgeries, pharmacies, police and schools have been affected by the outage.
Some GP surgeries have urged patients not to submit messages via their website while the problem remains, while several large police forces are urging members of the public to call if they want to make a report.