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.
More than 430 million people use Microsoft 365 apps according to Microsoft’s website.
The software 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.
Government agencies and services, including the Singaporean government and Australia’s spy agency, have also said they have been affected.
Several GP practices across the UK have warned of potential problems, with some instructing patient to make urgent queries over the phone instead of by email or web form.
