Tools such as messaging app Teams, Microsoft 365 (formerly Office) and artificial intelligence (AI) chatbot Copilot have also been hit by "connectivity issues".
Microsoft stated that the interruption in its services was not caused by a cybersecurity incident, but that work to correct the flaw has been unsuccessful.
In an update Friday morning, Microsoft said it is seeing "marginal success" in trying to resolve the issue - but that it is still unclear what caused the failure.
Previously, the company had stated that the problem was caused by an "unexpected unavailability" in a large number of the company's servers - powerful computers that process tasks such as work tools and other applications. This unavailability would, according to the company, have overloaded the remaining servers.
Microsoft also says it will provide details on the cause of the problem as soon as it is resolved. "We will continue to treat the incident with urgency and priority until a full resolution," he says.
The BBC found that there is no evidence that the interruption was caused by a cyber attack.
For international organizations, these tools are essential for communicating and collaborating with colleagues around the world.
"We appreciate our customers' patience as we work to restore service as soon as possible," the company said in a statement.
The company later warned that some services that had been restored "may become unavailable again", adding that customers "in any region" could be affected.
See below what we know about the failure in Microsoft services:
What services are affected?
The Outlook email service was the most affected by Microsoft's failure — Photo: Getty Images via BBC
A Microsoft spokesperson told Reuters that the problems were focused on Exchange Online, the service that manages Microsoft 365 mailboxes.
Among the issues, users may not be able to view certain files or details in their calendar applications, Microsoft said. Copilot (the company's artificial intelligence service) may also experience failures.
Some users are able to use Teams normally, both to send messages and to organize meetings and consult calendars.
How many people were affected?
The Teams service and the Microsoft 365 suite were also affected — Photo: Getty Images via BBC
The exact number of affected users has not yet been confirmed by Microsoft.
According to the internet failure monitoring website, 140,000 reports of problems with Outlook around 3pm (Brasília time) in 74 countries on Thursday night (12).
The number fell to around 34,490 in the late afternoon, according to website data cited by Reuters. At 7:47 pm, the site also registered around 13,000 reports about Microsoft 365 and 763 about Teams.
The number of affected users could reach millions on a global scale. According to Microsoft's own website, more than 430 million people use Microsoft 365 applications.
Because Microsoft software is used by companies around the world, there could be a ripple effect in other countries.
AI services with connections to Microsoft products were also impacted.
OpenAI, responsible for ChatGPT, reported that it was investigating an issue with integrations to Microsoft services, in which new connections could be interrupted, causing the tools to not work as expected.
For its part, Anthropic said it has identified an issue with a top-tier provider that affects its connection to Microsoft 365.
Government agencies and services, including the Singapore government and Australia's spy agency, said they were affected.
In the UK, several GP practices have warned of possible problems. Some have instructed patients to ask urgent questions over the phone rather than sending emails or filling out online forms.
What is the cause of the problem?
Microsoft denied that the failure in its services was caused by a cyber attack and attributed the problem to "high load" on its infrastructure.
The term refers to situations in which services are dealing with "a much greater volume of stress" than usual.
In the case of Microsoft's failure, a large number of servers became "unexpectedly unavailable," according to the company, which overloaded the remaining servers.
This Friday morning, Microsoft said it had seen "marginal success" in efforts to fix the global flaw.
In its last statement until this report was updated, Microsoft stated that it continues to monitor efforts to resolve the issue in all regions and evaluates the server status to determine what needs to be done.
In a previous statement, the company had said that correction attempts were "not having the expected effect."
What do the experts say?
Troy Hunt, an Australian cybersecurity expert, told the BBC that the outage could have far-reaching consequences.
"Microsoft is an essential part of the internet infrastructure and reminds us of the importance that email still has," he says.
"This disruption affects not only many people's ability to work, but also their ability to communicate about appointments, buy things that need verification, create or access accounts and services", he points out.
Paul Spain of New Zealand-based consultancy Gorilla Technology says that with emails and offline services, some companies have practically "lost a business day".
