Microsoft Corp's product suite that includes Word and Excel was down for more than 15,000 users on Monday, according to outage tracking website Downdetector.com.
"We're investigating an issue with accessing Outlook on the web," Microsoft tweeted.
The company did not immediately respond to a Reuters request for comment on the outage reports on Microsoft 365.
Microsoft services have faced at least three outages since the beginning of the year.
Downdetector tracks outages by collating status reports from several sources including users.
(Reporting by Jaspreet Singh in Bengaluru; Editing by Devika Syamnath)