Microsoft is currently having issues with Exchange Online, part of its Microsoft 365 cloud service. The main presentation currently is with delayed email or email that is bouncing completely and failing to send. This is affecting both inbound and outbound email.
The Office 365 support account on Twitter notes:
We’ve identified that a subset of infrastructure is unable to process requests for users served through North America, which is resulting in a message backlog and causing delivery delays or failures. More information can be found in the admin center under EX237654.
— Microsoft 365 Status (@MSFT365Status) February 3, 2021
As noted in the last tweet, the issue appears to be due to bad hardware, which Microsoft will hopefully be able to resolve in a short time.
DownDetector however suggests is has been ongoing for at least 2 hours now, so Microsoft better get a crack on.