I’ve seen scattered reports today that some mail to the Outlook 365 servers is failing. This has been confirmed by ZDNet. Only folks with a Office 365 account can log in and see the status messages, but there are some folks on the mailop list posting updates from the website.
Attempts to mail to affected domains results in this response:

421 4.3.2 The maximum number of concurrent server connections has exceeded a limit, closing transmission channel

MS is aware of this and is working to restore service. Senders should not remove addresses that bounce today with the 421 4.3.2 error message from Outlook hosted domains.

Microsoft • outage • outlook