Latest update (5:06 PM PST / 8:06 PM EST): Microsoft has found the cause and is testing a fix.

Still affected: email, Outlook, and calendars.
Mostly working: Teams chat and calls, and files in SharePoint and OneDrive.
What to do: nothing on your side; this is a Microsoft-wide issue, and emails you send will go out once service returns.

Next Microsoft update: by 6:00 PM PST / 9:00 PM EST.
Surf City Systems