As of yesterday morning (1/29/20), Outlook 365 would not sync my AT&T inbox. Other accounts I have worked fine. After an hour on the phone with Microsoft Support, he found that Norton Firewall was blocking that account by disabling the firewall. I went to Program Control in Norton and deleted that one for Outlook then loaded Outlook again and this time it worked with the firewall enabled. AT&T does use a new Email Security Key but Norton was allowing syncing of the inbox key before yesterday so I don't think that is a cause.
I just wanted to post this here in case someone else encountered the same issue.