The workaround is to turn Modern Authentication off for the clients that are experiencing the issue. Do this by by implementing the following registry key (DWORD (32-bit) Value) after closing Outlook: HKCU\SOFTWARE\Microsoft\Office\16.0\Common\Identity\EnableADAL Setting this key to 0 disables Modern Authentication within the Outlook client. No reboot is required. After opening Outlook back up, you will ..