Outlook 2016 onward can disconnect from Ms Exchange Self hosted. Set this DWORD data to below to force Outlook NOT to call the Office 365 Config Service to retrieve appropriate Autodiscover URLs.

Here is registry fix

1. Open up regedit
2. Navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover
3. Change value of DWORD:ExcludeExplicitO365Endpoint to 1, if key doesn’t exist add DWORD ExcludeExplicitO365Endpoint and then change value to 1
4. Change value of EnableOffice365ConfigService to 0
5. You may need to restart computer after those changes.