Outlook 2019 onwards seems to have a problem where it will aggressively default to using Microsoft 365 for all exchange servers even 3rd party one these reg keys seem to disable that behaviour.
Windows Registry Editor Version 5.00 [HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover] "ExcludeExplicitO365Endpoint"=dword:00000001 "ExcludeHttpsRootDomain"=dword:00000001 "DisableOffice365SimplifiedAccountCreation"=dword:00000001