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
The beast way to apply these on a non admin/standard user account is to create a text file, add these keys then save the file as <filename>.reg then you can run and add these keys without a UAC admin login
A couple of notes:
"ExcludeSrvRecord"=dword:00000000
The ExcludeSrvRecord key needs to not be set or set to 0 i.e. NOT Disabled. or the Autodiscovery will not get the setting from the SRV record
"ExcludeScpLookup"=dword:00000001
Might not be needed as this tries to get the settings from a server on the local domain