Older hosted Exchange mailboxes, are starting to fail Autodiscover in newer Outlook installs. Below is a registry fix which gets them working. Apply the change and then reboot, then try it again.
Windows Registry Editor Version 5.00 [HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\AutoDiscover] "ExcludeHttpsRootDomain"=dword:00000001 "PreferLocalXML"=dword:00000000 "ExcludeHttpRedirect"=dword:00000000 "ExcludeHttpsAutodiscoverDomain"=dword:00000001 "ExcludeScpLookup"=dword:00000001 "ExcludeSrvRecord"=dword:00000001 "ExcludeExplicitO365Endpoint"=dword:00000001