If a computer is in an Active Directory Domain environment with Exchange On-Prem installed, Outlook clients might connect to local Exchange instead of Exchange Online, because they query Active Directory first.
This Registry key will avoid SCP Lookup in Active Directory.
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover
DWORD ExcludeSCPLookup 1
If you are curious, SCP is located here (you can view it with ADSIEdit inside the Configuration Partition):
CN=Services, CN=Microsoft Exchange,CN=Organization, CN=Administrative Groups,CN=Exchange Administrative Group, CN=Servers,CN=YourServer,CN=Protocols,CN=Autodiscover,
References: