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.
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,