The MultiSendcon installer normally creates a send connector in Exchange automatically. If it fails (e.g. due to permissions or Group Policy restrictions), create it manually using the Exchange Management Shell:
new-sendconnector -name MultiSendcon -Addressspaces "SMTP:*;100" -port 2500 -SmarthostAuthMechanism none -smarthosts 127.0.0.1This creates a connector named "MultiSendcon" that routes all SMTP traffic (*) to 127.0.0.1 on port 2500 (the default MultiSendcon listening port) without authentication.
After creating the connector, verify it appears in Exchange Management Console under Organization Configuration > Hub Transport > Send Connectors. Change its cost from 100 to 1 to make it the preferred route.