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.
Background: Exchange selects a send connector by the recipient domain and the cost, never by the sender, which is why a single connector to MultiSendcon replaces several per-provider connectors. See the guide Exchange send connectors route by recipient domain, not sender.
If outgoing mail waits in the Unreachable queue with “A matching connector cannot be found to route the external recipient”, the connector is missing or still disabled: causes and fix.