Answer
This is typically a DNS resolution problem specific to the outbound path from the middle tier/Connect pods, often because the receiving 3rd party's own network doesn't accept or properly resolve requests referencing their domain name from outside their network. As an immediate workaround, use the target's IP address directly in the Routing Address instead of its hostname; the durable fix is for the 3rd party to correct their DNS/hosts mapping so their domain name resolves properly for external incoming requests, not just internally.