S2S is attempted even for subdomains that are known to belong to external components. This is the case for external components that are not connected and someone tries to send a packet to the external component. Instead of attempting a s2s we should follow the 'failed to deliver' logic.
1. Log into the admin console and create a configuration for an external component
2. From a client send an IQ packet to the subdomain of the external component
3. Check that no s2s is attempted and that the returned IQ error has the error 'service-unavailable'