Turn off the scoped option on Send Connector

When a send connector has the “scoped” option enabled, the connector can only be used by Hub servers in the same Active Directory site. If it is not selected, the connector can be used by all Hub servers in the entire environment. 

You can use the EMC to configure this in Organization Configuration>HubTransport>Send Connector, right click, properties.

 

Alternatively you can adjust this in Powershell using:

Set-SendConnector <sendconnectorname> –IsScopedConnector $true/$false