When decommissioning Exchange 2010 Hub servers, you sometimes run into a small issue where you forget to remove the server from all your send connectors and you are presented with the error below:
The error is self explanatory. Head over to the Exchange Management Console (EMC) -> Expand Organization Configuration and click on Hub transport. Click on Send Connectors on the right hand side and go through each of them and remove the HUB server.
Hope it helps.