All agents can run iperf tests with other agents. Generally when you start an iperf test, the NetBeez dashboard first instructs the destination agent to start iperf in server mode and then tells the client to start iperf in client mode.
If the destination is not an agent, but an external server, the dashboard will only instruct the sending agent to start the transfer.
In this case you have to check what ports the external server is expecting connections for TCP or UDP transfer. Another cause of a failed iperf test is a firewall in between the source and the destination blocking the TCP/UDP stream.
Hope that help.