Exchange 2010 550 5.7.1 unable to relay

First, I wouldn't have touched the default Receive Connector. In these instances I always create a new Receive Connector. Therefore I would put things back as they were. You don't want an authentication enabled receive connector on port 25 exposed to the internet - that is asking for authenticated user attacks.

Then create a new Receive Connector. You will need to use PWS for this because the ECP GUI currently only creates backend types, whereas you want a frontend. Lock it down to the specific IP address of the devices. You then need to enable the authentication types and Exchange Users under Permission groups - basically the same as Client Frontend Receive Connector, but on a different port and probably without TLS.

Then restart the MS Exchange Transport service.

Exchange is already setup to do this - that is what the Client Receive Connector is for. However the client receive connector uses TLS. Therefore if your application can use TLS, switch to the port on the Client Receive Connector and set the application to authenticate.

Otherwise just setup a new Receive Connector to listen to port 25. Configure the remote IP address to be the IP address of the host that needs to send email and copy the settings of the Client Receive Connector for everything else.