This KB will show to create External mail relay In Exchange Server 2013 which will allow applications to email external recipient on the Internet.
To start, Open EAC and to Mail Flow -> receive connectors
Select Server and click on the + sign
In the Receive Connector window type the name of the connector and select Front End Transport and select custom In the type section.
In the network binding select the IP addresses you need to connector to accept connection on
In Remote network settings, Add the servers \ application that needs to Relay and click next and Finish
Click on the edit connector and go security
In the security settings use the settings below.