Receive Connector External Relay by IP

I am trying to allow a server to relay through Exchange 2013 to an external email address. I added a new receive connector FrontendTranport port 25 and ran:

Get-ReceiveConnector -Identity "xxx" | Add-ADPermission -User "NT AUTHORITY\ANONYMOUS LOGON" -ExtendedRights "Ms-Exch-SMTP-Accept-Any-Recipient"

If I disable the default FrontendTransport connector that is on port 25 then the server will relay, but outside email stop coming in. How can I setup the default Fronendtransport connector on port 25 to allow this internal IP to relay? Do I add the IP in the scope and run the above command on that receiver? Is that safe?

Thanks,

Baze