Disable unauthorized SMTP Relay within Domain

Hi All,

We have Exchange 2013 SP1 infrastructure on top of Windows Server 2012 R2. We have a DAG and CAS NLB clusters.

Anyone can relay Emails from my SMTP to all users within my domain without authentication.However no one can send emails outside my domain (external Emails without authentication).

All these emails are sent through Default Front end Receive Connector, and we cannot Disable Anonymous Users from connector security settings as by doing this we will not receive emails from External World. Please refer the attached snapshot.

Need your suggestion to implement SMTP security to restrict Unauthorized SMTP Relay.

Thanks

VN