1- Create a new DL and add the users who will be blocked to send mail to Internet
2- Create a New transport rule on Hub Transport Server
Open EMC, Click on Organization Configuration, Hub Transport server,
Click on New Transport Rule in right hand Side to Action pane, to create new Transport Rule
In Name Type Transport Rule Name As “Block External Dilivery” in comments you can type information such as requester or realted information for record purpose, check the box enable rule to enable th the Transport rule as soon as it is created.
“From a Member of Distribution List”
“Sent to Users that are inside or outside the organization, or Partners”
Next click on Distributons list and slect the DL to apply rule on,
Click on Inside the organization you will see the below popup window
Select Outside the organization
Next click on “Send Rejection Message to Sender with Enhanced status code”
Defile the message and code.
And Specify Rejection Messages “ You Are not Authorized to Send email to receipents outside of this orgnization”
You can add any exception if you need any on Next Screen,
Click on New to finish.