Expand Global Settings and then right-click over Mail Delivery and select Properties. There is a Recipient Filtering Tab where you can add recipients that mail is to be blocked to.
You then need to enable the option on the SMTP virtual server.
ESM, Servers, , Protocols, SMTP. Right click on the default VS and choose Properties.
On the first tab click “Advanced” next to the IP address, then edit. Enable the option “Apply Sender Filter”.
Apply/OK out as appropriate.