Recipient Validation on CAS - Not ever going to work?

Good day sir/madame,

We have done a few Exchange 2013 migrations recently.  All seemed to have worked great. 

We are using Spam Experts for one of the clients and when checking the "Valid Mailbox Count" it was around 148 instead of the 17 it should be. 

I telnet'ed to the box and did the "mail from/rcpt to" with an invalid user and it came back as "Recipient ok". 

I installed the antimalware script an mentioned in other places but not realize that it has to be on an edge transport server which has not been setup. 

This seems ridiculous that there's no build in recipient validation by default. Some people have mentioned to have it validate with AD, but i have not seen any clear instructions. 

If someone could be so kind as to point me in the right direction it would be greatly appreciated.  

Thanks in advance

TL;DR - Need recipient validation on 2013 CAS.