Disallow Mails to multiple domains in single connection

   I have a custom milter setup on my shared postfix servers that allow
customers to maintain their own blacklists 

A domain admin can simply reject mails at the MTA. ( That gets rid of
all NDR's are backscatter automatically ) 
But that fails when a smtp client sends mails to multiple email ids of
different domains in a single transaction. Can I block this 

Thanks
Ram