Did you try the <a href=”http://blog.maysoft.org/blog.nsf/d6plinks/FPAO-7CNQVF”>DNS Blacklisting</a> and DNS Whitelisting for these ” Unrecognized email Domains”
You can do it the tough way by removing the Internet address from the user’s document for these suspecious users and add a ” Server Rule” to stop any email from going out of the SMTP server if there is no ” Internet Address”.
If you are using an External SMTP Gateway (Like : Symantec BM , IMSS of Trend Micro etc.) you can make a rule to restrict mail sending outside for your domain and also can get back ack mail. check this out ..