How to add top level domain filtering in Exchange 2010
To add a top level domain filter in Exchange 2010 open up the Exchange Management Console and select “Organization Configuration” and “Hub Transport” from the menu tree. Select the Anti-Spam tab and click on Sender Filtering and click properties on the right hand side. Select the Blocked Senders Tab and click add. In this example I am going to block all emails from a .ru (russia) domain address.
In the add blocked senders dialouge box, selct domain and add: ru and tick the include all subdomains box. Click OK.
You have now added a top level domain filter for .ru domains.
Below is an example from the agent logs of a .ru email being blocked as spam via the sender filter.
firstname.lastname@example.org;,email@example.com,1,Sender Filter Agent,OnEndOfHeaders,RejectMessage,554 5.1.0 Sender denied,SubdomainMatch,mail.ru,