Mac OS X Server 10.6 Block Domain from Reaching Server

I am running a Mac OS X 10.6 Server, but as of recently I get a huge amount of requests from a certain domain trying to log in using one of my accounts effectively locking it out after five failed attempts (password policy).

Therefore my questions:

Is there a way to tell the server in general to deny requests from certain domains entirely?

If not generally, is there a way to tell the Mail Server (or postfix, if only available through command-line) to blacklist domains from reaching this server?