How to stop Dictionary Name attack

[Belaird]

Trying to find away to change exim, mailscanner, or spamassasin to stop a sender from doing a dictionary name attack against a domain.
My default setting is :fail:, but they are stuffing my queue with thousands of bounce emails which are undeliverable. I've also notice they seem to be using 2-3 sending domains to determine allowable email addresses, or do DDoS

[dalem]

http://www.configserver.com/free/eximdeny.html