[exim] EXIM as bacup MX vs SPAM

Hello everyone,

can anyone help me with the next problem?
I use Exim 3 (Debian Woody) as backup MX mail server for two domains. Is it
possible to give to exim the list of the valid e-mail addresses? Some
spammers trying to use random user name with name our domain and I looking
for way to eliminate it. I Still use the DNSBL(spamhaus.org) and it seems it
is not enought.

Thank's for advice and any kick.

David