Wouldn't this be far easier accomplished by using the mydestination setting? That's how you control which hosts you accept mail for.

It's also a good idea to have a local caching resolver running anyway, especially if you have multiple machines in your local network. I use Bind, and have it forward unknown requests to my ISP. Those are then cached locally.

    forwarders {
        1.2.3.4;
        2.3.4.5;
    };
[download]

I can also heartily recommend implementing dns blacklists in your smtp daemon. I'm very, very happy with zen.spamhaus.org, which drops about 75% of incoming spam. I also use some of the rfc-ignorant.org blacklists, but haven't really seen much benefit of it.

Top it all off with bogofilter or another (bayesian) spam filter, and email life is good again. I see no spam in my inbox, and only have about 5 to 10 emails per day that the spam filter couldn't classify. I can live with those numbers!