Exim accepting mail from specific hosts

[Adam Dawes <adawes@clandestino.com>]

Hi,

I've implemented a spam service where a provider is filtering all my 
domain's mail before it hits my server. I want to lock down my mail 
server so it only accepts mail from those machines to prevent spammers 
from mailing directly to my host and doing directory harvests.

What do I need to stick in my exim.conf to accept mail from just those 
hosts that will be processing my mail?

thanks,
Adam