[exim] ACL's

I have the following redirect router

send2acc:
    driver = redirect
    domains = acc.darkstar.nom.za
    local_part_prefix = +acc
    data = user3@???
    no_more

I would like to limit the domains that can use this router, I have
created a db with domains allowed, but am not sure how to implement.
Should I create another router beforehand to check the domain and then
use 'redirect_router = send2acc' to process the redirect?

Paulo