[exim] Getting rid of local_domains

Hi,

I'm trying to get rid of the local_domains file, because I consider its
maintenance a redundant process. There should be three ways to do so:

1.)
A domain is a "local domain", if a nameserver answers the local host for
mx_any. Unfortunately, DNS records may be spoofed, so only trusted
nameservers are to be queried.
I manage about 200 domains which are all in the same nameserver which is
dedicated to these domains (i.e. it does not answer queries for other
domains). Is there a way to restrict mx_any to a special nameserver, or to
do some trick with dnsdb?
(It is not possible to set the private nameserver in resolv.conf because it
wouldn't answer other queries; this is needed when sending mail out.)

2.)
I have configured /etc/exim4/aliases like "localpart@domain:mailbox". Then,
a domain is a "local domain", if there is a key in the aliases file that
ends with @domain. Is there a way to do this dynamically in Exim?

3.)
The last resort is to force local deliveries for all unauthenticated mail,
and to see if it fails (because the recipient is not matched in
/etc/exim4/aliases). This would cause a "Unroutable address" message instead
of "Relay not permitted", but both is 550. Here, the problem is that I'm
not sure how to restrict the dnslookup routers to authenticated
connections.

Maybe, there's a much easier solution, or there's a reason to keep the
local_domains file. I'd be happy to see some comments on this.

Thanks,
Peter