[Exim] Exim 3.x - mail hub config?

I've seen tantalizing hints, but I can't seem to get it. I have two problems
to solve:

1. This is a mail hub, so there are no local users. I rewrite addresses like
'john_smith@???' to 'jsmith@???'. I'm doing that with a dbm
lookup in the rewrite rules; if the lookup fails, the address is not
modified. I'd like to verify these addresses and fail those that are not in
the lookup file. I tried setting up a router to do this, but it seems to get
ignored.

2. With address rewriting, I'm re-routing mail to internal hosts. How can I
get an address to fail verification if the domain name is a subdomain of
'abc.com', yet doesn't exist (by DNS lookup, I would think)?

Again, I've seen posts from people who say they've solved all sorts of
similar problems, yet I can't seem to make the leap to my particular
situation. Thanks in advance for any light you can shed.