[exim] Do not accept messages to <someuser@myname.company.or…

Hi all,

My exim acts as a mail relay for several internal domains. It does not
host any local users, and all in all it's working extremely well (I have
some acls and spamassassin that cut down spam quite a bit)
Today I checked with a relay test ("telnet relay-test.mail-abuse.org"
from your exim box) and that darn tool tried to send mail to
<user@???>; sure enough exim belched with a 451 "try
later" message.
Debugging via -bt revealed that "remote host address is the local host"
(as if I didn't know).

I installed an address rewriting rule that rewrites
"myhostname.company.org" to "company.org" and that worked fine...until
the test decided to reverse-lookup my IP which revealed the name the ISP
gave to our ADSL address and sent mail to "adslxyz.isp.com". I now could
write a 2nd rewriting rule...okay.
But if some smartass came and created some dyndns entry which again
points to my host? You see the point.

Question: Is there a simple acl that would deny all mail that would end
up at the local host? Or a catch-all rewrite rule that redirects all
"local" mail to one of the internal domains?

Considering the quality of thought and craftsmanship buried in exim code
and documentation I believe I'm missing something obvious...<sigh>

--
Ben