[Exim] failed to find host name

Hi,

I am using Exim 3.35 on Debian Woody as a relay for our Exchange server. The
relay is in our DMZ on a private IP and is accessible from the outside via a
mapped public IP address.
I am trying to get it to accept email to the postmaster when the IP of the
server is used in the address (postmaster@???). So far I have
not managed to succeed. All attempts have ended with;
550 - relaying to <postmaster@???> prohibited by administrator
(failed to find host name from IP address).

What am I missing?
Any advice on this would be welcome.

--
Patrick