[Exim] IP Masquerading and Exim

Hello

I am currently setting a new email system up in my company, and have so
far sucessfully managed to get the mail server up and running, allowing
mail to be sent and recieved from any machine on the local network.
However, I have an office internal network masquesaded behind the mail
server. This is the current setup:

local network: mh.9a.co.uk

mail server: rem.mh.9a.co.uk; set to handle mail for mh.9a.co.uk

As mentioned, by setting the localdomains to mh.9a.co.uk, I can deliver
mail from the mh.9a.co.uk networkno problem, via SMTP and POP.

However, my problem is with sending mial from the internal masqueraded
network. Whenever I try, I get an error in exim -

...failed to find hostname from IP address..

I assume this is because it is trying to do a DNS lookup on a private
IP. The only way around this I have found is to add the parameter
+alloe_unknown to host_accept_relay. Howvere, this essentailly gets rid
of any security.

So, Is there something fundemental I'm missing, or is there a way to
stop exim doing DNS lookups on certain domains

thanks

David Rigby