[exim] weird reject messages

Hi there,

I have had this 1 error coming up a couple of times on a firewall.

2005-05-04 15:16:30 1DTJjq-00016m-Mh lowest numbered MX record points to
local host: xxxxxx.co.za
2005-05-04 15:16:30 1DTJjq-00016m-Mh == xxxxxx@??? R=dnslookup
defer (-1): lowest numbered MX record points to local host
2005-05-04 15:16:30 1DTJjq-00016m-Mh ** xxxxxx@???: retry timeout
exceeded

The problem is this mail server handles emails for several other clients and
when they try send email via this firewall to the xxxxxx.co.za domain they
get this message.

I think it's because the xxxxxx.co.za domains resolves to an ip address that
the firewall has, this IP address is then natted using iptables to a
different mail server internally. Is there any way to rectify this or
disable exim from doing this and just to pass the mail on?

Thanks,
Michael