DNS resolution and default SMTP route

[et.favre at free.fr]

Hi,

I want to deploy a DNS solution using BIND on a private WAN network not
connected to the Internet. The network is composed of several domains,
but each domain can talk to each other only through specific and
limited gateways (SMTP MTA, HTTP proxy ...).
How could I configure BIND on a domain to resolve every SMTP domains of
the zone with their real IP address, and for all other SMTP domains,
the resolution should not give me the real IP address but the IP
address of the gateway ?
In other words, I would like the DNS to resolve every known name and to
give me a default value if the name could not be resolved instead of an
error message. And it's the gateway which will be able to forward the
SMTP message either to the correct recipient, either to another gateway
dependaing of the DNS resolution.
I don't want to get the correct IP address from the first MTA because
the topology of the network won't let me join the destination address.

Thanks for your responses.