Force Outlook to resolve to IP instead of hostname

I have a dual-domain environment at work. DNS, RRAS, etc, are set up to
handle network bridging. I have full communication across both domains and
IP ranges.

Both domains have their own Exchange servers and, of course, DNS does not
work across the network bridge although each domain has its own DNS servers.

I need to be able to set up users on Domain1 to access their exchange
mailboxes on Domain2. When I set up their second account to connect to
Domain2 I put the IP of Domain1.Exchange and it finds the server. However,
Outlook then resolves the IP to the hostname and stores that. Of course,
this will then break the connection as I can't resolve via DNS across the
domains.

Questin- how do I (or is it even possible to) force Outlook to connect using
the IP and not hostname of an Exchange server?