[Exim] Routing to a host before using smarthost

I have a mailserver running exim 3.35 (lets call that HostA) for a LAN
which typically uses a smarthost (lets call that relayhost) using SMTP
authentication to relay all outgoing messages, unless the mail
is local.

I've recently added another machine (lets call that HostB) inside the LAN
which I'd like to be able to receive messages from HostA. HostA and HostB
share the same domain name. The domain name is one which the outside world
does not know, only the internal LAN.

Whenever I attempt to send mail to HostB from HostA, it ends up being sent
to the relayhost, which bounces the message back to the sender on HostA
since it perceives the domain name as unresolvable.

How do I get HostA to send direct to HostB, rather than running it through
the relayhost or thinking the receiver is local? Any help is much
appreciated.

-Don