[Exim] Relay to another server

I have used sendmail for some years and for many reasons have just
switched to exim.

It is possible to relay all mail from a sendmail box to another
irrespective of MX records.

e.g. I have an exim box as a bastion host (the primary mx according to
DNS) to receive mail and want to forward it to an internal machine on a
private network. that will do local delivery. I have had a good look
through the documentation online but cannot find the correct terminology
for this routing.

If anyone can point me in the right direction I would be very happy.

Cheers
Xander