[Exim] Relay messages to Exchange...

I've been stumped with this for a while now.

We have Exim on a Unix box and an Exchange server on out local network.

Basically we want to support our two domains, domainA.com and domainB.com.
domianA.com's MX record points to our Exim Mail server which can be seen on
the internet. We have set-up domianB.com's MX record to also point to the
Exim mail server. We are trying to get Exim then to pass any messages for
domainB.com to the Exchange Server. What do we need to do? Currently
domainA.com is working fine. Also I can send from Exchange to anyone on
domainA.com. External messages to domainB.com are just getting stuck on the
Exim machine.

Please help as this is driving me mad.

Thanks

Henry