Route Unknown Local Mail to another server

Hi,

Our company has two offices, both running mail servers.  Our domain is hosted in one office so all mail sent to the relay.ie domain goes to this office (MDaemon Mail Server).  Our second office runs another mail server (MDaemon Mail Server), this uses pop to collect mail for the users in the second office for each user in the second office.  Because of this the second office uses internal e-mail address, not using relay.ie so that internal mail is routed locally.  I'm setting up an Exchange server and want to set it up so that if the name@relay.ie does not exist locally it will forward the mail externally.

I haven't really had time to look around exchange and figure this out but at the moment if I send mail to anyone internally it works fine, but if I send it to anyone in our head office it just comes back saying the address is unknown.

Can anyone let me know how to do this in exchange? I can do it in MDaemon, just haven't because I want to get rid of that server.

Thanks in advance for any help you can offer.