[exim] Help with Exim 3 required

Hi,

I am trying to configure our Exim 3 server to forward any mail that can't be
delivered on the server on to another server within our organisation. For
example, if user BOBA doesn't have a mailbox on Server A he'll have a
mailbox on Server B so deliver the message to Server B.

Can anyone explain what I need to do? I've tried Googling around but can't
find any concise info on my exact situation.

TIA,
Tim.