[Exim] Mail Gateway Function

Hi,

Being new to Exim, I would be grateful if someone could point me in the right
direction to achieve the following, or just laugh in my face and tell me it's
not possible...

We have a requirement where inbound e-mail for the domain, for example,
mywork.com gets sent to one of two machines depending on where the users
account is - None to be delivered locally.


To be more specific, we have a legacy mainframe mail system - IBM Officevision
with 17000 users, using 8 character ID's, currently all mail for the domain
gets delivered to the mainframe. Now, we will shortly be trialling an
exchange server & the powers that be have decided they don't want it to be
know as a subdomain, but still as the primary ie mywork.com - To further
complicate matters they wish to move away from the 8 character ID's to realnames.

So in a nutshell.. Is exim able to receive the item of mail, check the
destination userid in a database, if it exists pass to the exchange server,
if not, pass to the mainframe.

Sorry for the essay - Any pointers gratefully received!

Thanks.

Stuart.