[Exim] Routing Some Mail To Another Machine

Hello.

We have a central mailserver, which most of our users download mail from
using POP3. However, some users now wish to start using Exchange. For
these users, I need to tell Exim to route their mail along to the
Exchange machine. I can see this would probably involve some sort of
'smarthost' configuration, but how would I bind that to the particular
set of users who need to use this, without disrupting the rest?

Thanks in advance,

Matt.