[exim] Selective routing/delivery per user

Hi,

I have a requirement to be able to somehow set a flag (flat file or
database) and deliver mail to another server on a per user basis
depending on the state of the flag. Both servers handle the same domain
(e.g. partner.company.com).

So, a mail comes into server A for user1@??? - no flag
is set so its delivered locally on server A. Another mail comes in for
user2@??? but this time the flag is set, so I'd like to
deliver this to server B for local delivery there.

Any pointers/tips? I've found various examples, none of which seem to
work how I'd like when testing them.

TIA

D