[Exim] Select SMTP host based on local part only

Hello

I have a requirement that I feel Exim is able to resolve, but I can't
find the right documentation.

This is the thing: I want to make Exim act as a mail hub for an
internal network. This network has only one domain address, say
mydomain.com. Now, some of the users are running groupware clients and
their mail should be redirected to the grouware server for storage.
Other users are using POP-only clients or unix mailboxes and their
message should either be forwarded to a different smtp host, or stored
locally (depending if the mail hub will be the same box as the internal
mail server).

However, the address domain should be the same both for the groupware
users and others. What ways do I have of looking up the local part,
possibly in a SQL database, and deciding which transport to use?

I know this is probably all covered in the docs, but I would really
appreciate it if someone could whip up an example for me.

Thanks,
Arnar