[exim] Determining smarthoust address from message

Hi

I'm trying to setup an Exim configuration to allow users to test sending
emails from an application.  Users will run their own mail receiver
(such as Java's fakesmtp) but always send to the central Exim server.

So I would like the smarthost route_data to be derived from /something/
in each incoming mail message.  Originally I tried specifying a host
name as the user part of LOGIN authentication, then setting:

server_set_id = $auth1

and then referring to $authenticated_id in the route_data expression. 
But debug shows that $authenticated_id expands to empty in the
route_data setup, which I guess make sense since the authentication
information is not preserved with the spooled message.

I've tried various other variables, but none seem available when
route_data is derived.

Is there any simple way to achieve this?

Thanks

Richard