[exim] database lookups in auth client context

Hi there,
I would like to ask one question.

Is it true that there are only $host and $host_address available
to derive loginname-password pairs in the authenticators client
context, or, is there anything else valid, as, for example,
$sender_address and $return_path ?

The answer is of general importance for people who have to deal with
big providers. Namely, if only $host and $host_address are available,
i.e., something like a user name, $sender_address, or $return_path
can not be used for database lookups in the authenticators client
context, then, on a multi-user host where mail arrives via fetchmail
and is sent to the different providers in form of smarthosts, it seems
to be theoretically impossible to handle more than one user (per
authenticator) per provider ($host). The latter implies that the
providers do not allow to be used as ordinary relays, which is
frequently the case.
In fact, I tried some things, but without success.

Can anybody answer my question ?
Many thanks in advance.

Stephan