[exim] Using AUTH username in ACL

Hi,

I am not sure if this is stupid question, but I could not really figure it
out through the documentation.

We run a mail server for a number of domains. We also require our users to
use SMTP AUTH in order to send emails. We now would like to treat our
users different (i.e. apply different ACLs) depending on what domain they
do belong to.

In other words: I need to use the username (which is in the form of
localpart@???) to pick the ACL. I don't want the sender, as this
can freely be set by the user. Which variable would I expand in the
set_acl_rcpt for example?

Is this doable at all? Is the variable set at the point in time Exim will
decide what ACL to use?

Regards,
Torsten