[exim] exim with exiscan patch / per user learning

Hi,
I am using FreeBSD with Exim / Exican-patch.
I want to use per user filtering. That is, spamassassin should look
up the preferences for each user.
This is a closely out-of-scratch configuration, so I am not shure if
it is the best way to go to invoke spamassassin via an acl, and not
via transport.
Does someone have experience ?

My vexim-acl-check-content.conf has this statement:

# Always add X-Spam-Score and X-Spam-Report headers, using SA system- 
wide settings
   # (user "nobody"), no matter if over threshold or not.
   warn  message = X-Spam-Score: $spam_score ($spam_bar)
         spam = $localpart:true
   warn  message = X-Spam-Report: $spam_report
         spam = $localpart:true

The "$localpart" comes from me. I am trying to call spamc with the
username of the recipient, so that it knows where to lookup in the
database.
But that doesn't work.
Could someone please help me, with the right statement ?

Thank you very much for your help.

--
Michael