[exim] multiple domains

Hi! I 'm configuring exim to serve for multiple domains. I'm using
maildir_home for localdelivery, changed directory option to:
directory = /home/$domain/$local_part

How can i tell exim that local users are of the form usr@domain? ... i
see the option check_local_user that maps $local_part to system
account but i need something that maps $local_part@$domain to system
account.

Can anyone help me?
Thanks!