[Exim] local_part check in virtual domains envirovement

    Actually Im using my own virtual domain exim conf and its working pretty
fine, but have a little issue :

        Its looks like exim when receive a mail its check its against /etc/passwd or
use pam or something like.... so I have to have one user entry in etc/passwd
for each user of each domain, this is quite uncomfortable for me.

        So I need a way to change this check to look to
/etc/virtual-domain/${domain}/passwd or similar....

    Can you help me?

    Thanks in advance