[Exim] Quota and Virtual domains

    Hi all,
    I have dozen virtual domains at Exim and want to control only a virtual domain about quota (every mailbox in this domain will be controled with quota). 
    I have simple virtual domain. Just put them at local_domains and
configured at /etc/mail/domain* files. I'm  not puting every mail at every
virtual domain in separated /var/spool/mail/dir (just in /var/spool/mail).
    What is the best manner to do this? Consider that I'm using Qpopper.
    TIA,        Paulo Henrique