[exim] Configuring mailbox quotas on large servers

Hi,

what are people doing for setting mailbox quotas on systems with
very large numbers of mailboxes (hundreds of thousands)? I would
assume UNIX filesystem quotas aren't practicle as you´d need to have
all of those users in /etc/passwd or your directory service which I
think would kill performance. Anyone with any experience in this like
to share their thoughts??

thanks in advance, Andy.