Virtual Email Hosting, IMAP and LDAP - advise needed

[Ramin Motakef <ramin@motakef.de>]

Hi,
first of all i am quite new to all this stuff, so if anything below is
plain stupid, please tell me. (RTFM wellcome).

What i want to do is hosting virtual email domains on one server,
which should be accessible by IMAP and/or POP3 Clients. This actually
already works with exim as MTA and courier for POP and IMAP.

First question: 
I have exim deliver the mails in maildir format to 
/var/spool/virtual/<domain>/<user> with permissions "mail:mail 600".
Courier uses auth-userdb, the usernames are of the form
<user>.<domain> and all users have uid=gid=8 (mail).
Is this save or are there better ways to set the permissions/ids?

Now to the hard part:
I´ve heard (read) a lot of good thing things about LDAP, and having
all the infos at a central place sounds really fine to me. 

The next question:
How do people organize the LDAP tree? 
I´ ve searched around the Internet, found lot of infos on LDAP, but
not a concrete example of LDAP driven email solution. So, if you have
links, or like to share your setup, please respond.

Thanks,
        Ramin