[Exim] exim, virtual users & mysql

Greetings All,

I would like to install/configure exim to deliver mail to virtual users who
do not have an actual account on the machine (no entry for them in
/etc/passwd).

I currently have all their account information (username, password, domain
name etc) stored in a mysql database and have a web based login area that
authenticates/interacts with it fine. I would like to have exim set up so
when mail for joey@??? comes in, it checks to see if joey exists in
the mysql table and if so, it puts the mail into
/var/spool/mail/virtualuser/joey (Helping a friend set up a hotmail-like
service).

From the docs I have looked at, I know it is possible, I just haven't found
a clear-cut guide/doc detailing how to do so.

If you would be so kind to point me in the proper direction, it would be
much appreciated.

Thanks a bunch,
Joey.