openldap and mysql authentication

Hi, I have a simple question...

I want to use openldap as an address book, so it can be used with mail clients like thunderbird, outlook...

I already have a mysql database to store users for my mail server, and I dont want to duplicate these informations.

Can I use this sql database as a source for the openldap access control, or do I have to store all my users in the ldap ?

This way I could authenticate using user accounts stored in the mysql database and keep all my contact informations in the ldap...

Thanks for your answers!