ISPConfig 3 will use MySQL for authentication. We decided that we will not use ldap as it is much too complicated to setup for most users.
I hesitate to suggest this, BUT the easy way to do this would be to integrate Zimbra with ISPConfig. Zimbra is a HEAVY duty mail server that comes with LDAP, MySQL, Tomcat, Ajax, Java & etc. implementations. There would be no need to learn and implement a new LDAP system that integrates with mail system that use MySQL.
Please read the zimbra licence, it is not really open source even if they have a so named open source version. Additionally, using a project like zimbra as basis for ISPConfig is not an option for me as this would limit ISPConfigs functionality too much.