Hi,
Ok, I think that I understand your question now, what you want is to connect from a third-party application with the Zentyal LDAP server, isn't it?
In principle you could use any master or slave server, but as you need the mail information you'll have to connect to the slave with the mail module installed.
You can find the needed connection settings in "Users and Groups ->LDAP Settings". Remember also to open the ports in the firewall if needed. A slave Zentyal server has three LDAP instances (frontend, translucent and replica). You should try to connect with frontend (port 389) or replica (port 1389).
Regards,