Has anyone howto hints to share in authenticating Zimbra login against an external Apple OSX (10.5) OpenLDAP server?

I'm trialing ZCS 8.0.7 net on RHEL 6_64. I think I get the LDAP Authentication bulk provisioning instructions, and old posts. As yet I am finding no joy in attempts to directly authenticate.

Alternatively, the underlying RedHat PAM authenticates fine to the external OSX LDAP server. Performance issues might arise, but is there a Zimbra way to try that?