Zimbra permits the use of external LDAP servers per domain for end user authentication. Zimbra user accounts are mapped to LDAP accounts on an external host using an LDAP query filter. Though it is always a good idea to use an LDAP search base, it may not be required by your LDAP server.
Configuring external LDAP authentication
The easiest way to do LDAP authentication from a 3rd party application is to simply "bind" as the user intended.
Most applications do this by using a search, finding the valid user credentials and then binding as that specific user. Therefore the application doing the authentication needs to bind to the system first to do that search.
See LDAP Apache as a simple example