Zimbra provides its own authentication and doesn't require any additional LDAP backend (Be it AD, OpenLDAP, OpenDir) to function. This is how we have it set now.
However, Zimbra can be made to authenticate against other directory systems. PLEASE NOTE that the account has to be provisioned IN Zimbra and on your directory server but users will still actually authenticate against the external server.
As far as GALs are concerned, Zimbra can use its internal info, or look up external LDAP sources as well.
There are documents around, plus information at wiki.zimbra.com
Version 6.0.16_GA_2998.NETWORK Apr 4, 2012 MACOSXx86
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