I guess you stumbled upon this ressource as well? https://wiki.zimbra.com/wiki/LDAP_Authentication#LDAP_bind_DN
In Samba4 you would use cn=Administrator,cn=Users,dc=mydomain,dc=com and Zimbra says administrator@mydomain.com should work too if the password and permissions are right.
Here is a case for a bind to “mediavault” that maybe helpful: [url]LDAP Connection Issue]