These docs have more than their names might suggest:
Zimbra Directory Service (LDAP) - Zimbra :: Wiki LDAP Authentication - ZimbraWiki LDAP Active Directory - ZimbraWiki
For the search base you might only need DC=mydomain,DC=com
Some AD's need like: cn=administrator,cn=Users,dc=domain,dc=com
Filter examples:
(uid=%u)
or
(uid=%n)
or even as complex as: (&(|(cn=*%s*)(sn=*%s*)(gn=*%s*)(mail=*%s*)(zimbraM ai lDeliveryAddress=*%s*) (zimbraMailAlias=*%s*)(zimbraMailAddress=*%s*))(|( objectclass=zimbraAccount)(objectclass=zimbraDistr ibutionList)))
%n = username with @ (or without, if no @ was specified)
%u = username with @ removed
%d = domain as foo.com
%D = domain as dc=foo,dc=com