If you want to to deliver to a local virtual user drop the ldap here:

virtual_mailbox_maps = ldap:ldapvirtual

ldapvirtual_server_host = 127.0.0.1
ldapvirtual_server_port = 389
ldapvirtual_version = 2
ldapvirtual_bind = no
ldapvirtual_search_base = o=XXX
ldapvirtual_query_filter = (&(|(mail=%s)(mailalternateaddress=%s))
(objectclass=person))
ldapvirtual_result_attribute = homeDirectory