Lmtp redirection to other backend server

I have currently a working local delivery working ok on 2 backend servers. The backend servers use slapd/nslcd/nscd for local accounts. If I enable proxy=yes in the lmtp config section, I am getting this error 'auth: Debug: passdb out: FAIL#0111#011reason=Configured passdbs don't support credentials lookups' which is obvious because I do not have a passdb.

I am not sure if it is possible to get this value from the system environment eg. nslcd/nscd, without the need to configure some separate ldap lookup in dovecot? Or do I really need to configure a ldap lmtp lookup just for this host attribute? What would be an efficient solution?