Lmtp redirection to other backend server

[Marc Roos]

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?