[exim] verifying recipients on mailhub

Hi there,
I'm trying to setup a mailhub, and want to verify, if the user exists on
target server. I have this router:

prfmail:
driver = manualroute
route_list = natur.cuni.cz tao.natur.cuni.cz bydns
local_part_suffix=-*
local_part_suffix_optional
local_parts = lsearch;/etc/exim/natur
transport = remote_smtp

so when mail is sent to username@??? and username is found
in /etc/exim/natur, everything is OK and mail sent the right way (according
to exim -C testconf -bt username@???). But when username is not
found in /etc/exim/natur file, the mail is then processed further - but I
want to fail it with Unknown user error, and dont know how. Can anyone point
me in the right direction?

Ax
--
Vaclav Hula,
spravce postovniho serveru
Prirodovedecka fakulta
Univerzita Karlova v Praze