[exim] Gateway recipient verification (again)

Hello all.

I have a scenario that isn't uncommon I guess.
I have an Exim 4.60 server (Debian) acting as a gateway to several MS
Exchange servers, handling mail for about 10 domains/1000 users.

I want to do recipient verification during the SMTP phase. For
various reasons I cannot do neither callouts nor LDAP lookups, so I
want to verify recipients against a flat file. The flat file has one
valid and complete email address (local part + domain) per line, and
nothing more.

As my second router (after the domain literal router), I have this:

userlist_domains:
driver = redirect
allow_fail
domains = DEBCONFdc_relay_domainsDEBCONF
data = ${lookup{$local_part@$domain}lsearch{/etc/exim4/
valid_addresses}{$local_part@$domain}{:fail:Unknown user}

The above router makes Exim accepts all addresses, whether they're
found in the flat file or not. When I change the "data =" line to

data = ${lookup{$local_part}lsearch{/etc/exim4/valid_addresses}
{$local_part@$domain}{:fail:Unknown user}

...no addresses are accepted during the SMTP phase.

I'm quite new to Exim (being an old qmail guy) so I'm wondering
whether I'm barking up the wrong tree with regard to the router.

How can i verify the *complete* address against a local flat file?

--
Tor Willy Austerslått