[Exim] don't understand lsearch-lookup

Hi out there,

I have a problem doing a lsearch-lookup (or dbm).
As I told you before, there are 10 department-mail-servers and a central mx
distributing to them. An Alias-File on the mx-server would be OK to look up,
to which department hagen@domain has to be delivered, but I don't like this
design. :-(
Now I thought about putting this lookup in the Router-Config. Something like

---
domain:
driver = domainlist
transport = remote_smtp
condition = $local_part as key is found in lsearch, returning $value
route_list = "* $value bydns_a"

smarthost:
handle everything not handled by domain

---

Is this possible? And how can I a) do some lsearch lookup (how to specify
the key and a variable holding the return-value) in the first place and b)
what would the condition look like?

Well, perhaps I am just a little bit stupid, but I don't find the answer to
a) in Exim-Specification, chapter 6 or perhaps it is there but I fail to
understand.

--
CU,
Patrick.
"Never run on auto-pilot" - The Pragmatic Programmer