[exim] Condition for router

Hello,

I want some mails for a certain domain to be delivered locally and some to
be relayed over a second mailserver.
To do this I added a new router right before all other routers.
It works fine if I try to relay all mails for hole domains:

additional_route:
driver = manualroute
domains = domain1.com
route_data = 1.2.3.4
transport = remote_smtp

The problem is that not all mails at domain1.com should be relayed.
I tried a bit with lsearch and things like:

local_parts = lsearch;/etc/exim/relay/$domain/local_parts

But I did not find a working solution.
The best solution was if I could specify a file where the email adresses
that should be relayed are simply listed (one email per line).
How do I realize this?

Thanks in advance,
Martin