[exim] Condition for senders domain for use in a router

I need to add a condition to a router that is triggered by the senders domain. I am struggling to find an example in the Wiki and
the specification. All of the examples for routers seem to be based on destination rather than source :-(

I have a file called xxx-domains.txt that lists 20-50 domains owned by 1 customer. Mail sent by any user at this customer needs to
be passed through a transport filter. All other customers should be routed normally (their domains are held in files called
yyy-domains.txt, zzz-domains.txt, ...). I just don't know where to start.

Thanks

Priz