[exim] Routing: Accept mail from certain hosts

Hi,

I'm trying to configure a mail archive for some domains. The main server
sends a dup of all incoming / outgoing mail to the archive server, that
is also connected to the public Internet. The archive machine shall
accept these messages, regardless of their domain or local_part.
Therefore, I want my "archive router" (that routes to a given Maildir)
to accept only mail from this special host. How can I do that?

(If someone is further interested: The archive server shall not let any
mail in except that from that mail server, but it should of course be
able to forward local mail to me. Are there better ideas for realizing
this configuration?)

Thanks!
Peter