[Exim] how to act as a backup mx ?

Hello list,

I want to be a backup MX for a specific domain, e.g. myclient.de
and want to receive all mails for this domain in my mailqueue, if
the main MX for myclient.de is down, but when the first MX is up
again my queue should be flushed.

MY DNS looks like:

myclient.de.        IN MX 0    mail.myclient.de.
                    IN MX 20   mail.myserver.de.
mail.myclient.de.   IN A       192.168.1.10
mail.myserver.de.   IN A       192.168.55.10

I hope somebody can help me how to configure my exim.conf

Thanks for any help appreciated.

Thomas.

--
Thomas Baumann // tirili+list.exim@???