[Exim] Howto setup exim mail server as fallback

Hi I would like to setup my exim mailserver to act as a fallback
server for a friend ISP of mine. He has put a second MX record into
his DNS, I enabled relay domains include mx by setting the following
option:
relay_domains_include_local_mx = true

Are there more things I have to do to let it working properly?? Cause
I tried this but it didn't seem to work. Or is it also nesecary to
put the relay domains into:
relay_domains

Thanx for your help :)


N. Elsten