[Exim] MX host to bypass RBL...

Months ago I installed the RBL-configuration and for as far as I
could notice that helped to keep out a significant amount of spam
untill recently one of our users ended up in a spam-database.

The headers and logfiles show me that the spam is refused by the
(MX)hosts in our domain, but that the message arrives to us via the
mailhost at our backbone provider (which is the last fallback MX and
doesn't have RBL installed). Ofcourse our host accepts mail from this
friendly mailhost.

Is there anything I can do to keep spam out without having to take
this MX-record out of the DNS-table? I doubt I can find our backbone
provider willing to install RBL on their system, that is not only
used by us.

Jasper