[Exim] allow relaying via IP?

Hi all!

I have a quick question:
we have a couple of customers, who are running their own machine with any
kind of SMTP Daemon (we are an ISP). They all have a lot of domains, and we
provide the secondary and third MXers for them.
At the moment I have a list with all domains for which we are relaying.

the problem is that this list needs a lot of time in order to be up-to-date
(actually, I don't believe, that it is complete).

Is there a possibility that makes Exim relay all mails for which
mail.xxx.xxx (or itself) has a MX record?


Thanks,

Phil