[Exim] Setup for secondary MXes.

I might just be stupid here, but I was testing setting up Exim to be a
secondary MX for some domains, by using a non-existent host as primary
MX. First try was just using the already set up relay_domains, with
the result that Exim bounced the mails almost immediately, which
whatsn't exactly what I was looking for.

So how have people set up their secondaries out there?

Secondly, is it possible to make Exim treat any error as temporary? I
wouldn't put it past some of our customers to rebuild their primary by
setting up the box again on the original IP, and only then set up the
MTA. With the effect that Exim bounces all the mails.

--
Thomas
beast@???