[exim] Use DNS then fall back to local delivery

Hello,

I am setting up 2 VPS's for redundancy - I am trying to work this out:

Primary - exim, its the first MX preference
Secondary exim, second Mx preference + same local accounts as on Primary

Looking to setup the following router that would be global

mail arrives either locally or comes in that matches local user, then
looks at DNS first to see where to deliver, sees that primary is on
other server and off it goes.
if the primary server was down, the second MX record points to this
server, then the delivery is made there.

I don't want to relay, but actually have it delivered - so that its
readable during the primary server outage.

Thanks
Gregg