[exim] how to distribute outbound mail over multiple hosts?

Hey,

I'm planning on migrating one of my outbound mail servers to a new datacenter
so we will get a new IP as well. Judging from the past we should allow for
some "warm up" time before switching over to the new system completely and I'm
looking for ways how to achieve that with Exim.

Ideally (from my point of view) the old outbound server would get a new router
which forwards some messages to the new server. This "some" should be based on
a configurable percentage (starting with maybe 1% of the traffic and then
manually ramping up depending on the results).

I strongly prefer to configure the mail flow at the current outbound servers
instead of applying something at our application servers (diverse operating
systems and just a bigger number of systems to take care of).

I guess many more people experienced this problem as well, especially if you
need a "backup" outbound server (which should be "hot" all the time according
to best practices). How are you solving that problem?

Felix