[exim] multi-stage fallback

Hi all,

I'm trying to implement a multistage fallback as our current fallback
server's queue is too big, and I think this is causing a few issues
with things like greylisting (as it's not re-trying fast enough to hit
some peoples greylist cache).

I've had a bit of a google and I've not managed to find much on
implementing such a system.

Can anyone suggest a method?

Thanks,

Ian