If you are going for serious throughput optimisation then exim is
probably the wrong place to start - in many cases the vast majority of
messages could be switched through a box without touching the disk at
all (you would hold off acknowledging the incoming message until the
next hop had acked your onward transmission of it).
That sort of thing needs a hugely different architecture to that used by
exim.
Nigel.
--
[ Nigel Metheringham Nigel.Metheringham@??? ]
[ - Comments in this message are my own and not ITO opinion/policy - ]