[Exim] Using Exim on a large volume fallback MX

We're interested in replacing our fallback MX host (sendmail based) with
an exim based solution and were wondering if anyone had experience with a
large volume fallback MX using exim.

To give you some idea of the numbers, our current fallback MX processes
around 400k unique messages a day. Currently (off peak) we have several
queues with a total of 80k messages (1.5 GB). Our current hardware is a
Sun E3000 with 4 CPUs and 1.5GB of RAM with queues on a Netapp 630 filer.

Of main interest to us is whether the split_spool_directory will be enough
to balance the queues (ideally fewer than 4000 files per directory) ,
and what options there are for adding additional flushing queues.

We also would be interested in any tools people have to identify and
remove spam. Remember, the less spam we send the less you get :-).

Any other experiences people would like to share about Exim in a
large volume environment is welcome.

--
Michael Cope
Unix System Administrator
Earthlink, Pasadena