Large Mail Cluster Questions

[DGranath]

I'm setting up a large mail cluster and I'm trying to
hook up with other people who have done this. I need
advice on how to cluster boxes for about 250K external
msg/day. I'm using 750MHz AMD boxes I need an idea 
how many I need - including a backup capability.
I've only done single server units before and I need
to find out if there are different firewall configurations that I should examine...