Mail Servers

[Chris Greer <cgreer1@midsouth.rr.com>]

Ok, I've got a question about mail servers.  We've been running a mail
server (sendmail) with mirrored disk for a while now.  After mirroring
the disk, we've never had a problem with it (knock on wood) as far
as availibility.  

We are now at a point where we need to expand the capacity of the
server.  We are currently handling about 50 users and after
we consolidate with several other mail servers we will probably be
handling around 200 - 275 users, but need to plan to eventually support 
500 or so.

We are requesting new hardware to handle this, and management wants
everything to be completely fault tolerant.  Meaning if the machine
dies, they want a failover machine.  
Has anyone setup redundant mail servers like this?  Can you
possibly give me some info on your setup?   Ideally, I would like
to spread the pop3/imap load across two servers if we are actually
going to buy two servers instead of having a warm standby machine.

We've used DNS Round Robin before to loadbalance and provide
fault tolerance, but I didn't think I could do that with pop3 and
imap connections.  If I'm wrong about that, please let me know!  

Thanks,
Chris G.