I'm looking to have divided as much as possible. I'm using incoming and outgoing gateways (SPAM and AV scanning). I would also love to have some kind of SMTP server functionality which would be synchronised with main server which would allow divide POP/IMAP and SMTP services between systems. This would allow to handle more users on main server and maybe allow sending emails when main server would be down for some reason (users would have to be cached when main server is down).