I'm a fan of Zimbra. In reality...a Linode 4096 handles 5 "normal" users just fine, and a few here and there users, possibly more...that was all I needed. Tack on Z-push and you have a nice Activesync implementation as well. Then it is just like gmail just plugin the server and password and calendar, contacts, email, and whatnot all sync as you would expect. http://z-push.org/