Zimbra open-source works great..!
At Meditab we are using Zimbra - open source as our mailing suite. We almost never felt 99% up-time unless there is a Internet down due to any ISP trouble. Last week I have been asked to install Zimbra on one of our machine for test as we wanted to perform some r&d out but can't perform that on live server.
I am novice to Linux and so took help of our Network guy to get over with Red Hat Server Enterprise 5 on one of the machine. The Red Hat installation went smooth and then before starting installation of Zimbra we went over with some Quick-Start guides and Administrator Guide from Zimbra itself. Samba server let's us easily access Windows shared files from the Intranet.
After downloading latest build of Zimbra Collaboration Suite 5.0 we started installation, the first phase was to check pre-requisites required to successfully install Zimbra. We were missing some of them like libc and gmp but got all of them from installation disk and started again. Then installation went quite smooth; we just need to answer some of the questions.
Installation went successful without any error there on and we were able to access that from network without any problem. Creation of Admin account was required before we could start creating user accounts from Zimbra Admin UI. Zimbra Wiki is great help and we were able to find How to Create Zimbra Admin account from there itself using "zmprov" prompt. In short even for newbies it won't take more than 3 to 4 hours to setup basic things to get started for more advanced setup we can always digg into online Wiki or Forum to get through.
After everything is done we can access Admin UI by typing https://SUB.DOMAIN.COM:7071 in your browser address bar. You may need to add an exception to browser security for https certificate and you must type 7071 to goto Admin Console.
We created few accounts from the Admin Console and were able to send and receive mail successfully inside Intranet. We were even able to send mail on real web-mails or live mails without any problem and receiving mail is also just a one MX record change away for our testbed.
I am planning to install Zimbra on CentOS in my VMWare machine setup as it will be binary compitable with Red Hat package.