After trying a number of operating systems compatible with the latest release, none of which worked for one reason or another, I settled on FC7, which only needed:
1) "yum install fetchmail"
2) comment out the line in /etc/sudoers with "Default securetty"
3) use domain.com, not host.domain.com in order to pull the MX record
I run a split e-mail, split-dns network at home, with two commercial firewalls, and a DMZ where my external smtp and dns servers lie. The external smtp is a forwarder to the internal smtp server. The external dns server serves the outside world, while the internal servers the internal network.
Anyway, if you any of you do this also, note that you can still use whatever internal IP and dns scheme you desire. Because your internal users point at the internal DNS server, you can create a cname record (an alias), so both the external and internal DNS servers return the same host.domain.com name.
What this means is that you can tell all of your users, both those working from home or on the road, or those who only work in the office, the same exact domain name and it'll work fine.
I must say I really like Zimbra. My work uses this, while I was using Thunderbird and imap to get my personal e-mail. Already I can tell you the junk mail (amavist) is better in Zimbra than in Thunderbird, and I've been a loyal user of the latter for five years.
The downside is that my little hand-built AMD 2GHz server is thrashing itself. The java is killing it. So, one day I'm gonna have to do something about this. Fortunately, it's just me and a couple of other users so it's no big deal right now. I went ahead and turned off every service that I could find, and that helped a bit.
Overall I'm very, very impressed at the amount of work that went into this and the quality of the end result. I can administer multiple domains with surprising ease, the skins seem decent, and I'm still exploring and testing out the feature set, which is quite rich.
Beers all around!