I have Zimbra 4.04 open source installed on FC 5.
It's been working very smoothly for months and suddenly started misbehaving...
It crashes erratically. Sometimes once every 2 days and sometimes 2-3 times in one hour.
I can't see anything interesting in the logs, disk space seems to be fine and during a crash CPU and memory don't seem to show anything interesting either (top).
When a crash occurs it seems to be gradual. At first IMAP is not available, then other services start being inaccessible (including pop3, smtp httpd, ssh).
A couple of times I was able to restart Zimbra but after a short while the system crashed again and froze.
After reboot the system usually works fine for a day or an hour and then eventually crashes again.