Our installation is approximately one year old. We have a cron job that shuts zimbra down every evening, backs up the zimbra directories, and restarts zimbra. It worked flawlessly until a few weeks ago when we started getting:
Jul 1 20:08:30 mail postfix/postqueue: fatal: Queue report unavailable - mail system is down
A simple restart of zimbra clears the error.
We have made no changes to the system since install.
I have not tried any of the solutions, because I have been unable to find a thread that clearly states the problem, remains on track, and clearly presents a solution to the initial issue. I did find two threads that clearly state the same problem as mine, but no solutions are presented. I apologize for any inconvenience I have caused.