How to fix "Error: Queue report unavailable – mail system is down"

If Zimbra cannot send or receive mail, while webmail, POP, and IMAP still work normally, and checking the log shows “fatal: Queue report unavailable - mail system is down”, as in this example:

zimbra@zm:~$ tail /var/log/mail.log
May 22 11:08:00 zcs postfix/postqueue[19195]: fatal: Queue report unavailable - mail system is down

Cause An abnormal system restart, such as after a power outage, leaves master.pid, the file recording the MTA process id, undeleted. The system therefore believes that the MTA is still running. Solution Delete /opt/zimbra/data/postfix/spool/pid/master.pid and start the MTA service again with these commands:

# rm -f /opt/zimbra/data/postfix/spool/pid/master.pid
# su - zimbra
$ zmmtactl start