I have zimbra running on a Ubuntu 16.04. This server is Zimbra exclusive, so nothing installed beside it. It happens (more often than is should) that the MTA stops working, or will not start at all (even after reboot). In the log files are no traces of postfix or mta to be found.
zimbra@zimbra:~$ zmmtactl restart
Rewriting configuration files...done.
Stopping saslauthd...done.
Starting saslauthd...done.
/postfix-script: fatal: the Postfix mail system is not running
First of all, the last command you're running to start postfix is incorrect as you're trying to start the system version of postfix and that's not installed or used by zimbra - the ZCS postfix is started by the "zmmtactl restart" that you've already used.
Your use of the "zmmtactl restart" tells you that postfix is "not running" and you'll need to look in the log files for the reason it's not starting, see here: https://wiki.zimbra.com/wiki/Log_Files
Nov 20 12:21:41 zimbra zmconfigd[1528]: Processing rewrite request for antispam
Nov 20 12:21:41 zimbra zmconfigd[1528]: Processing rewrite request for antivirus
Nov 20 12:21:41 zimbra zmconfigd[1528]: Processing rewrite request for opendkim
Nov 20 12:21:41 zimbra zmconfigd[1528]: Processing rewrite request for mta
Nov 20 12:21:41 zimbra zmconfigd[1528]: Processing rewrite request for sasl
Nov 20 12:21:41 zimbra zmconfigd[1528]: Fetching All configs
Nov 20 12:21:41 zimbra zmconfigd[1528]: All configs fetched in 0.04 seconds
Nov 20 12:21:45 zimbra zmconfigd[1528]: Tracking service mta
Nov 20 12:21:46 zimbra zmconfigd[1528]: Watchdog: service antivirus status is OK.
Nov 20 12:21:46 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/conf/freshclam.conf with mode 600 (0.01 sec)
Nov 20 12:21:46 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/conf/opendkim.conf with mode 440 (0.01 sec)
Nov 20 12:21:46 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/conf/dspam.conf with mode 440 (0.01 sec)
Nov 20 12:21:46 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/conf/clamd.conf with mode 440 (0.01 sec)
Nov 20 12:21:46 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/common/conf/tag_as_originating.re with mode 440 (0.00 sec)
Nov 20 12:21:46 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/conf/sasl2/smtpd.conf with mode 440 (0.00 sec)
Nov 20 12:21:46 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/conf/postfix_header_checks with mode 440 (0.00 sec)
Nov 20 12:21:46 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/conf/opendkim-localnets.conf with mode 440 (0.00 sec)
Nov 20 12:21:46 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/common/conf/tag_as_foreign.re with mode 440 (0.00 sec)
Nov 20 12:21:46 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/conf/saslauthd.conf with mode 440 (0.00 sec)
Nov 20 12:21:46 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/common/conf/master.cf with mode 440 (0.01 sec)
Nov 20 12:21:46 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/data/spamassassin/localrules/salocal.cf with mode 440 (0.00 sec)
Nov 20 12:21:46 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/conf/mta_milter_options with mode 440 (0.00 sec)
Nov 20 12:22:00 zimbra zmconfigd[1528]: All rewrite threads completed in 13.95 sec
Nov 20 12:22:00 zimbra zmconfigd[1528]: All restarts completed in 0.00 sec
Nov 20 12:22:02 zimbra postfix/postqueue[18579]: fatal: Queue report unavailable - mail system is down
Nov 20 12:22:05 zimbra saslauthd[23773]: server_exit : master exited: 23773
Nov 20 12:22:05 zimbra systemd-logind[1115]: Removed session 13.
Nov 20 12:22:06 zimbra zmconfigd[1528]: Processing rewrite request for sasl
Nov 20 12:22:07 zimbra zmconfigd[1528]: Fetching All configs
Nov 20 12:22:07 zimbra zmconfigd[1528]: All configs fetched in 0.04 seconds
Nov 20 12:22:11 zimbra zmconfigd[1528]: Tracking service mta
Nov 20 12:22:11 zimbra zmconfigd[1528]: Service status change: zimbra.xxx.xxx sasl changed from running to stopped
Nov 20 12:22:12 zimbra zmconfigd[1528]: Watchdog: service antivirus status is OK.
Nov 20 12:22:12 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/conf/sasl2/smtpd.conf with mode 440 (0.00 sec)
Nov 20 12:22:12 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/conf/saslauthd.conf with mode 440 (0.00 sec)
Nov 20 12:22:12 zimbra zmconfigd[1528]: All rewrite threads completed in 0.01 sec
Nov 20 12:22:12 zimbra zmconfigd[1528]: All restarts completed in 0.00 sec
Nov 20 12:22:15 zimbra clamd[4639]: SelfCheck: Database status OK.
Nov 20 12:22:17 zimbra saslauthd[19382]: detach_tty : master pid is: 19382
Nov 20 12:22:17 zimbra saslauthd[19382]: ipc_init : listening on socket: /opt/zimbra/data/sasl2/state/mux
Nov 20 12:22:17 zimbra zmconfigd[1528]: Processing rewrite request for mta
Nov 20 12:22:18 zimbra zmconfigd[1528]: Fetching All configs
Nov 20 12:22:18 zimbra zmconfigd[1528]: All configs fetched in 0.03 seconds
Nov 20 12:22:22 zimbra zmconfigd[1528]: Tracking service mta
Nov 20 12:22:22 zimbra zmconfigd[1528]: Service status change: zimbra.xxx.xxx sasl changed from stopped to running
Nov 20 12:22:23 zimbra zmconfigd[1528]: Watchdog: service antivirus status is OK.
Nov 20 12:22:23 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/common/conf/tag_as_originating.re with mode 440 (0.00 sec)
Nov 20 12:22:23 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/conf/postfix_header_checks with mode 440 (0.00 sec)
Nov 20 12:22:23 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/common/conf/tag_as_foreign.re with mode 440 (0.00 sec)
Nov 20 12:22:23 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/common/conf/master.cf with mode 440 (0.01 sec)
Nov 20 12:22:23 zimbra zmconfigd[1528]: Rewrote: /opt/zimbra/conf/mta_milter_options with mode 440 (0.00 sec)
Nov 20 12:22:36 zimbra zmconfigd[1528]: All rewrite threads completed in 13.35 sec
Nov 20 12:22:36 zimbra zmconfigd[1528]: All restarts completed in 0.00 sec
Nov 20 12:22:38 zimbra postfix/postqueue[20403]: fatal: Queue report unavailable - mail system is down
Nov 20 12:22:40 zimbra sshd[20747]: Connection closed by 192.168.90.42 port 56164 [preauth]
Nov 20 12:22:41 zimbra /postfix-script[20758]: fatal: the Postfix mail system is not running
Nov 20 12:23:08 zimbra postfix/postqueue[20765]: fatal: Queue report unavailable - mail system is down
/opt/zimbra/libexec/zmmtastatus (perl script) is run as root to see if the MTA is running... which should do this:
/opt/zimbra/common/sbin/postfix status which calls: /opt/zimbra/common/libexec/postfix-script status (bash script) ... but Zimbra chose the kill -0 pattern instead to determine if postfix is running and ignore the logic already built into postfix that uses the pid file as an advisory lock ... Zimbra doesn't account for pid wrap so it can be confused and not start the MTA if you are really unlucky even on reboot depending on the value of the pid in that master.pid ... If you cat /opt/zimbra/data/postfix/spool/pid/master.pid and it contains a really low pid number, you are a victim of this bug. Some people blow the master.pid file away, but a better approach is to call /opt/zimbra/common/sbin/postfix status since it knows how to handle this file and do the correct thing.
BTW, zimbra will eventually call /opt/zimbra/common/sbin/postfix start which uses that postfix-script again as the logic that starts it. Not sure why they chose to ignore status but called start???
Note: they did use this patch for 8.8.15 but I don't believe it made it's way to older versions. I run with the patch I describe above in 8.7.11+ without issues given I have to restart often unattended with letsencrypt certs.
