Mail cannot be sent or delivered: mail transport unavailable

Symptoms

• Mail cannot be sent or delivered;

• Following error can be found in the /var/log/maillog:

  postfix/pickup[1002]: C579B70771A5: uid=10004 [email protected]
   postfix/cleanup[2160]: C579B70771A5: [email protected]
   postfix/qmgr[25399]: C579B70771A5: [email protected], size=356, nrcpt=1 (queue active)
   postfix/qmgr[25399]: warning: connect to transport private/plesk-192.0.2.2-: Connection refused
   postfix/error[2162]: C579B70771A5: [email protected], relay=none, delay=0.07, delays=0.05/0.01/0/0.01, dsn=4.3.0, status=deferred (mail transport unavailable)

• Postfix cannot be started through the systemd, as it reaches the failed state:

  # systemctl status postfix.service
  ● postfix.service – Postfix Mail Transport Agent
  Loaded: loaded (/usr/lib/systemd/system/postfix.service; enabled; vendor preset: disabled)
  Active: failed (Result: exit-code) since Sat 2018-04-28 06:46:18 UTC; 8s ago
  Process: 2908 ExecStart=/usr/sbin/postfix start (code=exited, status=1/FAILURE)
  <…>

  postfix/master[2981]: fatal: open lock file /var/lib/postfix/master.lock: unable to set exclusive lock: Resource temporarily unavailable
  postfix/master[2980]: fatal: daemon initialization failure
  systemd[1]: postfix.service: control process exited, code=exited status=1
  systemd[1]: Failed to start Postfix Mail Transport Agent.

• There is an instance of Postfix master daemon running on the server, which is not tracked by the systemd:

  # ps -eo cmd | grep [p]ostfix
  /usr/lib64/plesk-9.0/psa-pc-remote -p inet:[email protected] -t 7210 -P /run/psa-pc-remote.pid -u postfix -g popuser -n
  /usr/libexec/postfix/master -w

Cause

A master daemon is already running on the server, but cannot spawn required transport daemons.

Resolution

1. Connect to the server via SSH.

2. Stop Postfix process:

   # kill -9 $(pgrep master)

3. (Optional) Restore permissions for the master lock file:

   # chown postfix:postfix /var/lib/postfix/master.lock

   # chmod 600 /var/lib/postfix/master.lock

4. Start the Postfix service again:

   # service postfix start