Unable to start postfix on Plesk server: Address already in use

Symptoms

• It is not possible to start postfix, it fails with the error below in /var/log/maillog log file:

  CONFIG_TEXT: postfix/postfix-script[9889]: starting the Postfix mail system
  postfix/master[9890]: fatal: bind 0.0.0.0 port 25: Address already in use

• Manual start for smtp service fails:

  # plesk bin service --start smtp
  ERR [util_exec] proc_close() failed ['/opt/psa/admin/bin/mailmng-service' '--start-service' '--mail-component=mta'] with exit code [1] mailmng-service failed: exit status 1

Cause

Wrong configuration entries in /etc/postfix/master.cf , duplicate entries exist.

Resolution

1. Connect to the server via SSH

2. Verify that another application is not using port 25 (output should be exactly as below):

   # netstat -ntpl | grep 25
   tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN 21078/master
   tcp6 0 0 :::25 :::* LISTEN 21078/master

3. If output is different and a 3rd-party service is running, kill the process:

   # kill -9 <id>

4. Edit file /etc/postfix/master.cf and comment out all duplicate lines in the configuration, for example:

   CONFIG_TEXT: 203.0.113.2- unix - n n - - smtp -o smtp_bind_address=203.0.113.2 -o
   smtp_bind_address6= -o smtp_address_preference=ipv4

   Note: The line with the IP address should exist only once in the /etc/postfix/master.cf configuration file.