Step:1 Edit ‘/etc/postfix/main.cf’ file & uncomment below line:
#header_checks = regexp:/etc/postfix/header_checks
Step:2 Append the below line in ‘/etc/postfix/header_checks’ [at the bottom]
/^Subject:/ WARN
Step:3 Run postmap to apply the new configuration in /etc/postfix/header_checks
[root@server1 ~]# postmap /etc/postfix/header_checks
Step:4 Restart or Reload the postfix server
[root@server1 ~]# systemctl restart postfix
[root@server1 ~]# postfix reload
Step:5 Login to your Roundcube webmail and send an test with the subject line: "Fwd: New! Machine Learning: Algorithms in the Real World"
Perhaps you want to regect junk mails.
But this docker image is your private send-only email server, so I think no one attempts to send a junk mail to it.
configure postfix with log header with subject