How to Enable logging of Email’s Subject Line in Postfix Maillog ?

By default Postfix MTA only capture ‘From’ and ‘To’ details in the log file (/var/log/maillog). There are some scenarios where we want that email’s subject should be capture in the maillog. This will very useful for email administrators when performing troubleshooting of problems related with email. There are simple steps to enable logging of the email subject in postfix maillog.

We are assuming that postfix is already up and running and will make below changes.
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"
OR do the following

Now see the mail logs using the command ‘tailf -f /var/log/maillog | grep @‘

[root@server1 ~]# tail -f /var/log/maillog | grep @

Default log without subject line