Implement Postfix header checks

Edit main configuration file /etc/postfix/main.cf and add:

...
smtp_header_checks = regexp:/etc/postfix/header_checks
...

Add regexp to the /etc/postfix/header_checks:

...
/^Subject:.+?Your website$/ REJECT 554
...

Restart Postfix:

systemctl restart postfix

Test:

postmap -q "Subject: Your website" regexp:/etc/postfix/header_checks

You should get:

REJECT 554