What does the complete file /etc/postfix/headerstring look like? Does another line match a filter action and override the desired result? Have you defined any other header_checks (body for example)?
That should be enough, too: /^Subject: .*\[cleartext\]/ FILTER cleartext: -- Sent from: http://postfix.1071664.n5.nabble.com/Postfix-Users-f2.html
