[CentOS] Postfix header check help

Hi list,

I have a requirement for a mail server that only allows email to pass
with a particular word in the subject line.

Reading the header checks docs for Postfix I thought I may be able to
add this rule:

!/^Subject: .*dingdong/ REJECT Need to add dingdong to subject line to
send

Which should reject all mail unless the subject line contains
"dingdong". Unfortunately this seems to block all mail from passing.

Removing the "!" works as expected and the server allows all mail to
pass, unless dingdong is in the subject line which is rejected.

Is this rule valid? Or is there a better way to make this work?

Thanks

Dean