[korlja]

in postfix.conf, set header_checks = pcre:/etc/postfix/header_checks and in /etc/postfix/header_checks add:

/In-Reply-To:.*[e-mail]/ REJECT reply-storm

However, blocking the whole thread that will originate is not as easy.

[bo1024]

That msgid should be at the top of the “references” header in all thread replies, right?

[korlja]

Should be, but I'm not so sure about Outlook, which is usually involved in all such reply-all-storms. Replies and threads in Outlook are very weird.