[Dovecot] Question about "extracting" unwanted e-mails from mdbox

Imagine the following scenario

Last Saturday, 3:00 AM a big phishing attack hits our e-mail inboxes. Spamassassin does not mark them as spam, and our 50.000+ users have in their mdbox a very credible phishing attack. What doveadm-fu could I use to delete (or move to spam) that e-mail from each user INBOX (let´s imagine the Subject or a Header is known)?

I repeat: already delivered e-mail, how to filter/prune it.

Regards

Maria