Why is a DKIM error thrown when adding a Message ID header?

Question

PHP

Sending an email using PHP Mailer. In DKIM "h" tag: h=From:To:Date:Subject. When adding the Message ID header (h=From:To:Date:Subject:Message-ID) - it breaks and an error is generated dkim=fail reason=signature_incorrect header.
Message ID is generated automatically. Example:-

Answer 1

The Message-ID and Date must be generated along with the email before you apply the DKIM signature. Otherwise, the Message-ID and Date can be added by any intermediate relay after the email has been signed with DKIM, and this will result in an incorrect signature.