When a message arrives in Exchange Online Protection (EOP) with an existing X-Forefront-Antispam-Report header, it is renamed to X-Forefront-Antispam-Report-Untrusted. If the message is then detected as spam and stored in the optional quarantine, upon release it will go back into EOP. Upon entering EOP the previously set X-Forefront-Antispam-Report header is renamed to X-Forefront-Antispam-Report-Untrusted.