EV 8.0.4 - Mailbox Archiving messages where recipient was BBC'd

Hi,

We want to archive Leavers mailboxes in Ev 8.0.4, so we have zero day achiving policy, added all message classes to archive everything, however some users have items in their inbox with no To: or CC: field, maybe these mails were sent BBC'd to a DL or set of recepients, 

Why EV is not archiving those messages? Is there any settings we need to apply? 

Many Thanks