CAS evaluate the message after it has been processed by the SMG
SMG with CAS
This is the way the system is designed to work.
When the SMG is taking all the actions specified by the various verdicts, it has no way of knowing if (to address the specific case you mentioned) the notification is that the message is in the quarantine, or if the notification is that the message has been deleted. For actions that are obviously over-ridden by the 'delete' action (such as 'Disarm'), those actions will not be performed.
The expectation is that the administrator, in constructing the policies and policy actions, takes into account the different policies and actions that they have configured and plan accordingly.
In this specific case it would be expected the admin to add an action to the spam policy of 'Bypass Threat Defense scanning'. By doing so, the SMG will not spend CPU cycles testing for a verdict that will not affect the message anyway, given that the message is being deleted (due to the 'Spam' policy action)."