There is a setting in the Proofpoint Essentials 'Spam' page called;
'Include an easy-spam-reporting disclaimer in passed email:'
Just tick that box and then users will get a link in the footer of very email which they can use to report an email as spam.
I wouldn't turn it on until users have been given the standard educational speech first around NOT marking newsletters and things they _may_ have subscribed to but to unsubscribe instead.
It creates havoc with spam filters when people get lazy and think that marking something as spam is easier than unsubscribing. Always Unsubscribe.
The old adage, 'one mans newsletter is another mans spam' is in fact untrue, very misleading and shows a basic lack of understanding of the spam issue we all face.