Select Settings and Configurations at the top right of your page. This will take you here:
Within the horizontal navigation bar, select System. From the drop down, select Messages. This will take you here:
Within the New Message Notification box, use the on/off toggle to the left of Inclusion Of Message Content In Notifications.
Selecting on will allow the recipient of the message from the system to be view within their email.
Select off will not allow the recipient to view the contents of the message.
The recipient will not be able to reply to the message if they are not within the system. The email notification is to give them the alert that they have a message within the system.