As you may know, Mdaemon has the Read Confirmation and Delivery Confirmation. However, these features may not be affective if the users decide to click on "Cancel" or not even click on the broadcast email that you send to them.
If you want your users to read a very important message or announcement, the only way that you can do is to force them to read it. Therefore, Acknowledgement Message is the way to go.
Right after a users log in to his/her email account, the whole WorldClient email interface will be replaced by a small windows that contains the important message that you want your users to read. At the bottom of that window, there is a "Acknowledge" button that will become active after 5 seconds (In other word, the Button will only be click-able 5 seconds after the Ackknowledgement Windows appears). This is to prevent lazy or irresponsible users not to read the message and just simply click on the Button to get away.
After the users click on the "Acknowledgement", the users will be able to see his/her normal WorldClient interface and an automated email acknowledgement confirmation will send to the admin or whoever has the permission to setup the acknowledgement message.
Thank you for submitting your idea to require users to acknowledge reading a message. It will be considered for future versions.