Delivery Status Notifications as described by Roundcube, allow you to request a notification, stating that the recipient received your message:
Request a notification message when the message is delivered.
This will be sent by the email server upon message delivery.
Please note that this doesn't confirm that the actual recipient has received the message.
The feature can be enabled via Roundcube:
Settings >> Preferences >> Composing Messages >> "Always request
a delivery status notification"
However, in order for this to function as intended, you must first enable this via Exim at the server level.
WHM >> Service Configuration >> Exim Configuration Manager >>
Basic Editor >> Mail >> "Hosts to which to advertise the SMTP DSN
You will want to advertise to all hosts using the "*" character. In the Exim configuration this will be inserted as follows: