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.
More information on sending emails with Delivery Status Notifications via Roundcube can be found at Roundcube - Sending Email Messages
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 option"
To advertise to all hosts, you would input the "*" as the value and Save the changes.
In the Exim configuration this will be inserted as follows: