You can request a Notification when your message is read by the recipient(s) (is displayed to the recipient(s)).
Select the Disposition Notification (Notify when Read) option to request an MDN. MDN messages sent back to your INBOX tell you that your messages have been displayed to the recipients.
Note: The MDN is not guaranteed to work. MDN messages are generated with the mail client software, and many mail applications simply do not support MDNs. The client mailers that support MDN may have it disabled, or may ask the user if the MDN message should be sent, and the user may tell the mail application not to send an MDN.