Many email clients such as outlook has a feature that allows you to request a receipt when your email is read by the recipient(s).
It can be achieved in asp.net generated emails using additional header called Disposition-Notification-To
Here is simple code
MailMessage email = new MailMessage(fromAddress, toAddress);
email.Headers.Add("Disposition-Notification-To", receiptToEmailAddress); //Use email address on which you want receive the receipt