In a nutshell: The vast majority of servers may not have the DSN feature enabled. Some servers do not allow return receipts. So user will not see it. Some users do not choose to use/respond to return receipts. Some people have tried to use various email tracking/reading methods by adding remote content, but Remote content can be auto disabled. It is auto disabled in Thunderbird. Some servers controlling internal mail can employ a read receipt when email is downloaded or moved etc. There are a few ways around this, but not so easy as it involves getting email copied to a non imap folder to view it. So, it is not that simple to know whether something was received or read. http://kb.mozillazine.org/Figuring_out_whether_the_recipient_read_your_message For example: Google help says Read receipts are not delivered in the following cases: * The message is opened using a POP client or a G Suite Sync client. * The message is opened using an IMAP client, but your administrator has not configured the feature to send receipts automatically. * The message is sent to a user through a group mailing list or alias. The message needs to be addressed directly to a user’s email address in order to receive a read receipt from that user. The RFC's allow for delivery status notifications. Most servers have them turned off to control spam see http://en.wikipedia.org/wiki/Backscatter_%28email%29.