We all like certainty, return receipts and certified mail. Unfortunately, internet email does not provide a way to know a message has arrived at the recipient's mailbox or has been picked up. All you can do is assume it did arrive all right, and you have to assume the recipient opened it, too.
Images on Web Pages
HTML does not only allow you to use fancy fonts and gorgeous graphics in emails. HTML provides a way to retrieve images from a remote server to display them inline in a document, locally.
Your browser does that when it displays an image on a web page. The HTML source tells it where to fetch the image. The browser makes a connection to the specified server and asks it to send the image. After it has received the image from the server, the Web browser shows it on the screen.
Hidden Images in Email Messages
The same happens when the HTML code used in an email message includes a reference to an image at a remote server. The email client downloads the image from that server and displays it on your screen.
Services like SentThere exploit this capability. They introduce HTML code in an email message sent through them that includes a reference to a tiny transparent image on one of their servers.
While the recipient of the message does not notice the image or the connection, the request is logged at the server. It is a proof that an email message has been received and opened by the recipient.
Since a unique image address is created for every message, it is also clear which message was opened and what time. The image is downloaded anew every time somebody opens the message, so it is possible to even count how often a message is opened.
When a request for the hidden image is recorded at the server, the sender connected with the specific message is notified that their message has been received and opened.
HTML Return Receipts Do Not Always Work
Since this concept of return receipts relies on an image being retrieved from a remote server, this is also its weak spot. If the image is not requested, the message has not been read officially.
There are a number of reasons that can prevent the image from being retrieved.
The recipient's email program may not understand HTML code and thus ignores the instructions to request the image.
The recipient may not be connected to the internet when they open the message. No connection to the server can then be established and consequently the image cannot be requested.
The recipient may also be concerned about their privacy. They do not want anybody to know when they open and read which email messages. Consequently, the recipient has configured the email client not to make any connections to remote servers without asking first.