Read receipt
Have you ever wondered why email clients have you click on something before displaying the images? There are a couple reasons why they do this.
One reason is because the images can take a while to download and delaying the images can make reading the email faster. You can decide at a glance whether you actually want to see the pictures associated with the email. This is a good lesson to anyone writing an email that includes images. It's a good idea to include text that will entice the reader to want to download the associated images.
Another, more sinister reason why images are delayed is because they can actually divulge information about the person who is reading them. When you load the image, it might require a download from a web server. When you connect to the web server you divulge your IP address which can offer some insight as to who you are. Additionally, reference to the image might contain information that was unique to the email that was sent to you. It might contain a unique identifier. In this case simply loading the image will tell the web server that you've read the email, acting as a read receipt.
Consider the following code example for an HTML email. When the sender writes the email, he includes the email address of the person he is sending it to. This can be done in an automatic script. When the recipient loads the image it sends their email back to the web server. This could be tied to a database that is used to track whether an email address is valid or whether to record when the recipient first read the email.
<html> <head></head> <body> <img src="http://www.qicboy.com:80/receipt.php?eml=them@someisp.com" border="1" /> </body> </html>