Tracking email opens

posted: February 13, 2025

tl;dr: A peak under the hood of email opens...

To interpret the “opens” and “links clicked” statistics collected by email senders, and how those statistics might be skewed, it helps to understand how individual opens and link clicks are actually tracked. I’ll start with opens.

Almost all emails these days are formatted as HyperText Markup Language (HTML), the same as a web page. If you actually look at the raw source of an email, you will see this. However, few users look at the raw source. They rely upon the email tool they are using to “render” or “interpret” the HTML in order to display the email to the user.

The email tool (sometimes called “client”) turns the raw HTML into a nice looking email, with text in different fonts, images, links, and buttons. This rendering happens whether you are viewing the email on a website such as Google Mail, Outlook or Hotmail, or whether you are viewing the email by using an app on your smartphone. To render the email, the email client has to issue requests to servers on the Internet to obtain necessary information, such as images. Images are not typically embedded inside of the email itself, as that would make the emails very large in size. Instead the email client fetches the images from servers when it is time to render the email.

Embedded within the HTML are the tricks that the email system which sent the email uses to track email opens and clicks. Most email systems which send emails, such as Salesforce Marketing Cloud, use the same tricks. Email opens are tracked with something called an “invisible tracking pixel”. When the email system sends each email to each recipient, an extra image (in addition to all the other regular images in the email) is inserted into the email, typically towards the top. Here is an example from an email I received from realtor.com:

On a white background a blue cloud on the left with the word 'salesforce' inside it in white, with the words 'marketing cloud' in black to the right of the cloud, all lettering in lower case

A visible, non-tracking image of 724 pixels by 292 pixels

“img” means this is an image, “src” means source, and what follows is a URL. The first part of the URL looks like a typical website address, but this address at e.e.mail.realtor.com does not serve up a web page; instead, it serves up an image. The long string of letters and numbers in the URL, which is computed by the email system, uniquely identifies this particular invisible tracking pixel for an individual email sent to an individual recipient. Every copy of the email sent to every recipient will have a different URL, and every email sent by realtor.com to the same recipient will have a different URL. A “height” and “width” of 1 means the image is tiny, just one pixel by one pixel, which is effectively invisible to the human eye.

When the recipient opens the email, the email client goes to work on rendering the email’s HTML to turn it into a pretty looking email. Since it needs to obtain the images to be displayed, it issues a request to a server on the Internet for each image, using the URL for that image. The invisible tracking pixel image is no different: in this example the email client issues a request to e.e.mail.realtor.com using the entire URL. The server at e.e.mail.realtor.com sees this request and it does several things. It decodes the long string of letters and numbers in the URL and records an open event for that particular email sent to that particular user. It also returns a tiny 1x1 pixel image to the email client.

This trick for tracking opens is not foolproof. Some email clients contain anti-tracking features that won’t bother trying to retrieve the image if it is only 1 pixel by 1 pixel. Such an image size is highly likely to be a tracking pixel, and the image is not visible to the human eye anyway. This will decrease the number of open events received by the email system. Some email clients, such as Apple Mail, will prefetch email images or all the components needed to render an email before the user opens it, so that if the user actually does open the email it can be displayed more rapidly. This will increase the number of open events received by the email system.

Bottom line: an email open event does not necessarily mean the user actually opened the email, and the overall count of email opens may be higher or lower than the actual number of times the email was opened. There’s no way to know for certain how many times actual human beings opened a given email. The email system may also interpret an open event as an indication that a particular user has actively engaged with a particular email, and that is not necessarily the case.