How Email Open Tracking Works

How Email Open Tracking Works

Note: This is Part II in a three part series about email open tracking. Before you read this post, you should check out What is Email Open Tracking?

Many sales and marketing tools offer the ability to track when people open your emails, including RocketBolt. Because we have email open tracking -- via Gmail and Outlook plugins -- we get a firsthand look at the difference between what email open tracking actually is and what people think it is.

Let’s start by clearing up the most important misconception: email open tracking is nowhere close to 100% accurate. Any company trying to convince you otherwise isn’t being honest and should be avoided.

Despite not being perfect, email open tracking is a powerful tool that, when used properly, can alert you to valuable strategic selling opportunities. Since we believe the better you understand how something works, the better you’ll be able to use it, here’s a quick overview of how email open tracking technology actually works.

The Background

The email protocol has no mechanism for dynamic logic. That means nothing can change inside the content of an email after it’s delivered, and readers can’t interact with any parts of their emails (aside from clicking links) once emails are rendered by servers and sent to their recipients.

Contrast this with sites like Facebook where you can enter text, upload pictures, and “Like” the posts of other people, all without refreshing the page.

In terms of email tracking, this means emails don’t natively have the ability to notify their senders when they get opened.1

The Concept

Even though email content can’t change after being sent, emails can include images. This adds a pseudo-dynamic element because an image – when rendered by an email client – is actually just a link pointing to the image file’s URL. In order to render the image, the email client has to load it from the remote server.

If you control the server that’s being contacted for an image file, you can listen for an email client requesting a certain image and, by doing so, you can guess that an email containing a certain image was opened.

The Execution

If you’re already using email tracking, you’re probably thinking: “What do images have to do with anything? I rarely ever send images in my emails.”


If you’re using email tracking, you're always sending images in your emails. They’re just tiny (usually 1px) and invisible.

Email tracking software inserts that tiny, invisible, uniquely identifiable pixel somewhere in your email. It’s impossible to see, but the email client rendering your message doesn’t know that, so, like every other image, it calls the server where the image is stored. Your email tracking software listens for that unique image pixel to be called, and, when it is, it can guess that the associated email was opened.

In addition, because the image file was accessed on a server, that server can tell certain things about the computer calling the file such as the IP address (which is connected to a user’s location) and the kind of web browser being used. This is why email tracking software occasionally tells you where an email was opened and what kind of device was being used to view it.

The Limitations

The concept of email open tracking is simple, but lots of variables can impact reliability. Here’s a list of the more common issues:

  • Individual recipients can’t be differentiated: This is probably the biggest limitation of email tracking that most people don’t realize. When an email client calls a server to access an image, there’s no way to tell what person’s email client requested it. This isn’t an issue if you're email was sent to one person (unless that person forwards your email), but it’s a problem if you send an email to multiple people – even when using CC or BCC. The only thing email tracking can do is tell you someone is (probably) viewing an email.

  • Images aren’t rendered by default: Some email clients – including Microsoft Outlook – don’t show images by default. With default image rendering disabled, the recipient has to click a link in order to download images for that particular email. If they don’t, you won’t know your email was opened.

  • The email client isn’t accepting HTML: Rendering an image in an email requires the recipient’s email client to accept HTML emails. Not all clients accept HTML emails. In addition, some people intentionally disable the option. In either case, email open tracking won’t work.

  • Location data is unreliable: Even though, in theory, a server should be able to identify the IP address (and thus, location information) about a server accessing an image, email routing structures often rely on remote resources to access accounts. This is particularly true when people open email on mobile devices, and you’ll often see an email register as being opened by someone a few states away when you literally saw the person 30 minutes ago.

  • Image pre-caching by email providers: Some services (primarily Gmail), automatically download and store all images whenever they detect new emails. Because of this, an email can register as being opened when it was actually the email recipient’s provider automatically opening and storing a copy of the tracking pixel. In addition, because the tracking pixel has already been downloaded and saved by the email provider, an actual email open might not require another call to the tracking pixel’s original server, which will also prevent an email open from registering.

  • Can’t distinguish between you and your recipients: If you have experience with email tracking, you’ve likely received notifications when you open an email you sent or you read someone’s reply to your message. That’s because the limitations of email tracking prevent the system from being able to differentiate between an email open by you and an open by someone else. Technically, the email tracking is working because it’s notifying you of a genuine email open, but seeing notifications about your own actions can be annoying if you don't understand why they're happening.

[1] The email protocol does include the concept of a “read receipt,” however it’s not widely supported by email clients.

Tim Bachmann

As RocketBolt's Social Media Manager, Tim likes writing about social marketing and social selling. In his free time, he writes about zombies, which is sort of what social media turns people into.