How Email Tracking Pixels Work: A Plain English Guide

If you have ever wondered how an app can tell you that the email you sent this morning was opened at 2:14pm, the answer is a single invisible image. There is no secret channel into the recipient’s mailbox and no cooperation from their mail provider. This guide explains what a tracking pixel is, what it can honestly tell you, and why the same email can produce a clear result for one recipient and complete silence for another.

What a tracking pixel actually is

A tracking pixel is a tiny image file referenced by a web address inside the email. The European Data Protection Board, in its Guidelines 2/2023 on the technical scope of the ePrivacy Directive, describes it as a hyperlink to a resource, usually an image file, embedded into a piece of content like a website or an email. The UK Information Commissioner’s Office uses almost the same description in its guidance on storage and access technologies, calling tracking pixels small pieces of code, usually an image file, embedded into a website or an email.

The image is normally transparent and about one pixel across, so it sits invisibly in the message. It serves no design purpose. As the EDPB puts it, its sole purpose is to automatically establish a communication by the client to the host of the pixel, which would otherwise not have occurred.

What happens the moment the email is opened

When a mail app displays a message, it downloads every remote image that message points to. The tracking pixel is one of those images. That download is an ordinary web request, and like any web request it arrives at the server with three useful pieces of information attached.

The time. The server records when the request arrived, down to the second.

An IP address. Every request comes from somewhere on the internet. The IP address maps to an approximate location, which is where the city and region in your tracking app come from. It is a rough signal, and it is covered in more detail in how accurate email open location is.

A user agent string. MDN describes the User-Agent request header as a characteristic string that lets servers identify the application, operating system, vendor, and version of the requesting software. In an email context that is often enough to say the message was opened in a particular mail client on a phone rather than a desktop, though many mail providers strip or generalize it.

Each tracked email gets its own pixel address, so the server knows which message was opened rather than just that some message was.

What the pixel can tell you

• That the images in the message were loaded at least once.
• Roughly when that happened.
• An approximate location, when the IP address is not hidden behind a proxy.
• The mail client or device family, when the request includes enough detail.
• Whether it happened again later, which usually means the message was reopened or forwarded.

What a tracking pixel cannot tell you

This is where honest tracking apps and optimistic marketing copy part ways.

• Not whether it was read. Loading images and reading words are different events. A one second preview loads the pixel. A careful read in a plain text view loads nothing.
• Not who opened it. If your recipient forwards the message, the forward’s opens look like more opens on your email. There is no name attached to the request.
• Not how long they looked. There is no timer, only the moment the image loaded.
• Not what they clicked or typed. A pixel is an image request, not a script. It sees nothing inside the mailbox and nothing about other messages.
• Not the street address. IP geolocation is a neighborhood level guess at best, and often a city level guess.

If you want a comparison with the built in feature people usually reach for first, see read receipts versus email tracking.

Image proxies changed the picture

For years the pixel request came straight from the recipient’s device. That is no longer true for two of the biggest mail providers.

Gmail routes images through Google. Google’s Workspace documentation states that when users open email messages, Gmail uses Google’s secure proxy servers to serve images that might be included in those messages. Gmail’s own help page for users says that by default, when you get an email with an image, you see the image automatically, and that senders cannot use image loading to get information about your computer or location. The practical result for a sender is specific and worth remembering: with Gmail you usually do get the open and the time, but the location is Google’s, not the reader’s, so an honest app shows it as unknown.

Apple pre-downloads images whether or not anyone opens the message. Apple’s Mail Privacy Protection page explains that Mail downloads remote content in the background by default, regardless of whether you engage with the email, and routes it through two separate relays so that no single party knows both who you are and what content you received. That is the single biggest reason pixel tracking is less reliable than it was, and it has its own article: Apple Mail Privacy Protection and email tracking.

Why the same email gives different results in different mail apps

Microsoft’s support page on blocking picture downloads states plainly that Outlook is configured by default to block automatic picture downloads from the internet, and lists avoiding tracking pixels as one of the reasons. Microsoft even describes them as invisible images that can tell a sender you have read the email. So a silent result from an Outlook recipient is normal, not a fault. The details are in tracking Outlook emails on iPhone.

Why some opens never show up at all

Beyond image blocking, a few ordinary situations produce silence:

• The recipient reads a plain text version, which has no images to load.
• Their employer’s security system strips remote images before delivery.
• The message went to spam and was never opened.
• They read the subject line in a notification, decided it could wait, and never opened the message.

None of these are bugs in tracking. They are the honest limits of a technique that depends entirely on an image being fetched.

How Email Tracker handles the messy parts

Email Tracker is an iPhone app that adds a pixel to emails you compose through it, using Apple Mail’s own compose sheet, and notifies you when that pixel loads. Because false opens are worse than no opens, it makes a few deliberate choices:

• It ignores Apple’s automatic pre-fetch instead of reporting it as an open, so you will not see a confident open that never happened.
• It ignores requests coming from your own device and network, and anything in the first 60 seconds after the email is created, so your own compose and send activity does not register.
• It counts repeated loads from the same reader within five minutes as a single open.
• With Gmail recipients it shows the open and the time, and marks the location unknown rather than reporting a Google data centre.

That means quieter results than some tools report, and results you can act on. If you are setting this up for the first time, start with how to track email opens on iPhone, and if your opens are not appearing, work through why email opens are not showing.

Email Tracker is a free download on the App Store for iPhone, with an optional subscription. It works with Gmail, Outlook, iCloud, Yahoo and any other address you have added to Apple Mail.