What is a tracking pixel?
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A tracking pixel is a tiny, invisible image embedded in an email's HTML. When your email client loads that image, it pings the sender's server, which logs the open. That's how ESPs like Mailchimp, Klaviyo, and SendGrid calculate open rates.
Technically, it's a 1x1 pixel image (often a transparent GIF or PNG) with a unique URL for each recipient. When the email is opened and images load, that URL is fetched from the sender's server. The server sees the request, matches the unique URL to the recipient, and marks the email as opened.
But The problem: many email clients block images by default. Gmail proxies images through its own servers (which can still register the open, but strips IP and location data). Outlook and Yahoo Mail often block external images until the user clicks "show images." Apple Mail (since Mail Privacy Protection in iOS 15) prefetches all images when the email arrives, whether or not the user actually reads it. That means opens get recorded even if the email sits unread in the inbox.
What this means for you: open rates are directional, not absolute. They tell you trends ("this subject line worked better than that one"), but they don't tell you who actually read your email. A 40% open rate doesn't mean 40% of people read your email. It means 40% of people had images load (or Apple Mail prefetched them automatically).
You can't control whether tracking pixels work. That's entirely up to the recipient's email client and their settings. What you can control is how much weight you put on open rate data. If you're making decisions based on opens alone (like removing inactive subscribers after six months of no opens), you're probably cutting engaged readers who have images blocked. Better strategy: look at clicks, replies, and conversions alongside opens.
One more thing: tracking pixels only work in HTML emails. Plain text emails have no images, so there's nothing to load and no way to track opens. If you send plain text (or if the recipient's client displays the plain text version), you're flying blind on opens. That's fine. Clicks and replies still work.
And if you want to see what your tracking pixel looks like, view the HTML source of any marketing email you've received. Look for an <img> tag with a 1x1 dimension and a long, complex URL. That's the pixel.
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