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What is an email tracking pixel? What senders learn

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What is an email tracking pixel? What senders learn

Francesco Sappa

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Marketing email can tell the sender that you opened it, when, and roughly where you were. It uses a hidden one-pixel image called a tracking pixel.

In short

  • A tracking pixel is a hidden image. When your mail app loads it, the sender's server records the open.[1]
  • The sender sees the time, how often you open the message, your IP address and your mail app.[6]
  • An alias hides your address, not this. Blocking the pixel is a separate step.
  • Skudo removes the common trackers and lists what it leaves alone.

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How it works

Marketing email is HTML, and HTML can load images from someone else's server. A tracking pixel is one of them. In the message source it looks like this:

<img src="https://mail.example-mailer.com/o/7f3a91c2d4.gif" width="1" height="1" alt="">

The image is transparent, so you never see it. The part after /o/ identifies your copy of the message. Mailchimp says its version "loads a tiny, transparent image into each email".[1]

The sender puts a hidden image in the message. Your mail app asks for it when you open the message, and the sender's server writes down the request.
The sender puts a hidden image in the message. Your mail app asks for it when you open the message, and the sender's server writes down the request.

What the sender sees

The sender can see How
That the message was opened, and when The image request arrives at their server
How many times it was opened Each load is one more request
Roughly where you are The IP address of whoever made the request
Which mail app you use The user agent sent with any web request

Apple's privacy page lists the same things: when and how many times you opened the message, and your IP address and location.[6] The sender does not get your real address from this.

A link in the message can add tracking too:

https://shop.example/summer-sale?utm_source=newsletter&utm_campaign=summer-sale&fbclid=IwAR0abc123
Parameter Where it comes from What it says
utm_source, utm_medium, utm_campaign Google Analytics convention[2] Where the visit came from
fbclid Generated by Meta[3] Which ad click it belongs to

If a link also carries an identifier unique to you, the sender knows you opened the message and looked at the product.

What Skudo removes

Skudo removes Skudo leaves alone
Remote images of 3 pixels or smaller Large remote images
Images from known tracking domains, or with paths like /open or /pixel.gif Unsubscribe links
Link parameters like utm_*, fbclid, gclid, msclkid Redirect links

It is on by default and runs before your mail app sees the message. Messages that arrive already end-to-end encrypted are left as they are. addy.io lists "Block Tracking Pixels" on its Lite and Pro plans and not on Free.[8]

Removal reduces tracking, it does not end it

A sender can still use a large image with a unique address, and Skudo will pass it through. It takes the cautious side because it would rather leave a tracker in than break a message.

In your own mail app

  1. Do not load remote images by default. In Gmail this is "Ask before displaying external images". Even then, "sometimes, senders may know whether you've opened an email that has an image".[4] Gmail loads images through Google's proxy servers, so the sender sees Google's request, not yours.[5]
  2. Turn on Apple's Protect Mail Activity. Apple says "your IP address is hidden from senders and remote content is privately downloaded in the background when you receive a message (instead of when you view it)".[7]
  3. Strip trackers before they arrive. That is what a forwarding service can do, and it complements the two steps above.

What this does not change

Blocking a pixel does not remove you from a sender's list. The clean answer is one address per sender, so you can switch off the one that starts tracking you: see email aliases for online shopping.

Sources

  1. Mailchimp, About Open and Click Rates. https://mailchimp.com/help/about-open-and-click-rates/. Checked on 19 September 2026. ↑
  2. Google Analytics Help, Collect campaign data with custom URLs (legacy Universal Analytics page). https://support.google.com/analytics/answer/1033863. Checked on 19 September 2026. ↑
  3. Meta for Developers, ClickID and the fbp and fbc Parameters. https://developers.facebook.com/docs/marketing-api/conversions-api/parameters/fbp-and-fbc/. Checked on 19 September 2026. ↑
  4. Gmail Help, Turn images on or off in Gmail. https://support.google.com/mail/answer/145919. Checked on 19 September 2026. ↑
  5. Google Workspace Admin Help, Set up an image URL proxy allowlist. https://knowledge.workspace.google.com/admin/gmail/advanced/set-up-an-image-url-proxy-allowlist. Checked on 19 September 2026. ↑
  6. Apple Legal, Mail Privacy Protection & Privacy. https://www.apple.com/legal/privacy/data/en/mail-privacy-protection/. Checked on 19 September 2026. ↑
  7. Apple Support, Protect email privacy in Mail on Mac. https://support.apple.com/guide/mail/protect-email-privacy-mlhlp1205/mac. Checked on 19 September 2026. ↑
  8. addy.io, home page pricing section. https://addy.io/. Checked on 19 September 2026. ↑