This app claims to know when recipients opened the emails that were sent to them. Tracking clicked URLs is fairly straightforward. What I don't understand is how a 3rd party can possibly know when I open their email?

Does web-based email clients automatically provide reading-proof to all emails? Because I remember Outlook was asking me whether I wanted to actually send proof of reading or not. If so, how do I disable it?

  • Sidekick sends you alerts even when you open your own sent mail. This is a flaw.
    – user80579
    Jul 8, 2015 at 21:33
  • @GurvinderS. Seems to be fixed now.
    – AStopher
    Aug 20, 2015 at 22:46

4 Answers 4


They track opens the same way every other email sending/analytics company does it: by inserting a tracking pixel within the HTML of the email. If your email client blocks image loading by default, then you won't be tracked. If you load the images, or your client automatically downloads the images (iPhone email client) then you're being tracked.

You can see more information about how this works precisely here: http://en.wikipedia.org/wiki/Web_bug

  • 1
    Note that not only images can be used. This tool demonstrates a few different techniques, and some of them worked even though remote images were disabled in my (crappy) mail client.
    – user42178
    Oct 16, 2014 at 13:42
  • Am I safe if I use mutt? It can't read html or load images anyway.
    – MadTux
    Oct 16, 2014 at 14:01
  • @MadTux I'm pretty sure any open-source client is safe, but to be sure just test it, enter your email (create a throwaway one if you don't trust the site), open the mail in Mutt and see if the site detects anything.
    – user42178
    Oct 16, 2014 at 14:33
  • @AndréDaniel Nothing. Good :)
    – MadTux
    Oct 16, 2014 at 14:56
  • How does Sidekick know who opened the email? I've received forwarded emails, and I show up in my mate's stream. Shouldn't it only know which image has been loaded, and maybe the URL it was loaded from (if it's a web mail client)?
    – Jayen
    Sep 3, 2015 at 2:33

I'm not familiar with Sidekick, but another way to verify that an email has been opened is to use an image tag in the body of an HTML email.

Let us pretend that the image tag looks like:

<img src="server.com/images/[email protected]" />

The server delivering tracker.jpg can then record the query string ([email protected]) and report that the email has been opened.

Normally, marketing campaigners use a long string of random characters that translates back to your email but you get the idea.

You can block it by blocking the downloading of images in your email client. Instructions for Outlook.

As to your second question, it depends on your webmail. Search Google for "read receipt" and the name of your webmail client.


As @MrDCGN and @saltface pointed out, the tracking is done using hidden objects in the HTML view of the email.

Blocking this kind of tracking can be achieved in a few ways:

  • Stop using HTML view, and use only plain text. This would prevent loading of any non-text object, and will damage RTF formatting.
  • Disable image loading in the settings of the email service/app.
  • Use 3rd party extensions, such as Ghostery for Firefox, or PixelBlock for Chrome.
  • Custom formatting script, as described here.

Sidekick is designed to track emails sent to individual email addresses.

Sidekick tracks emails by placing an invisible, 1-pixel image inside the message. When the email is opened, the image loads, and Sidekick notifies you that the email was received.

  • 2
    I'm not sure that this answer adds anything to the other answers listed.
    – schroeder
    Apr 7, 2016 at 3:47

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