How does someone with HubSpot Sidekick knows when I opened their sent email?

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

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.