Not all emails are what they seem. Many messages come with embedded code designed to tell the sender when (and even where) you open them up. It's a trick often used by marketing companies to work out if you're actually paying any attention to them, but there are ways of spotting this kind of email tracking.
Please note: There is no 100 per cent effective method of avoiding email tracking, not least because the methods used and email technology themselves are constantly evolving. However, for a quick and largely effective solution, the browser extension Ugly Email (Chrome only) is the tool you want.
Once you've added the extension to Chrome and reloaded Gmail, you'll see tell-tale eye symbols next to all the messages with some kind of tracking software embedded in them. You can delete these without opening them or at least get an idea of which companies want to know most about your email-opening activities.
The tracker itself is usually an invisible, single pixel image. When the email is opened, the image is retrieved from wherever it's hosted, and the senders have the information they're looking for. One of the more old-school methods of blocking email trackers is to not load images by default (under General in Gmail's settings) but that's not an ideal solution.
Another similar Chrome extension we like is PixelBlock. In this case you have to open up your emails to see the eye icons, though you do get extra information such as the number of tracking attempts and the source of the tracking widget for each message. For the most complete protection, you might want to consider installing both tools.