UglyEmail Chrome extension could help Google users to discover which are the companies on the web that track their email messages. Companies like Yesware, Bananatag, and Streak are aware of your existence, to be more specific, they know when you open e-mail sent by one of their clients. When that email is opened, the image pings the server it originated from with information like the time, your location, and the device youre using. The emails identified with tracking pixels will appear with an eye icon next to the subject.”]
Source: https://securityaffairs.co/wordpress/35292/security/discovering-tracking-emails-uglyemail.html