Chrome: It's possible for people to track whether you've seen an email using tracking pixels, and if that idea makes you cringe a bit, Ugly Mail warns you when an email you receive can track you.
Ugly Mail monitors your email for tracking pixels from services like Yeswear, Bananatag and Streak. When you open up an email with tracking pixels, the sender will get information back that you opened it, whether you clicked a link, where you are, and what sort of device you're on. With Ugly Mail, when you receive an email that's tracking you, you'll see a little eye icon so you can decide whether or not you want to open it.
Ugly Mail only works with Gmail and Chrome at the moment. If you want to actually block those tracking pixels, another Chrome extension, PixelBlock will do it for you.