You can be tracked by email.
A free Gmail plug-in called Streak lets the sender know where you are when you open email he or she sent you.
Streak doesn't explain how its system works, but most email trackers embed a tiny image file in the email, and when it's opened, your computer or device has to ping the server where the picture is stored, and that way trackers can find your IP address and locate you.
IP addresses aren't always able to specify your exact location, but they can be pretty close. The best way to protect yourself is to not let your email program autoload any images.