The usage of HTML emails offers many tracking features for the sender. By using such tracking features like webbugs the sender may get information about the time you opened the mail, your IP address, used software and a list of forwarded recipients.
The tracking features are not blocked at all by webinterfaces. Even if you see a message about blocked tracking elements the protections is not safe but only partially. You may use the E-Mail Privacy Test for testing the webinterface of your preferred email provider. Open the test page and send a mail to your mail account. Read the received message in a new browser tab (in most cases you will find it in the spam folder) and go back to the E-Mail Privacy Test page. You will see a list of not blocked tracking features (red marked):
The result depends on the configuration of your browser too, but JonDoFox can’t protect you against all possible email tracking features. We highly reommend the usage of Thunderbird + TorBirdy for email communication to stay privat.
from here