As far as I have searched, there are many third-party email tracking systems; and many of them are in the form of add-ons for gmail.
I have tested two of them, and they work fine when the recipients are using gmail or yahoo web clients. But when they use MUAs (Mail User Agents) like Outlook or Thunderbird, a warning message is shown that a "remote content is blocked", and as a result, no delivery report is sent. They can open the emails without any notifications.
Is there a way to track emails undetectably, so no one can detect that emails are being tracked?
I should mention that I am only concerned about the mail being read, not gathering data.
Best Answer
Is there a way to track emails, so no one can detect that emails are being tracked?
No.
If email is tracked it must communicate that information somewhere and therefore must leave a footprint of some kind.
You can use such tricks as a unique image (i.e. the image is only in the email to one recipient ever) and watching the web server to see if the image is accessed, but this will only work if the user loads the images in email, which doesn't always happen, which you've experienced with thunderbird. How much content is loaded is decided by a users settings and so isn't something you can control.
Some companies will provide a link in an email 'can't read this email? click here to view online' which is basically the same thing, but instead of a image online they present the entire emails content again, under the guise of 'helping you' when in fact it's all about tracking you.