If your recipients have ‘showing images’ enabled, then there is almost zero the margin of error of our tool. Still, there is a second method by which we can keep track of your e-mails: through the links contained in your email.
So it is suggested to always include a link into your email like (i.e., add something like please read document here, or please see this screenshot, etc.)
Limitations such as this one should not worry. Nowadays, many of the email clients open images by default. It is also increasingly widespread the use of emails with images, which helps the recipient to accept those more and more to enable them. This reduces the number of emails not tracked, significantly reducing our margin of error.
In fact, Gmail decided to open images by default, a good indicator of this tendency among email clients.