Email tracking, though works across most modern email clients, not fully accurate. In this article we have provided the details of How Email Tracking Works.

In short, a tracking pixel (a very small invisible image) is added to your email by a tracking software like MailerPlex, when you send your email. This image gets downloaded from MailerPlex server, when your recipient opens the email. This pixel is unique using which MailerPlex gets to know the opens.

But in some email clients, image download is disabled by default (like some versions of outlook). Or in some cases the recipient may have blocked auto image download. So the email can’t be tracked in these cases even if the user has opened.
The good news is most of the modern emails, allow downloading images by default (like Gmail, Yahoo, Apple Mail etc). This is to give their users a good experience. So majority of cases should be covered.

For more details on how email open tracking works, you can follow this link.
Short Guide on How Email Tracking Works