I'm using TestFlight for my apps.
When I uploaded a new ipa file to testflight, testflight sent me 1 email.
I only read this email by Mail App on my MAC. I didn't open testFlight app on any smartphone.
And the status is "Email Opened" (following pic)
How can they do that? Anyone like me :(, they sent virus via email ?????
If this is HTML mail they probably have attached an unique identifier to an image that is loaded externally.
If you have a image in your email that is loaded from the resource
they change the URL in each email to something like
When that image is loaded they know that you have opened the email because of your unique identification is in the request.
There are many more ways to do this but this is one way.