What if the recipient disables images and HTML/script ?

If the recipient blocks images it will not work unless either you persuade them to unblock images by including an image they would want to see or they click on a link (link tracking only for pro users). Most users won't have images blocked automatically especially if it's someone you commonly email. No email tracking can work all the time but this will work in as much cases as possible, is completely free, and is so seamless you don't even know it's there until you need it. Enable it and I guarantee you will get some interesting stats about the people you email. It's definitely helped with my follow ups and replies.

Most of these tools don't work. How does this one work differently?

No email tracking can work all the time but this will work in as much cases as possible, is completely free, and is so seamless you don't even know it's there until you need it. Enable it and I guarantee you will get some interesting stats about the people you email. It's definitely helped with my follow ups and replies.

You kind of dodged the person's question. What do you use to check that emails are opened?

Sorry. The extension uses a hidden tracking pixel to check if emails get opened.

This would be interesting if it was a Thunderbird plug-in.

Thanks for the suggestion. I'll definitely keep it in mind for the future.