At the moment the classic read receipt model is designed in a way so the recipient of the email needs to open the email in an email client or webmail that also supports read receipt, after that, they have to agree to send the read receipt or not. This is annoying (for the receiver) and not accurate because it depends on the receiver intent on sending the read receipt, basically, it's not very useful.
The second one is the more recent read receipt model where the sender embedded a link and a transparent picture with tracking code on each email that he sent. When the recipient opens this email, this picture will send data back to the sender so that they know when, where, how often, etc the recipient interacts with the email. This looks like a brilliant solution to read receipt, but the issue is, all decent email clients will block images that are embedded in the body of an email. This is due to a security issue since images and links embedded in an email is one of the ways you can get infected by a virus. But even this solution is not 100% accurate.
What we wanted to do is to develop a read receipt that is 100% accurate. SO yes, we are planning to add this feature, but not just now. We know how important this feature is for some businesses, so having an unreliable read receipt is just as bad or even worse than not having one at all. I
If you still want to use read receipt in Mailbird, there are free 3rd party app that you can use, one of them is this one