Hello Paul. I just tried to follow the same instructions for requesting receipts, which I assume you probably found at LiveWire, or at some other site which copied the instructions from LiveWire.
To be honest, I am not sure if it is working or not, because while I did get the error message which they mention -- that is, "The domain/default pair of (com.apple.mail, UserHeaders) does not exist," -- and then followed what they said to do after that, in the Terminal feedback, I am getting nothing like "{Bcc = "bcc@example.com"; },". So as I said, I am not sure.
In addition, I am wondering if that same command can be run for three different email addresses, or if it is just restricted to one address.
The reason why I am doubtful that it is working, is because I sent myself three messages from one of my email addresses to another one of my email address. That is, from two different email servers. I did not receive a single read request.
While I am sure you probably already know this, in the above, you are replacing "John Doe <sender@example.com>" with your actual name and email address, right? I imagine you typed that way to protect your own privacy, right?