No longer receive return receipts or delivery status on emails

I used to get delivery status notification and return receipts for an email when I selected those options.

Early in January, I stopped receiving them, This happens with more than one email client, all of which I have uninstalled and reinstalled. The notifications messages do not appear on the Cox as they used to.

On various forums, the consensus seems to be that ISPs can pass these messages back to the sender or not; if they are not being received it is probably because the ISP has decided to block them. I would like to know if Cox allows these notifications to pass to email senders, and if so whether there is any known cause for them not being received.