request acknowledgement of email receipt

As far as I can see, you need the menu to be visible in the compose window. Then you can find it under '''Options''' ("Return Receipt"). I'm not sure how the designers of Thunderbird anticipate you seeing this option, since the menu is almost certainly invisible by default. So, with the Write window open and focussed, the <alt> key on your keyboard on a PC running Windows or Linux will reveal the menu, albeit temporarily. 'O' will then open the '''Options''' sub-menu. (I'm not sure how Mac users do this.) If you want to make the menu permanent, use <alt> then V for '''View''' and then select '''Toolbars''' and tick the checkboxes.