Under account settings for an account is 'Return Receipts' in left column. All settings for that are there. BUT, participating by recipient is voluntary and may ignore return receipt requests. For active email tracking tools, there are some free and fee-paid services available and can be located with internet search.

Thank you, David. I already tried 'Return Receipts' option before sending this request without knowing that it is voluntary. Can you please provide me some examples of some free and fee-paid services available for mailtrack that work with Thunderbird? Thx

Mail tracking relies on remote images, customized URL links or the return receipts you have already explored.

So any tracking information you get will largely depend on if you recipient either clicking a link or allowed remote images. Neither is guaranteed.

You probably need to look at some of the Internets mailing house specialists. Perhaps try searching Google for the term "email marketing" to determine what works and what does not.