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