I can't find the checkbox for marking read receipts! Please help!

I can't find the checkbox for marking read receipts! Please help!

I am using Zoho email to send one by one emails to my prospects. 

I need to mark read receipts in the email. I did read on the forum that we need to mark the read receipts but i can not locate the check box in the send email window!

Please help!

Thanks
Amol