Microsoft itself suggests asking for receipts only on important messages, because recipients who are asked rarely are more likely to answer. It also notes that the option is not available for IMAP or POP accounts such as Gmail, Yahoo or iCloud in Outlook, and that recipients can decline, so a receipt that never comes proves nothing.
HawkOpen does not use read receipts. It records image loads and clicks for each tracked email: from Outlook on the web through the extension for Chrome or Firefox, or from the Outlook desktop app by sending through the HawkOpen dashboard with your connected Microsoft mailbox. Each open gets a category, and none is more than “likely” a person.